[v2,2/3] userfaultfd: protect mmap_changing with rw_sem in userfaulfd_ctx

  Increments and loads to mmap_changing are always in mmap_lock
critical section. This ensures that if userspace requests event
notification for non-cooperative operations (e.g. mremap), userfaultfd
operations don't occur concurrently.

This can be achieved by using a separate read-write semaphore in
userfaultfd_ctx such that increments are done in write-mode and loads
in read-mode, thereby eliminating the dependency on mmap_lock for this
purpose.

This is a preparatory step before we replace mmap_lock usage with
per-vma locks in fill/move ioctls.

Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
---
 fs/userfaultfd.c              | 40 ++++++++++++----------
 include/linux/userfaultfd_k.h | 31 ++++++++++--------
 mm/userfaultfd.c              | 62 ++++++++++++++++++++---------------
 3 files changed, 75 insertions(+), 58 deletions(-)
  

* Lokesh Gidra <lokeshgidra@google.com> [240129 14:35]:
> Increments and loads to mmap_changing are always in mmap_lock
> critical section.

Read or write?


> This ensures that if userspace requests event
> notification for non-cooperative operations (e.g. mremap), userfaultfd
> operations don't occur concurrently.
> 
> This can be achieved by using a separate read-write semaphore in
> userfaultfd_ctx such that increments are done in write-mode and loads
> in read-mode, thereby eliminating the dependency on mmap_lock for this
> purpose.
> 
> This is a preparatory step before we replace mmap_lock usage with
> per-vma locks in fill/move ioctls.
> 
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> ---
>  fs/userfaultfd.c              | 40 ++++++++++++----------
>  include/linux/userfaultfd_k.h | 31 ++++++++++--------
>  mm/userfaultfd.c              | 62 ++++++++++++++++++++---------------
>  3 files changed, 75 insertions(+), 58 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 58331b83d648..c00a021bcce4 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -685,12 +685,15 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
>  		ctx->flags = octx->flags;
>  		ctx->features = octx->features;
>  		ctx->released = false;
> +		init_rwsem(&ctx->map_changing_lock);
>  		atomic_set(&ctx->mmap_changing, 0);
>  		ctx->mm = vma->vm_mm;
>  		mmgrab(ctx->mm);
>  
>  		userfaultfd_ctx_get(octx);
> +		down_write(&octx->map_changing_lock);
>  		atomic_inc(&octx->mmap_changing);
> +		up_write(&octx->map_changing_lock);

This can potentially hold up your writer as the readers execute.  I
think this will change your priority (ie: priority inversion)?

You could use the first bit of the atomic_inc as indication of a write.
So if the mmap_changing is even, then there are no writers.  If it
didn't change and it's even then you know no modification has happened
(or it overflowed and hit the same number which would be rare, but
maybe okay?).

>  		fctx->orig = octx;
>  		fctx->new = ctx;
>  		list_add_tail(&fctx->list, fcs);
> @@ -737,7 +740,9 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
>  	if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
>  		vm_ctx->ctx = ctx;
>  		userfaultfd_ctx_get(ctx);
> +		down_write(&ctx->map_changing_lock);
>  		atomic_inc(&ctx->mmap_changing);
> +		up_write(&ctx->map_changing_lock);
>  	} else {
>  		/* Drop uffd context if remap feature not enabled */
>  		vma_start_write(vma);
> @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
>  		return true;
>  
>  	userfaultfd_ctx_get(ctx);
> +	down_write(&ctx->map_changing_lock);
>  	atomic_inc(&ctx->mmap_changing);
> +	up_write(&ctx->map_changing_lock);
>  	mmap_read_unlock(mm);
>  
>  	msg_init(&ewq.msg);
> @@ -825,7 +832,9 @@ int userfaultfd_unmap_prep(struct vm_area_struct *vma, unsigned long start,
>  		return -ENOMEM;
>  
>  	userfaultfd_ctx_get(ctx);
> +	down_write(&ctx->map_changing_lock);
>  	atomic_inc(&ctx->mmap_changing);
> +	up_write(&ctx->map_changing_lock);
>  	unmap_ctx->ctx = ctx;
>  	unmap_ctx->start = start;
>  	unmap_ctx->end = end;
> @@ -1709,9 +1718,8 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
>  	if (uffdio_copy.mode & UFFDIO_COPY_MODE_WP)
>  		flags |= MFILL_ATOMIC_WP;
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mfill_atomic_copy(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
> -					uffdio_copy.len, &ctx->mmap_changing,
> -					flags);
> +		ret = mfill_atomic_copy(ctx, uffdio_copy.dst, uffdio_copy.src,
> +					uffdio_copy.len, flags);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -1761,9 +1769,8 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
>  		goto out;
>  
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mfill_atomic_zeropage(ctx->mm, uffdio_zeropage.range.start,
> -					   uffdio_zeropage.range.len,
> -					   &ctx->mmap_changing);
> +		ret = mfill_atomic_zeropage(ctx, uffdio_zeropage.range.start,
> +					   uffdio_zeropage.range.len);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -1818,9 +1825,8 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
>  		return -EINVAL;
>  
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
> -					  uffdio_wp.range.len, mode_wp,
> -					  &ctx->mmap_changing);
> +		ret = mwriteprotect_range(ctx, uffdio_wp.range.start,
> +					  uffdio_wp.range.len, mode_wp);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -1870,9 +1876,8 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
>  		flags |= MFILL_ATOMIC_WP;
>  
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mfill_atomic_continue(ctx->mm, uffdio_continue.range.start,
> -					    uffdio_continue.range.len,
> -					    &ctx->mmap_changing, flags);
> +		ret = mfill_atomic_continue(ctx, uffdio_continue.range.start,
> +					    uffdio_continue.range.len, flags);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -1925,9 +1930,8 @@ static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long
>  		goto out;
>  
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mfill_atomic_poison(ctx->mm, uffdio_poison.range.start,
> -					  uffdio_poison.range.len,
> -					  &ctx->mmap_changing, 0);
> +		ret = mfill_atomic_poison(ctx, uffdio_poison.range.start,
> +					  uffdio_poison.range.len, 0);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -2003,13 +2007,14 @@ static int userfaultfd_move(struct userfaultfd_ctx *ctx,
>  	if (mmget_not_zero(mm)) {
>  		mmap_read_lock(mm);
>  
> -		/* Re-check after taking mmap_lock */
> +		/* Re-check after taking map_changing_lock */
> +		down_read(&ctx->map_changing_lock);
>  		if (likely(!atomic_read(&ctx->mmap_changing)))
>  			ret = move_pages(ctx, mm, uffdio_move.dst, uffdio_move.src,
>  					 uffdio_move.len, uffdio_move.mode);
>  		else
>  			ret = -EAGAIN;
> -
> +		up_read(&ctx->map_changing_lock);
>  		mmap_read_unlock(mm);
>  		mmput(mm);
>  	} else {
> @@ -2216,6 +2221,7 @@ static int new_userfaultfd(int flags)
>  	ctx->flags = flags;
>  	ctx->features = 0;
>  	ctx->released = false;
> +	init_rwsem(&ctx->map_changing_lock);
>  	atomic_set(&ctx->mmap_changing, 0);
>  	ctx->mm = current->mm;
>  	/* prevent the mm struct to be freed */
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 691d928ee864..3210c3552976 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -69,6 +69,13 @@ struct userfaultfd_ctx {
>  	unsigned int features;
>  	/* released */
>  	bool released;
> +	/*
> +	 * Prevents userfaultfd operations (fill/move/wp) from happening while
> +	 * some non-cooperative event(s) is taking place. Increments are done
> +	 * in write-mode. Whereas, userfaultfd operations, which includes
> +	 * reading mmap_changing, is done under read-mode.
> +	 */
> +	struct rw_semaphore map_changing_lock;
>  	/* memory mappings are changing because of non-cooperative event */
>  	atomic_t mmap_changing;
>  	/* mm with one ore more vmas attached to this userfaultfd_ctx */
> @@ -113,22 +120,18 @@ extern int mfill_atomic_install_pte(pmd_t *dst_pmd,
>  				    unsigned long dst_addr, struct page *page,
>  				    bool newly_allocated, uffd_flags_t flags);
>  
> -extern ssize_t mfill_atomic_copy(struct mm_struct *dst_mm, unsigned long dst_start,
> +extern ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
>  				 unsigned long src_start, unsigned long len,
> -				 atomic_t *mmap_changing, uffd_flags_t flags);
> -extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
> +				 uffd_flags_t flags);
> +extern ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
>  				     unsigned long dst_start,
> -				     unsigned long len,
> -				     atomic_t *mmap_changing);
> -extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
> -				     unsigned long len, atomic_t *mmap_changing,
> -				     uffd_flags_t flags);
> -extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> -				   unsigned long len, atomic_t *mmap_changing,
> -				   uffd_flags_t flags);
> -extern int mwriteprotect_range(struct mm_struct *dst_mm,
> -			       unsigned long start, unsigned long len,
> -			       bool enable_wp, atomic_t *mmap_changing);
> +				     unsigned long len);
> +extern ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> +				     unsigned long len, uffd_flags_t flags);
> +extern ssize_t mfill_atomic_poison(struct userfaultfd_ctx *ctx, unsigned long start,
> +				   unsigned long len, uffd_flags_t flags);
> +extern int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
> +			       unsigned long len, bool enable_wp);
>  extern long uffd_wp_range(struct vm_area_struct *vma,
>  			  unsigned long start, unsigned long len, bool enable_wp);
>  
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index e3a91871462a..6e2ca04ab04d 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -353,11 +353,11 @@ static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
>   * called with mmap_lock held, it will release mmap_lock before returning.
>   */
>  static __always_inline ssize_t mfill_atomic_hugetlb(
> +					      struct userfaultfd_ctx *ctx,
>  					      struct vm_area_struct *dst_vma,
>  					      unsigned long dst_start,
>  					      unsigned long src_start,
>  					      unsigned long len,
> -					      atomic_t *mmap_changing,
>  					      uffd_flags_t flags)
>  {
>  	struct mm_struct *dst_mm = dst_vma->vm_mm;
> @@ -379,6 +379,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  	 * feature is not supported.
>  	 */
>  	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
> +		up_read(&ctx->map_changing_lock);
>  		mmap_read_unlock(dst_mm);
>  		return -EINVAL;
>  	}
> @@ -463,6 +464,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  		cond_resched();
>  
>  		if (unlikely(err == -ENOENT)) {
> +			up_read(&ctx->map_changing_lock);
>  			mmap_read_unlock(dst_mm);
>  			BUG_ON(!folio);
>  
> @@ -473,12 +475,13 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  				goto out;
>  			}
>  			mmap_read_lock(dst_mm);
> +			down_read(&ctx->map_changing_lock);
>  			/*
>  			 * If memory mappings are changing because of non-cooperative
>  			 * operation (e.g. mremap) running in parallel, bail out and
>  			 * request the user to retry later
>  			 */
> -			if (mmap_changing && atomic_read(mmap_changing)) {
> +			if (atomic_read(ctx->mmap_changing)) {
>  				err = -EAGAIN;
>  				break;
>  			}
> @@ -501,6 +504,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  	}
>  
>  out_unlock:
> +	up_read(&ctx->map_changing_lock);
>  	mmap_read_unlock(dst_mm);
>  out:
>  	if (folio)
> @@ -512,11 +516,11 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  }
>  #else /* !CONFIG_HUGETLB_PAGE */
>  /* fail at build time if gcc attempts to use this */
> -extern ssize_t mfill_atomic_hugetlb(struct vm_area_struct *dst_vma,
> +extern ssize_t mfill_atomic_hugetlb(struct userfaultfd_ctx *ctx,
> +				    struct vm_area_struct *dst_vma,
>  				    unsigned long dst_start,
>  				    unsigned long src_start,
>  				    unsigned long len,
> -				    atomic_t *mmap_changing,
>  				    uffd_flags_t flags);
>  #endif /* CONFIG_HUGETLB_PAGE */
>  
> @@ -564,13 +568,13 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
>  	return err;
>  }
>  
> -static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
> +static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
>  					    unsigned long dst_start,
>  					    unsigned long src_start,
>  					    unsigned long len,
> -					    atomic_t *mmap_changing,
>  					    uffd_flags_t flags)
>  {
> +	struct mm_struct *dst_mm = ctx->mm;
>  	struct vm_area_struct *dst_vma;
>  	ssize_t err;
>  	pmd_t *dst_pmd;
> @@ -600,8 +604,9 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  	 * operation (e.g. mremap) running in parallel, bail out and
>  	 * request the user to retry later
>  	 */
> +	down_read(&ctx->map_changing_lock);
>  	err = -EAGAIN;
> -	if (mmap_changing && atomic_read(mmap_changing))
> +	if (atomic_read(&ctx->mmap_changing))
>  		goto out_unlock;
>  
>  	/*
> @@ -633,8 +638,8 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  	 * If this is a HUGETLB vma, pass off to appropriate routine
>  	 */
>  	if (is_vm_hugetlb_page(dst_vma))
> -		return  mfill_atomic_hugetlb(dst_vma, dst_start, src_start,
> -					     len, mmap_changing, flags);
> +		return  mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
> +					     src_start, len, flags);
>  
>  	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
>  		goto out_unlock;
> @@ -693,6 +698,7 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  		if (unlikely(err == -ENOENT)) {
>  			void *kaddr;
>  
> +			up_read(&ctx->map_changing_lock);
>  			mmap_read_unlock(dst_mm);
>  			BUG_ON(!folio);
>  
> @@ -723,6 +729,7 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  	}
>  
>  out_unlock:
> +	up_read(&ctx->map_changing_lock);
>  	mmap_read_unlock(dst_mm);
>  out:
>  	if (folio)
> @@ -733,34 +740,33 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  	return copied ? copied : err;
>  }
>  
> -ssize_t mfill_atomic_copy(struct mm_struct *dst_mm, unsigned long dst_start,
> +ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
>  			  unsigned long src_start, unsigned long len,
> -			  atomic_t *mmap_changing, uffd_flags_t flags)
> +			  uffd_flags_t flags)
>  {
> -	return mfill_atomic(dst_mm, dst_start, src_start, len, mmap_changing,
> +	return mfill_atomic(ctx, dst_start, src_start, len,
>  			    uffd_flags_set_mode(flags, MFILL_ATOMIC_COPY));
>  }
>  
> -ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm, unsigned long start,
> -			      unsigned long len, atomic_t *mmap_changing)
> +ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
> +			      unsigned long start,
> +			      unsigned long len)
>  {
> -	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> +	return mfill_atomic(ctx, start, 0, len,
>  			    uffd_flags_set_mode(0, MFILL_ATOMIC_ZEROPAGE));
>  }
>  
> -ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
> -			      unsigned long len, atomic_t *mmap_changing,
> -			      uffd_flags_t flags)
> +ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long start,
> +			      unsigned long len, uffd_flags_t flags)
>  {
> -	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> +	return mfill_atomic(ctx, start, 0, len,
>  			    uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
>  }
>  
> -ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> -			    unsigned long len, atomic_t *mmap_changing,
> -			    uffd_flags_t flags)
> +ssize_t mfill_atomic_poison(struct userfaultfd_ctx *ctx, unsigned long start,
> +			    unsigned long len, uffd_flags_t flags)
>  {
> -	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> +	return mfill_atomic(ctx, start, 0, len,
>  			    uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
>  }
>  
> @@ -793,10 +799,10 @@ long uffd_wp_range(struct vm_area_struct *dst_vma,
>  	return ret;
>  }
>  
> -int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> -			unsigned long len, bool enable_wp,
> -			atomic_t *mmap_changing)
> +int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
> +			unsigned long len, bool enable_wp)
>  {
> +	struct mm_struct *dst_mm = ctx->mm;
>  	unsigned long end = start + len;
>  	unsigned long _start, _end;
>  	struct vm_area_struct *dst_vma;
> @@ -820,8 +826,9 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
>  	 * operation (e.g. mremap) running in parallel, bail out and
>  	 * request the user to retry later
>  	 */
> +	down_read(&ctx->map_changing_lock);
>  	err = -EAGAIN;
> -	if (mmap_changing && atomic_read(mmap_changing))
> +	if (atomic_read(&ctx->mmap_changing))
>  		goto out_unlock;
>  
>  	err = -ENOENT;
> @@ -850,6 +857,7 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
>  		err = 0;
>  	}
>  out_unlock:
> +	up_read(&ctx->map_changing_lock);
>  	mmap_read_unlock(dst_mm);
>  	return err;
>  }
> -- 
> 2.43.0.429.g432eaa2c6b-goog
> 
>
  

On Mon, Jan 29, 2024 at 1:00 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
>
> * Lokesh Gidra <lokeshgidra@google.com> [240129 14:35]:
> > Increments and loads to mmap_changing are always in mmap_lock
> > critical section.
>
> Read or write?
>
It's write-mode when incrementing (except in case of
userfaultfd_remove() where it's done in read-mode) and loads are in
mmap_lock (read-mode). I'll clarify this in the next version.
>
> > This ensures that if userspace requests event
> > notification for non-cooperative operations (e.g. mremap), userfaultfd
> > operations don't occur concurrently.
> >
> > This can be achieved by using a separate read-write semaphore in
> > userfaultfd_ctx such that increments are done in write-mode and loads
> > in read-mode, thereby eliminating the dependency on mmap_lock for this
> > purpose.
> >
> > This is a preparatory step before we replace mmap_lock usage with
> > per-vma locks in fill/move ioctls.
> >
> > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> > ---
> >  fs/userfaultfd.c              | 40 ++++++++++++----------
> >  include/linux/userfaultfd_k.h | 31 ++++++++++--------
> >  mm/userfaultfd.c              | 62 ++++++++++++++++++++---------------
> >  3 files changed, 75 insertions(+), 58 deletions(-)
> >
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 58331b83d648..c00a021bcce4 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -685,12 +685,15 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> >               ctx->flags = octx->flags;
> >               ctx->features = octx->features;
> >               ctx->released = false;
> > +             init_rwsem(&ctx->map_changing_lock);
> >               atomic_set(&ctx->mmap_changing, 0);
> >               ctx->mm = vma->vm_mm;
> >               mmgrab(ctx->mm);
> >
> >               userfaultfd_ctx_get(octx);
> > +             down_write(&octx->map_changing_lock);
> >               atomic_inc(&octx->mmap_changing);
> > +             up_write(&octx->map_changing_lock);
>
> This can potentially hold up your writer as the readers execute.  I
> think this will change your priority (ie: priority inversion)?

Priority inversion, if any, is already happening due to mmap_lock, no?
Also, I thought rw_semaphore implementation is fair, so the writer
will eventually get the lock right? Please correct me if I'm wrong.

At this patch: there can't be any readers as they need to acquire
mmap_lock in read-mode first. While writers, at the point of
incrementing mmap_changing, already hold mmap_lock in write-mode.

With per-vma locks, the same synchronization that mmap_lock achieved
around mmap_changing, will be achieved by ctx->map_changing_lock.
>
> You could use the first bit of the atomic_inc as indication of a write.
> So if the mmap_changing is even, then there are no writers.  If it
> didn't change and it's even then you know no modification has happened
> (or it overflowed and hit the same number which would be rare, but
> maybe okay?).

This is already achievable, right? If mmap_changing is >0 then we know
there are writers. The problem is that we want writers (like mremap
operations) to block as long as there is a userfaultfd operation (also
reader of mmap_changing) going on. Please note that I'm inferring this
from current implementation.

AFAIU, mmap_changing isn't required for correctness, because all
operations are happening under the right mode of mmap_lock. It's used
to ensure that while a non-cooperative operations is happening, if the
user has asked it to be notified, then no other userfaultfd operations
should take place until the user gets the event notification.
>
> >               fctx->orig = octx;
> >               fctx->new = ctx;
> >               list_add_tail(&fctx->list, fcs);
> > @@ -737,7 +740,9 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
> >       if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
> >               vm_ctx->ctx = ctx;
> >               userfaultfd_ctx_get(ctx);
> > +             down_write(&ctx->map_changing_lock);
> >               atomic_inc(&ctx->mmap_changing);
> > +             up_write(&ctx->map_changing_lock);
> >       } else {
> >               /* Drop uffd context if remap feature not enabled */
> >               vma_start_write(vma);
> > @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
> >               return true;
> >
> >       userfaultfd_ctx_get(ctx);
> > +     down_write(&ctx->map_changing_lock);
> >       atomic_inc(&ctx->mmap_changing);
> > +     up_write(&ctx->map_changing_lock);
> >       mmap_read_unlock(mm);
> >
> >       msg_init(&ewq.msg);
> > @@ -825,7 +832,9 @@ int userfaultfd_unmap_prep(struct vm_area_struct *vma, unsigned long start,
> >               return -ENOMEM;
> >
> >       userfaultfd_ctx_get(ctx);
> > +     down_write(&ctx->map_changing_lock);
> >       atomic_inc(&ctx->mmap_changing);
> > +     up_write(&ctx->map_changing_lock);
> >       unmap_ctx->ctx = ctx;
> >       unmap_ctx->start = start;
> >       unmap_ctx->end = end;
> > @@ -1709,9 +1718,8 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
> >       if (uffdio_copy.mode & UFFDIO_COPY_MODE_WP)
> >               flags |= MFILL_ATOMIC_WP;
> >       if (mmget_not_zero(ctx->mm)) {
> > -             ret = mfill_atomic_copy(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
> > -                                     uffdio_copy.len, &ctx->mmap_changing,
> > -                                     flags);
> > +             ret = mfill_atomic_copy(ctx, uffdio_copy.dst, uffdio_copy.src,
> > +                                     uffdio_copy.len, flags);
> >               mmput(ctx->mm);
> >       } else {
> >               return -ESRCH;
> > @@ -1761,9 +1769,8 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
> >               goto out;
> >
> >       if (mmget_not_zero(ctx->mm)) {
> > -             ret = mfill_atomic_zeropage(ctx->mm, uffdio_zeropage.range.start,
> > -                                        uffdio_zeropage.range.len,
> > -                                        &ctx->mmap_changing);
> > +             ret = mfill_atomic_zeropage(ctx, uffdio_zeropage.range.start,
> > +                                        uffdio_zeropage.range.len);
> >               mmput(ctx->mm);
> >       } else {
> >               return -ESRCH;
> > @@ -1818,9 +1825,8 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
> >               return -EINVAL;
> >
> >       if (mmget_not_zero(ctx->mm)) {
> > -             ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
> > -                                       uffdio_wp.range.len, mode_wp,
> > -                                       &ctx->mmap_changing);
> > +             ret = mwriteprotect_range(ctx, uffdio_wp.range.start,
> > +                                       uffdio_wp.range.len, mode_wp);
> >               mmput(ctx->mm);
> >       } else {
> >               return -ESRCH;
> > @@ -1870,9 +1876,8 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
> >               flags |= MFILL_ATOMIC_WP;
> >
> >       if (mmget_not_zero(ctx->mm)) {
> > -             ret = mfill_atomic_continue(ctx->mm, uffdio_continue.range.start,
> > -                                         uffdio_continue.range.len,
> > -                                         &ctx->mmap_changing, flags);
> > +             ret = mfill_atomic_continue(ctx, uffdio_continue.range.start,
> > +                                         uffdio_continue.range.len, flags);
> >               mmput(ctx->mm);
> >       } else {
> >               return -ESRCH;
> > @@ -1925,9 +1930,8 @@ static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long
> >               goto out;
> >
> >       if (mmget_not_zero(ctx->mm)) {
> > -             ret = mfill_atomic_poison(ctx->mm, uffdio_poison.range.start,
> > -                                       uffdio_poison.range.len,
> > -                                       &ctx->mmap_changing, 0);
> > +             ret = mfill_atomic_poison(ctx, uffdio_poison.range.start,
> > +                                       uffdio_poison.range.len, 0);
> >               mmput(ctx->mm);
> >       } else {
> >               return -ESRCH;
> > @@ -2003,13 +2007,14 @@ static int userfaultfd_move(struct userfaultfd_ctx *ctx,
> >       if (mmget_not_zero(mm)) {
> >               mmap_read_lock(mm);
> >
> > -             /* Re-check after taking mmap_lock */
> > +             /* Re-check after taking map_changing_lock */
> > +             down_read(&ctx->map_changing_lock);
> >               if (likely(!atomic_read(&ctx->mmap_changing)))
> >                       ret = move_pages(ctx, mm, uffdio_move.dst, uffdio_move.src,
> >                                        uffdio_move.len, uffdio_move.mode);
> >               else
> >                       ret = -EAGAIN;
> > -
> > +             up_read(&ctx->map_changing_lock);
> >               mmap_read_unlock(mm);
> >               mmput(mm);
> >       } else {
> > @@ -2216,6 +2221,7 @@ static int new_userfaultfd(int flags)
> >       ctx->flags = flags;
> >       ctx->features = 0;
> >       ctx->released = false;
> > +     init_rwsem(&ctx->map_changing_lock);
> >       atomic_set(&ctx->mmap_changing, 0);
> >       ctx->mm = current->mm;
> >       /* prevent the mm struct to be freed */
> > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > index 691d928ee864..3210c3552976 100644
> > --- a/include/linux/userfaultfd_k.h
> > +++ b/include/linux/userfaultfd_k.h
> > @@ -69,6 +69,13 @@ struct userfaultfd_ctx {
> >       unsigned int features;
> >       /* released */
> >       bool released;
> > +     /*
> > +      * Prevents userfaultfd operations (fill/move/wp) from happening while
> > +      * some non-cooperative event(s) is taking place. Increments are done
> > +      * in write-mode. Whereas, userfaultfd operations, which includes
> > +      * reading mmap_changing, is done under read-mode.
> > +      */
> > +     struct rw_semaphore map_changing_lock;
> >       /* memory mappings are changing because of non-cooperative event */
> >       atomic_t mmap_changing;
> >       /* mm with one ore more vmas attached to this userfaultfd_ctx */
> > @@ -113,22 +120,18 @@ extern int mfill_atomic_install_pte(pmd_t *dst_pmd,
> >                                   unsigned long dst_addr, struct page *page,
> >                                   bool newly_allocated, uffd_flags_t flags);
> >
> > -extern ssize_t mfill_atomic_copy(struct mm_struct *dst_mm, unsigned long dst_start,
> > +extern ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> >                                unsigned long src_start, unsigned long len,
> > -                              atomic_t *mmap_changing, uffd_flags_t flags);
> > -extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
> > +                              uffd_flags_t flags);
> > +extern ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
> >                                    unsigned long dst_start,
> > -                                  unsigned long len,
> > -                                  atomic_t *mmap_changing);
> > -extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
> > -                                  unsigned long len, atomic_t *mmap_changing,
> > -                                  uffd_flags_t flags);
> > -extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> > -                                unsigned long len, atomic_t *mmap_changing,
> > -                                uffd_flags_t flags);
> > -extern int mwriteprotect_range(struct mm_struct *dst_mm,
> > -                            unsigned long start, unsigned long len,
> > -                            bool enable_wp, atomic_t *mmap_changing);
> > +                                  unsigned long len);
> > +extern ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> > +                                  unsigned long len, uffd_flags_t flags);
> > +extern ssize_t mfill_atomic_poison(struct userfaultfd_ctx *ctx, unsigned long start,
> > +                                unsigned long len, uffd_flags_t flags);
> > +extern int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
> > +                            unsigned long len, bool enable_wp);
> >  extern long uffd_wp_range(struct vm_area_struct *vma,
> >                         unsigned long start, unsigned long len, bool enable_wp);
> >
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index e3a91871462a..6e2ca04ab04d 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -353,11 +353,11 @@ static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
> >   * called with mmap_lock held, it will release mmap_lock before returning.
> >   */
> >  static __always_inline ssize_t mfill_atomic_hugetlb(
> > +                                           struct userfaultfd_ctx *ctx,
> >                                             struct vm_area_struct *dst_vma,
> >                                             unsigned long dst_start,
> >                                             unsigned long src_start,
> >                                             unsigned long len,
> > -                                           atomic_t *mmap_changing,
> >                                             uffd_flags_t flags)
> >  {
> >       struct mm_struct *dst_mm = dst_vma->vm_mm;
> > @@ -379,6 +379,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> >        * feature is not supported.
> >        */
> >       if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
> > +             up_read(&ctx->map_changing_lock);
> >               mmap_read_unlock(dst_mm);
> >               return -EINVAL;
> >       }
> > @@ -463,6 +464,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> >               cond_resched();
> >
> >               if (unlikely(err == -ENOENT)) {
> > +                     up_read(&ctx->map_changing_lock);
> >                       mmap_read_unlock(dst_mm);
> >                       BUG_ON(!folio);
> >
> > @@ -473,12 +475,13 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> >                               goto out;
> >                       }
> >                       mmap_read_lock(dst_mm);
> > +                     down_read(&ctx->map_changing_lock);
> >                       /*
> >                        * If memory mappings are changing because of non-cooperative
> >                        * operation (e.g. mremap) running in parallel, bail out and
> >                        * request the user to retry later
> >                        */
> > -                     if (mmap_changing && atomic_read(mmap_changing)) {
> > +                     if (atomic_read(ctx->mmap_changing)) {
> >                               err = -EAGAIN;
> >                               break;
> >                       }
> > @@ -501,6 +504,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> >       }
> >
> >  out_unlock:
> > +     up_read(&ctx->map_changing_lock);
> >       mmap_read_unlock(dst_mm);
> >  out:
> >       if (folio)
> > @@ -512,11 +516,11 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> >  }
> >  #else /* !CONFIG_HUGETLB_PAGE */
> >  /* fail at build time if gcc attempts to use this */
> > -extern ssize_t mfill_atomic_hugetlb(struct vm_area_struct *dst_vma,
> > +extern ssize_t mfill_atomic_hugetlb(struct userfaultfd_ctx *ctx,
> > +                                 struct vm_area_struct *dst_vma,
> >                                   unsigned long dst_start,
> >                                   unsigned long src_start,
> >                                   unsigned long len,
> > -                                 atomic_t *mmap_changing,
> >                                   uffd_flags_t flags);
> >  #endif /* CONFIG_HUGETLB_PAGE */
> >
> > @@ -564,13 +568,13 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
> >       return err;
> >  }
> >
> > -static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
> > +static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> >                                           unsigned long dst_start,
> >                                           unsigned long src_start,
> >                                           unsigned long len,
> > -                                         atomic_t *mmap_changing,
> >                                           uffd_flags_t flags)
> >  {
> > +     struct mm_struct *dst_mm = ctx->mm;
> >       struct vm_area_struct *dst_vma;
> >       ssize_t err;
> >       pmd_t *dst_pmd;
> > @@ -600,8 +604,9 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
> >        * operation (e.g. mremap) running in parallel, bail out and
> >        * request the user to retry later
> >        */
> > +     down_read(&ctx->map_changing_lock);
> >       err = -EAGAIN;
> > -     if (mmap_changing && atomic_read(mmap_changing))
> > +     if (atomic_read(&ctx->mmap_changing))
> >               goto out_unlock;
> >
> >       /*
> > @@ -633,8 +638,8 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
> >        * If this is a HUGETLB vma, pass off to appropriate routine
> >        */
> >       if (is_vm_hugetlb_page(dst_vma))
> > -             return  mfill_atomic_hugetlb(dst_vma, dst_start, src_start,
> > -                                          len, mmap_changing, flags);
> > +             return  mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
> > +                                          src_start, len, flags);
> >
> >       if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
> >               goto out_unlock;
> > @@ -693,6 +698,7 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
> >               if (unlikely(err == -ENOENT)) {
> >                       void *kaddr;
> >
> > +                     up_read(&ctx->map_changing_lock);
> >                       mmap_read_unlock(dst_mm);
> >                       BUG_ON(!folio);
> >
> > @@ -723,6 +729,7 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
> >       }
> >
> >  out_unlock:
> > +     up_read(&ctx->map_changing_lock);
> >       mmap_read_unlock(dst_mm);
> >  out:
> >       if (folio)
> > @@ -733,34 +740,33 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
> >       return copied ? copied : err;
> >  }
> >
> > -ssize_t mfill_atomic_copy(struct mm_struct *dst_mm, unsigned long dst_start,
> > +ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> >                         unsigned long src_start, unsigned long len,
> > -                       atomic_t *mmap_changing, uffd_flags_t flags)
> > +                       uffd_flags_t flags)
> >  {
> > -     return mfill_atomic(dst_mm, dst_start, src_start, len, mmap_changing,
> > +     return mfill_atomic(ctx, dst_start, src_start, len,
> >                           uffd_flags_set_mode(flags, MFILL_ATOMIC_COPY));
> >  }
> >
> > -ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm, unsigned long start,
> > -                           unsigned long len, atomic_t *mmap_changing)
> > +ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
> > +                           unsigned long start,
> > +                           unsigned long len)
> >  {
> > -     return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> > +     return mfill_atomic(ctx, start, 0, len,
> >                           uffd_flags_set_mode(0, MFILL_ATOMIC_ZEROPAGE));
> >  }
> >
> > -ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
> > -                           unsigned long len, atomic_t *mmap_changing,
> > -                           uffd_flags_t flags)
> > +ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long start,
> > +                           unsigned long len, uffd_flags_t flags)
> >  {
> > -     return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> > +     return mfill_atomic(ctx, start, 0, len,
> >                           uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
> >  }
> >
> > -ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> > -                         unsigned long len, atomic_t *mmap_changing,
> > -                         uffd_flags_t flags)
> > +ssize_t mfill_atomic_poison(struct userfaultfd_ctx *ctx, unsigned long start,
> > +                         unsigned long len, uffd_flags_t flags)
> >  {
> > -     return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> > +     return mfill_atomic(ctx, start, 0, len,
> >                           uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
> >  }
> >
> > @@ -793,10 +799,10 @@ long uffd_wp_range(struct vm_area_struct *dst_vma,
> >       return ret;
> >  }
> >
> > -int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> > -                     unsigned long len, bool enable_wp,
> > -                     atomic_t *mmap_changing)
> > +int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
> > +                     unsigned long len, bool enable_wp)
> >  {
> > +     struct mm_struct *dst_mm = ctx->mm;
> >       unsigned long end = start + len;
> >       unsigned long _start, _end;
> >       struct vm_area_struct *dst_vma;
> > @@ -820,8 +826,9 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> >        * operation (e.g. mremap) running in parallel, bail out and
> >        * request the user to retry later
> >        */
> > +     down_read(&ctx->map_changing_lock);
> >       err = -EAGAIN;
> > -     if (mmap_changing && atomic_read(mmap_changing))
> > +     if (atomic_read(&ctx->mmap_changing))
> >               goto out_unlock;
> >
> >       err = -ENOENT;
> > @@ -850,6 +857,7 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> >               err = 0;
> >       }
> >  out_unlock:
> > +     up_read(&ctx->map_changing_lock);
> >       mmap_read_unlock(dst_mm);
> >       return err;
> >  }
> > --
> > 2.43.0.429.g432eaa2c6b-goog
> >
> >
  

* Lokesh Gidra <lokeshgidra@google.com> [240129 17:35]:
> On Mon, Jan 29, 2024 at 1:00 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> >
> > * Lokesh Gidra <lokeshgidra@google.com> [240129 14:35]:
> > > Increments and loads to mmap_changing are always in mmap_lock
> > > critical section.
> >
> > Read or write?
> >
> It's write-mode when incrementing (except in case of
> userfaultfd_remove() where it's done in read-mode) and loads are in
> mmap_lock (read-mode). I'll clarify this in the next version.
> >
> > > This ensures that if userspace requests event
> > > notification for non-cooperative operations (e.g. mremap), userfaultfd
> > > operations don't occur concurrently.
> > >
> > > This can be achieved by using a separate read-write semaphore in
> > > userfaultfd_ctx such that increments are done in write-mode and loads
> > > in read-mode, thereby eliminating the dependency on mmap_lock for this
> > > purpose.
> > >
> > > This is a preparatory step before we replace mmap_lock usage with
> > > per-vma locks in fill/move ioctls.
> > >
> > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> > > ---
> > >  fs/userfaultfd.c              | 40 ++++++++++++----------
> > >  include/linux/userfaultfd_k.h | 31 ++++++++++--------
> > >  mm/userfaultfd.c              | 62 ++++++++++++++++++++---------------
> > >  3 files changed, 75 insertions(+), 58 deletions(-)
> > >
> > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > index 58331b83d648..c00a021bcce4 100644
> > > --- a/fs/userfaultfd.c
> > > +++ b/fs/userfaultfd.c
> > > @@ -685,12 +685,15 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> > >               ctx->flags = octx->flags;
> > >               ctx->features = octx->features;
> > >               ctx->released = false;
> > > +             init_rwsem(&ctx->map_changing_lock);
> > >               atomic_set(&ctx->mmap_changing, 0);
> > >               ctx->mm = vma->vm_mm;
> > >               mmgrab(ctx->mm);
> > >
> > >               userfaultfd_ctx_get(octx);
> > > +             down_write(&octx->map_changing_lock);
> > >               atomic_inc(&octx->mmap_changing);
> > > +             up_write(&octx->map_changing_lock);

On init, I don't think taking the lock is strictly necessary - unless
there is a way to access it before this increment?  Not that it would
cost much.

> >
> > This can potentially hold up your writer as the readers execute.  I
> > think this will change your priority (ie: priority inversion)?
> 
> Priority inversion, if any, is already happening due to mmap_lock, no?
> Also, I thought rw_semaphore implementation is fair, so the writer
> will eventually get the lock right? Please correct me if I'm wrong.

You are correct.  Any writer will stop any new readers, but readers
currently in the section must finish before the writer.

> 
> At this patch: there can't be any readers as they need to acquire
> mmap_lock in read-mode first. While writers, at the point of
> incrementing mmap_changing, already hold mmap_lock in write-mode.
> 
> With per-vma locks, the same synchronization that mmap_lock achieved
> around mmap_changing, will be achieved by ctx->map_changing_lock.

The inversion I was thinking was that the writer cannot complete the
write until the reader is done failing because the atomic_inc has
happened..?  I see the writer as a priority since readers cannot
complete within the write, but I read it wrong.  I think the readers are
fine if the happen before, during, or after a write.  The work is thrown
out if the reader happens during the transition between those states,
which is detected through the atomic.  This makes sense now.

> >
> > You could use the first bit of the atomic_inc as indication of a write.
> > So if the mmap_changing is even, then there are no writers.  If it
> > didn't change and it's even then you know no modification has happened
> > (or it overflowed and hit the same number which would be rare, but
> > maybe okay?).
> 
> This is already achievable, right? If mmap_changing is >0 then we know
> there are writers. The problem is that we want writers (like mremap
> operations) to block as long as there is a userfaultfd operation (also
> reader of mmap_changing) going on. Please note that I'm inferring this
> from current implementation.
> 
> AFAIU, mmap_changing isn't required for correctness, because all
> operations are happening under the right mode of mmap_lock. It's used
> to ensure that while a non-cooperative operations is happening, if the
> user has asked it to be notified, then no other userfaultfd operations
> should take place until the user gets the event notification.

I think it is needed, mmap_changing is read before the mmap_lock is
taken, then compared after the mmap_lock is taken (both read mode) to
ensure nothing has changed.

..

> > > @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
> > >               return true;
> > >
> > >       userfaultfd_ctx_get(ctx);
> > > +     down_write(&ctx->map_changing_lock);
> > >       atomic_inc(&ctx->mmap_changing);
> > > +     up_write(&ctx->map_changing_lock);
> > >       mmap_read_unlock(mm);
> > >
> > >       msg_init(&ewq.msg);

If this happens in read mode, then why are you waiting for the readers
to leave?  Can't you just increment the atomic?  It's fine happening in
read mode today, so it should be fine with this new rwsem.

Thanks,
Liam

..
  

On Mon, Jan 29, 2024 at 11:35:11AM -0800, Lokesh Gidra wrote:
> Increments and loads to mmap_changing are always in mmap_lock
> critical section. This ensures that if userspace requests event
> notification for non-cooperative operations (e.g. mremap), userfaultfd
> operations don't occur concurrently.
> 
> This can be achieved by using a separate read-write semaphore in
> userfaultfd_ctx such that increments are done in write-mode and loads
> in read-mode, thereby eliminating the dependency on mmap_lock for this
> purpose.
> 
> This is a preparatory step before we replace mmap_lock usage with
> per-vma locks in fill/move ioctls.
> 
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>

Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>

> ---
>  fs/userfaultfd.c              | 40 ++++++++++++----------
>  include/linux/userfaultfd_k.h | 31 ++++++++++--------
>  mm/userfaultfd.c              | 62 ++++++++++++++++++++---------------
>  3 files changed, 75 insertions(+), 58 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 58331b83d648..c00a021bcce4 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -685,12 +685,15 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
>  		ctx->flags = octx->flags;
>  		ctx->features = octx->features;
>  		ctx->released = false;
> +		init_rwsem(&ctx->map_changing_lock);
>  		atomic_set(&ctx->mmap_changing, 0);
>  		ctx->mm = vma->vm_mm;
>  		mmgrab(ctx->mm);
>  
>  		userfaultfd_ctx_get(octx);
> +		down_write(&octx->map_changing_lock);
>  		atomic_inc(&octx->mmap_changing);
> +		up_write(&octx->map_changing_lock);
>  		fctx->orig = octx;
>  		fctx->new = ctx;
>  		list_add_tail(&fctx->list, fcs);
> @@ -737,7 +740,9 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
>  	if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
>  		vm_ctx->ctx = ctx;
>  		userfaultfd_ctx_get(ctx);
> +		down_write(&ctx->map_changing_lock);
>  		atomic_inc(&ctx->mmap_changing);
> +		up_write(&ctx->map_changing_lock);
>  	} else {
>  		/* Drop uffd context if remap feature not enabled */
>  		vma_start_write(vma);
> @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
>  		return true;
>  
>  	userfaultfd_ctx_get(ctx);
> +	down_write(&ctx->map_changing_lock);
>  	atomic_inc(&ctx->mmap_changing);
> +	up_write(&ctx->map_changing_lock);
>  	mmap_read_unlock(mm);
>  
>  	msg_init(&ewq.msg);
> @@ -825,7 +832,9 @@ int userfaultfd_unmap_prep(struct vm_area_struct *vma, unsigned long start,
>  		return -ENOMEM;
>  
>  	userfaultfd_ctx_get(ctx);
> +	down_write(&ctx->map_changing_lock);
>  	atomic_inc(&ctx->mmap_changing);
> +	up_write(&ctx->map_changing_lock);
>  	unmap_ctx->ctx = ctx;
>  	unmap_ctx->start = start;
>  	unmap_ctx->end = end;
> @@ -1709,9 +1718,8 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
>  	if (uffdio_copy.mode & UFFDIO_COPY_MODE_WP)
>  		flags |= MFILL_ATOMIC_WP;
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mfill_atomic_copy(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
> -					uffdio_copy.len, &ctx->mmap_changing,
> -					flags);
> +		ret = mfill_atomic_copy(ctx, uffdio_copy.dst, uffdio_copy.src,
> +					uffdio_copy.len, flags);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -1761,9 +1769,8 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
>  		goto out;
>  
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mfill_atomic_zeropage(ctx->mm, uffdio_zeropage.range.start,
> -					   uffdio_zeropage.range.len,
> -					   &ctx->mmap_changing);
> +		ret = mfill_atomic_zeropage(ctx, uffdio_zeropage.range.start,
> +					   uffdio_zeropage.range.len);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -1818,9 +1825,8 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
>  		return -EINVAL;
>  
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
> -					  uffdio_wp.range.len, mode_wp,
> -					  &ctx->mmap_changing);
> +		ret = mwriteprotect_range(ctx, uffdio_wp.range.start,
> +					  uffdio_wp.range.len, mode_wp);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -1870,9 +1876,8 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
>  		flags |= MFILL_ATOMIC_WP;
>  
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mfill_atomic_continue(ctx->mm, uffdio_continue.range.start,
> -					    uffdio_continue.range.len,
> -					    &ctx->mmap_changing, flags);
> +		ret = mfill_atomic_continue(ctx, uffdio_continue.range.start,
> +					    uffdio_continue.range.len, flags);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -1925,9 +1930,8 @@ static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long
>  		goto out;
>  
>  	if (mmget_not_zero(ctx->mm)) {
> -		ret = mfill_atomic_poison(ctx->mm, uffdio_poison.range.start,
> -					  uffdio_poison.range.len,
> -					  &ctx->mmap_changing, 0);
> +		ret = mfill_atomic_poison(ctx, uffdio_poison.range.start,
> +					  uffdio_poison.range.len, 0);
>  		mmput(ctx->mm);
>  	} else {
>  		return -ESRCH;
> @@ -2003,13 +2007,14 @@ static int userfaultfd_move(struct userfaultfd_ctx *ctx,
>  	if (mmget_not_zero(mm)) {
>  		mmap_read_lock(mm);
>  
> -		/* Re-check after taking mmap_lock */
> +		/* Re-check after taking map_changing_lock */
> +		down_read(&ctx->map_changing_lock);
>  		if (likely(!atomic_read(&ctx->mmap_changing)))
>  			ret = move_pages(ctx, mm, uffdio_move.dst, uffdio_move.src,
>  					 uffdio_move.len, uffdio_move.mode);
>  		else
>  			ret = -EAGAIN;
> -
> +		up_read(&ctx->map_changing_lock);
>  		mmap_read_unlock(mm);
>  		mmput(mm);
>  	} else {
> @@ -2216,6 +2221,7 @@ static int new_userfaultfd(int flags)
>  	ctx->flags = flags;
>  	ctx->features = 0;
>  	ctx->released = false;
> +	init_rwsem(&ctx->map_changing_lock);
>  	atomic_set(&ctx->mmap_changing, 0);
>  	ctx->mm = current->mm;
>  	/* prevent the mm struct to be freed */
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 691d928ee864..3210c3552976 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -69,6 +69,13 @@ struct userfaultfd_ctx {
>  	unsigned int features;
>  	/* released */
>  	bool released;
> +	/*
> +	 * Prevents userfaultfd operations (fill/move/wp) from happening while
> +	 * some non-cooperative event(s) is taking place. Increments are done
> +	 * in write-mode. Whereas, userfaultfd operations, which includes
> +	 * reading mmap_changing, is done under read-mode.
> +	 */
> +	struct rw_semaphore map_changing_lock;
>  	/* memory mappings are changing because of non-cooperative event */
>  	atomic_t mmap_changing;
>  	/* mm with one ore more vmas attached to this userfaultfd_ctx */
> @@ -113,22 +120,18 @@ extern int mfill_atomic_install_pte(pmd_t *dst_pmd,
>  				    unsigned long dst_addr, struct page *page,
>  				    bool newly_allocated, uffd_flags_t flags);
>  
> -extern ssize_t mfill_atomic_copy(struct mm_struct *dst_mm, unsigned long dst_start,
> +extern ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
>  				 unsigned long src_start, unsigned long len,
> -				 atomic_t *mmap_changing, uffd_flags_t flags);
> -extern ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm,
> +				 uffd_flags_t flags);
> +extern ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
>  				     unsigned long dst_start,
> -				     unsigned long len,
> -				     atomic_t *mmap_changing);
> -extern ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long dst_start,
> -				     unsigned long len, atomic_t *mmap_changing,
> -				     uffd_flags_t flags);
> -extern ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> -				   unsigned long len, atomic_t *mmap_changing,
> -				   uffd_flags_t flags);
> -extern int mwriteprotect_range(struct mm_struct *dst_mm,
> -			       unsigned long start, unsigned long len,
> -			       bool enable_wp, atomic_t *mmap_changing);
> +				     unsigned long len);
> +extern ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> +				     unsigned long len, uffd_flags_t flags);
> +extern ssize_t mfill_atomic_poison(struct userfaultfd_ctx *ctx, unsigned long start,
> +				   unsigned long len, uffd_flags_t flags);
> +extern int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
> +			       unsigned long len, bool enable_wp);
>  extern long uffd_wp_range(struct vm_area_struct *vma,
>  			  unsigned long start, unsigned long len, bool enable_wp);
>  
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index e3a91871462a..6e2ca04ab04d 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -353,11 +353,11 @@ static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
>   * called with mmap_lock held, it will release mmap_lock before returning.
>   */
>  static __always_inline ssize_t mfill_atomic_hugetlb(
> +					      struct userfaultfd_ctx *ctx,
>  					      struct vm_area_struct *dst_vma,
>  					      unsigned long dst_start,
>  					      unsigned long src_start,
>  					      unsigned long len,
> -					      atomic_t *mmap_changing,
>  					      uffd_flags_t flags)
>  {
>  	struct mm_struct *dst_mm = dst_vma->vm_mm;
> @@ -379,6 +379,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  	 * feature is not supported.
>  	 */
>  	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
> +		up_read(&ctx->map_changing_lock);
>  		mmap_read_unlock(dst_mm);
>  		return -EINVAL;
>  	}
> @@ -463,6 +464,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  		cond_resched();
>  
>  		if (unlikely(err == -ENOENT)) {
> +			up_read(&ctx->map_changing_lock);
>  			mmap_read_unlock(dst_mm);
>  			BUG_ON(!folio);
>  
> @@ -473,12 +475,13 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  				goto out;
>  			}
>  			mmap_read_lock(dst_mm);
> +			down_read(&ctx->map_changing_lock);
>  			/*
>  			 * If memory mappings are changing because of non-cooperative
>  			 * operation (e.g. mremap) running in parallel, bail out and
>  			 * request the user to retry later
>  			 */
> -			if (mmap_changing && atomic_read(mmap_changing)) {
> +			if (atomic_read(ctx->mmap_changing)) {
>  				err = -EAGAIN;
>  				break;
>  			}
> @@ -501,6 +504,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  	}
>  
>  out_unlock:
> +	up_read(&ctx->map_changing_lock);
>  	mmap_read_unlock(dst_mm);
>  out:
>  	if (folio)
> @@ -512,11 +516,11 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  }
>  #else /* !CONFIG_HUGETLB_PAGE */
>  /* fail at build time if gcc attempts to use this */
> -extern ssize_t mfill_atomic_hugetlb(struct vm_area_struct *dst_vma,
> +extern ssize_t mfill_atomic_hugetlb(struct userfaultfd_ctx *ctx,
> +				    struct vm_area_struct *dst_vma,
>  				    unsigned long dst_start,
>  				    unsigned long src_start,
>  				    unsigned long len,
> -				    atomic_t *mmap_changing,
>  				    uffd_flags_t flags);
>  #endif /* CONFIG_HUGETLB_PAGE */
>  
> @@ -564,13 +568,13 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
>  	return err;
>  }
>  
> -static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
> +static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
>  					    unsigned long dst_start,
>  					    unsigned long src_start,
>  					    unsigned long len,
> -					    atomic_t *mmap_changing,
>  					    uffd_flags_t flags)
>  {
> +	struct mm_struct *dst_mm = ctx->mm;
>  	struct vm_area_struct *dst_vma;
>  	ssize_t err;
>  	pmd_t *dst_pmd;
> @@ -600,8 +604,9 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  	 * operation (e.g. mremap) running in parallel, bail out and
>  	 * request the user to retry later
>  	 */
> +	down_read(&ctx->map_changing_lock);
>  	err = -EAGAIN;
> -	if (mmap_changing && atomic_read(mmap_changing))
> +	if (atomic_read(&ctx->mmap_changing))
>  		goto out_unlock;
>  
>  	/*
> @@ -633,8 +638,8 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  	 * If this is a HUGETLB vma, pass off to appropriate routine
>  	 */
>  	if (is_vm_hugetlb_page(dst_vma))
> -		return  mfill_atomic_hugetlb(dst_vma, dst_start, src_start,
> -					     len, mmap_changing, flags);
> +		return  mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
> +					     src_start, len, flags);
>  
>  	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
>  		goto out_unlock;
> @@ -693,6 +698,7 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  		if (unlikely(err == -ENOENT)) {
>  			void *kaddr;
>  
> +			up_read(&ctx->map_changing_lock);
>  			mmap_read_unlock(dst_mm);
>  			BUG_ON(!folio);
>  
> @@ -723,6 +729,7 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  	}
>  
>  out_unlock:
> +	up_read(&ctx->map_changing_lock);
>  	mmap_read_unlock(dst_mm);
>  out:
>  	if (folio)
> @@ -733,34 +740,33 @@ static __always_inline ssize_t mfill_atomic(struct mm_struct *dst_mm,
>  	return copied ? copied : err;
>  }
>  
> -ssize_t mfill_atomic_copy(struct mm_struct *dst_mm, unsigned long dst_start,
> +ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
>  			  unsigned long src_start, unsigned long len,
> -			  atomic_t *mmap_changing, uffd_flags_t flags)
> +			  uffd_flags_t flags)
>  {
> -	return mfill_atomic(dst_mm, dst_start, src_start, len, mmap_changing,
> +	return mfill_atomic(ctx, dst_start, src_start, len,
>  			    uffd_flags_set_mode(flags, MFILL_ATOMIC_COPY));
>  }
>  
> -ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm, unsigned long start,
> -			      unsigned long len, atomic_t *mmap_changing)
> +ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
> +			      unsigned long start,
> +			      unsigned long len)
>  {
> -	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> +	return mfill_atomic(ctx, start, 0, len,
>  			    uffd_flags_set_mode(0, MFILL_ATOMIC_ZEROPAGE));
>  }
>  
> -ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
> -			      unsigned long len, atomic_t *mmap_changing,
> -			      uffd_flags_t flags)
> +ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long start,
> +			      unsigned long len, uffd_flags_t flags)
>  {
> -	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> +	return mfill_atomic(ctx, start, 0, len,
>  			    uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
>  }
>  
> -ssize_t mfill_atomic_poison(struct mm_struct *dst_mm, unsigned long start,
> -			    unsigned long len, atomic_t *mmap_changing,
> -			    uffd_flags_t flags)
> +ssize_t mfill_atomic_poison(struct userfaultfd_ctx *ctx, unsigned long start,
> +			    unsigned long len, uffd_flags_t flags)
>  {
> -	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
> +	return mfill_atomic(ctx, start, 0, len,
>  			    uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
>  }
>  
> @@ -793,10 +799,10 @@ long uffd_wp_range(struct vm_area_struct *dst_vma,
>  	return ret;
>  }
>  
> -int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> -			unsigned long len, bool enable_wp,
> -			atomic_t *mmap_changing)
> +int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
> +			unsigned long len, bool enable_wp)
>  {
> +	struct mm_struct *dst_mm = ctx->mm;
>  	unsigned long end = start + len;
>  	unsigned long _start, _end;
>  	struct vm_area_struct *dst_vma;
> @@ -820,8 +826,9 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
>  	 * operation (e.g. mremap) running in parallel, bail out and
>  	 * request the user to retry later
>  	 */
> +	down_read(&ctx->map_changing_lock);
>  	err = -EAGAIN;
> -	if (mmap_changing && atomic_read(mmap_changing))
> +	if (atomic_read(&ctx->mmap_changing))
>  		goto out_unlock;
>  
>  	err = -ENOENT;
> @@ -850,6 +857,7 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
>  		err = 0;
>  	}
>  out_unlock:
> +	up_read(&ctx->map_changing_lock);
>  	mmap_read_unlock(dst_mm);
>  	return err;
>  }
> -- 
> 2.43.0.429.g432eaa2c6b-goog
>
  

On Mon, Jan 29, 2024 at 10:46:27PM -0500, Liam R. Howlett wrote:
> * Lokesh Gidra <lokeshgidra@google.com> [240129 17:35]:
> > On Mon, Jan 29, 2024 at 1:00 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> > >
> > > * Lokesh Gidra <lokeshgidra@google.com> [240129 14:35]:
> > > > Increments and loads to mmap_changing are always in mmap_lock
> > > > critical section.
> > >
> > > Read or write?
> > >
> > It's write-mode when incrementing (except in case of
> > userfaultfd_remove() where it's done in read-mode) and loads are in
> > mmap_lock (read-mode). I'll clarify this in the next version.
> > >
> > > > This ensures that if userspace requests event
> > > > notification for non-cooperative operations (e.g. mremap), userfaultfd
> > > > operations don't occur concurrently.
> > > >
> > > > This can be achieved by using a separate read-write semaphore in
> > > > userfaultfd_ctx such that increments are done in write-mode and loads
> > > > in read-mode, thereby eliminating the dependency on mmap_lock for this
> > > > purpose.
> > > >
> > > > This is a preparatory step before we replace mmap_lock usage with
> > > > per-vma locks in fill/move ioctls.
> > > >
> > > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> > > > ---
> > > >  fs/userfaultfd.c              | 40 ++++++++++++----------
> > > >  include/linux/userfaultfd_k.h | 31 ++++++++++--------
> > > >  mm/userfaultfd.c              | 62 ++++++++++++++++++++---------------
> > > >  3 files changed, 75 insertions(+), 58 deletions(-)
> > > >
> > > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > > index 58331b83d648..c00a021bcce4 100644
> > > > --- a/fs/userfaultfd.c
> > > > +++ b/fs/userfaultfd.c
> > > > @@ -685,12 +685,15 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> > > >               ctx->flags = octx->flags;
> > > >               ctx->features = octx->features;
> > > >               ctx->released = false;
> > > > +             init_rwsem(&ctx->map_changing_lock);
> > > >               atomic_set(&ctx->mmap_changing, 0);
> > > >               ctx->mm = vma->vm_mm;
> > > >               mmgrab(ctx->mm);
> > > >
> > > >               userfaultfd_ctx_get(octx);
> > > > +             down_write(&octx->map_changing_lock);
> > > >               atomic_inc(&octx->mmap_changing);
> > > > +             up_write(&octx->map_changing_lock);
> 
> On init, I don't think taking the lock is strictly necessary - unless
> there is a way to access it before this increment?  Not that it would
> cost much.

It's fork, the lock is for the context of the parent process and there
could be uffdio ops running in parallel on its VM.
 
> > > You could use the first bit of the atomic_inc as indication of a write.
> > > So if the mmap_changing is even, then there are no writers.  If it
> > > didn't change and it's even then you know no modification has happened
> > > (or it overflowed and hit the same number which would be rare, but
> > > maybe okay?).
> > 
> > This is already achievable, right? If mmap_changing is >0 then we know
> > there are writers. The problem is that we want writers (like mremap
> > operations) to block as long as there is a userfaultfd operation (also
> > reader of mmap_changing) going on. Please note that I'm inferring this
> > from current implementation.
> > 
> > AFAIU, mmap_changing isn't required for correctness, because all
> > operations are happening under the right mode of mmap_lock. It's used
> > to ensure that while a non-cooperative operations is happening, if the
> > user has asked it to be notified, then no other userfaultfd operations
> > should take place until the user gets the event notification.
> 
> I think it is needed, mmap_changing is read before the mmap_lock is
> taken, then compared after the mmap_lock is taken (both read mode) to
> ensure nothing has changed.

mmap_changing is required to ensure that no uffdio operation runs in
parallel with operations that modify the memory map, like fork, mremap,
munmap and some of madvise calls. 
And we do need the writers to block if there is an uffdio operation going
on, so I think an rwsem is the right way to protect mmap_chaniging.

> > > > @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
> > > >               return true;
> > > >
> > > >       userfaultfd_ctx_get(ctx);
> > > > +     down_write(&ctx->map_changing_lock);
> > > >       atomic_inc(&ctx->mmap_changing);
> > > > +     up_write(&ctx->map_changing_lock);
> > > >       mmap_read_unlock(mm);
> > > >
> > > >       msg_init(&ewq.msg);
> 
> If this happens in read mode, then why are you waiting for the readers
> to leave?  Can't you just increment the atomic?  It's fine happening in
> read mode today, so it should be fine with this new rwsem.

It's been a while and the details are blurred now, but if I remember
correctly, having this in read mode forced non-cooperative uffd monitor to
be single threaded. If a monitor runs, say uffdio_copy, and in parallel a
thread in the monitored process does MADV_DONTNEED, the latter will wait
for userfaultfd_remove notification to be processed in the monitor and drop
the VMA contents only afterwards. If a non-cooperative monitor would
process notification in parallel with uffdio ops, MADV_DONTNEED could
continue and race with uffdio_copy, so read mode wouldn't be enough.

There was no much sense to make MADV_DONTNEED take mmap_lock in write mode
just for this, but now taking the rwsem in write mode here sounds
reasonable.
 
> Thanks,
> Liam
> 
> ...
  

* Mike Rapoport <rppt@kernel.org> [240130 03:55]:
> On Mon, Jan 29, 2024 at 10:46:27PM -0500, Liam R. Howlett wrote:
> > * Lokesh Gidra <lokeshgidra@google.com> [240129 17:35]:
> > > On Mon, Jan 29, 2024 at 1:00 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> > > >
> > > > * Lokesh Gidra <lokeshgidra@google.com> [240129 14:35]:
> > > > > Increments and loads to mmap_changing are always in mmap_lock
> > > > > critical section.
> > > >
> > > > Read or write?
> > > >
> > > It's write-mode when incrementing (except in case of
> > > userfaultfd_remove() where it's done in read-mode) and loads are in
> > > mmap_lock (read-mode). I'll clarify this in the next version.
> > > >
> > > > > This ensures that if userspace requests event
> > > > > notification for non-cooperative operations (e.g. mremap), userfaultfd
> > > > > operations don't occur concurrently.
> > > > >
> > > > > This can be achieved by using a separate read-write semaphore in
> > > > > userfaultfd_ctx such that increments are done in write-mode and loads
> > > > > in read-mode, thereby eliminating the dependency on mmap_lock for this
> > > > > purpose.
> > > > >
> > > > > This is a preparatory step before we replace mmap_lock usage with
> > > > > per-vma locks in fill/move ioctls.
> > > > >
> > > > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> > > > > ---
> > > > >  fs/userfaultfd.c              | 40 ++++++++++++----------
> > > > >  include/linux/userfaultfd_k.h | 31 ++++++++++--------
> > > > >  mm/userfaultfd.c              | 62 ++++++++++++++++++++---------------
> > > > >  3 files changed, 75 insertions(+), 58 deletions(-)
> > > > >
> > > > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > > > index 58331b83d648..c00a021bcce4 100644
> > > > > --- a/fs/userfaultfd.c
> > > > > +++ b/fs/userfaultfd.c
> > > > > @@ -685,12 +685,15 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> > > > >               ctx->flags = octx->flags;
> > > > >               ctx->features = octx->features;
> > > > >               ctx->released = false;
> > > > > +             init_rwsem(&ctx->map_changing_lock);
> > > > >               atomic_set(&ctx->mmap_changing, 0);
> > > > >               ctx->mm = vma->vm_mm;
> > > > >               mmgrab(ctx->mm);
> > > > >
> > > > >               userfaultfd_ctx_get(octx);
> > > > > +             down_write(&octx->map_changing_lock);
> > > > >               atomic_inc(&octx->mmap_changing);
> > > > > +             up_write(&octx->map_changing_lock);
> > 
> > On init, I don't think taking the lock is strictly necessary - unless
> > there is a way to access it before this increment?  Not that it would
> > cost much.
> 
> It's fork, the lock is for the context of the parent process and there
> could be uffdio ops running in parallel on its VM.

Is this necessary then?  We are getting the octx from another mm but the
mm is locked for forking.  Why does it matter if there are readers of
the octx?

I assume, currently, there is no way the userfaultfd ctx can
be altered under mmap_lock held for writing. I would think it matters if
there are writers (which, I presume are blocked by the mmap_lock for
now?)  Shouldn't we hold the write lock for the entire dup process, I
mean, if we remove the userfaultfd from the mmap_lock, we cannot let the
structure being duplicated change half way through the dup process?

I must be missing something with where this is headed?

>  
> > > > You could use the first bit of the atomic_inc as indication of a write.
> > > > So if the mmap_changing is even, then there are no writers.  If it
> > > > didn't change and it's even then you know no modification has happened
> > > > (or it overflowed and hit the same number which would be rare, but
> > > > maybe okay?).
> > > 
> > > This is already achievable, right? If mmap_changing is >0 then we know
> > > there are writers. The problem is that we want writers (like mremap
> > > operations) to block as long as there is a userfaultfd operation (also
> > > reader of mmap_changing) going on. Please note that I'm inferring this
> > > from current implementation.
> > > 
> > > AFAIU, mmap_changing isn't required for correctness, because all
> > > operations are happening under the right mode of mmap_lock. It's used
> > > to ensure that while a non-cooperative operations is happening, if the
> > > user has asked it to be notified, then no other userfaultfd operations
> > > should take place until the user gets the event notification.
> > 
> > I think it is needed, mmap_changing is read before the mmap_lock is
> > taken, then compared after the mmap_lock is taken (both read mode) to
> > ensure nothing has changed.
> 
> mmap_changing is required to ensure that no uffdio operation runs in
> parallel with operations that modify the memory map, like fork, mremap,
> munmap and some of madvise calls. 
> And we do need the writers to block if there is an uffdio operation going
> on, so I think an rwsem is the right way to protect mmap_chaniging.
> 
> > > > > @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
> > > > >               return true;
> > > > >
> > > > >       userfaultfd_ctx_get(ctx);
> > > > > +     down_write(&ctx->map_changing_lock);
> > > > >       atomic_inc(&ctx->mmap_changing);
> > > > > +     up_write(&ctx->map_changing_lock);
> > > > >       mmap_read_unlock(mm);
> > > > >
> > > > >       msg_init(&ewq.msg);
> > 
> > If this happens in read mode, then why are you waiting for the readers
> > to leave?  Can't you just increment the atomic?  It's fine happening in
> > read mode today, so it should be fine with this new rwsem.
> 
> It's been a while and the details are blurred now, but if I remember
> correctly, having this in read mode forced non-cooperative uffd monitor to
> be single threaded. If a monitor runs, say uffdio_copy, and in parallel a
> thread in the monitored process does MADV_DONTNEED, the latter will wait
> for userfaultfd_remove notification to be processed in the monitor and drop
> the VMA contents only afterwards. If a non-cooperative monitor would
> process notification in parallel with uffdio ops, MADV_DONTNEED could
> continue and race with uffdio_copy, so read mode wouldn't be enough.
> 

Right now this function won't stop to wait for readers to exit the
critical section, but with this change there will be a pause (since the
down_write() will need to wait for the readers with the read lock).  So
this is adding a delay in this call path that isn't necessary (?) nor
existed before.  If you have non-cooperative uffd monitors, then you
will have to wait for them to finish to mark the uffd as being removed,
where as before it was a fire & forget, this is now a wait to tell.


> There was no much sense to make MADV_DONTNEED take mmap_lock in write mode
> just for this, but now taking the rwsem in write mode here sounds
> reasonable.
>  

I see why there was no need for a mmap_lock in write mode, but I think
taking the new rwsem in write mode is unnecessary.

Basically, I see this as a signal to new readers to abort, but we don't
need to wait for current readers to finish before this one increments
the atomic.  

Unless I missed something, I don't think you want to take the write lock
here.

Thanks,
Liam
  

On Tue, Jan 30, 2024 at 9:28 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
>
> * Mike Rapoport <rppt@kernel.org> [240130 03:55]:
> > On Mon, Jan 29, 2024 at 10:46:27PM -0500, Liam R. Howlett wrote:
> > > * Lokesh Gidra <lokeshgidra@google.com> [240129 17:35]:
> > > > On Mon, Jan 29, 2024 at 1:00 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> > > > >
> > > > > * Lokesh Gidra <lokeshgidra@google.com> [240129 14:35]:
> > > > > > Increments and loads to mmap_changing are always in mmap_lock
> > > > > > critical section.
> > > > >
> > > > > Read or write?
> > > > >
> > > > It's write-mode when incrementing (except in case of
> > > > userfaultfd_remove() where it's done in read-mode) and loads are in
> > > > mmap_lock (read-mode). I'll clarify this in the next version.
> > > > >
> > > > > > This ensures that if userspace requests event
> > > > > > notification for non-cooperative operations (e.g. mremap), userfaultfd
> > > > > > operations don't occur concurrently.
> > > > > >
> > > > > > This can be achieved by using a separate read-write semaphore in
> > > > > > userfaultfd_ctx such that increments are done in write-mode and loads
> > > > > > in read-mode, thereby eliminating the dependency on mmap_lock for this
> > > > > > purpose.
> > > > > >
> > > > > > This is a preparatory step before we replace mmap_lock usage with
> > > > > > per-vma locks in fill/move ioctls.
> > > > > >
> > > > > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> > > > > > ---
> > > > > >  fs/userfaultfd.c              | 40 ++++++++++++----------
> > > > > >  include/linux/userfaultfd_k.h | 31 ++++++++++--------
> > > > > >  mm/userfaultfd.c              | 62 ++++++++++++++++++++---------------
> > > > > >  3 files changed, 75 insertions(+), 58 deletions(-)
> > > > > >
> > > > > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > > > > index 58331b83d648..c00a021bcce4 100644
> > > > > > --- a/fs/userfaultfd.c
> > > > > > +++ b/fs/userfaultfd.c
> > > > > > @@ -685,12 +685,15 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> > > > > >               ctx->flags = octx->flags;
> > > > > >               ctx->features = octx->features;
> > > > > >               ctx->released = false;
> > > > > > +             init_rwsem(&ctx->map_changing_lock);
> > > > > >               atomic_set(&ctx->mmap_changing, 0);
> > > > > >               ctx->mm = vma->vm_mm;
> > > > > >               mmgrab(ctx->mm);
> > > > > >
> > > > > >               userfaultfd_ctx_get(octx);
> > > > > > +             down_write(&octx->map_changing_lock);
> > > > > >               atomic_inc(&octx->mmap_changing);
> > > > > > +             up_write(&octx->map_changing_lock);
> > >
> > > On init, I don't think taking the lock is strictly necessary - unless
> > > there is a way to access it before this increment?  Not that it would
> > > cost much.
> >
> > It's fork, the lock is for the context of the parent process and there
> > could be uffdio ops running in parallel on its VM.
>
> Is this necessary then?  We are getting the octx from another mm but the
> mm is locked for forking.  Why does it matter if there are readers of
> the octx?
>
> I assume, currently, there is no way the userfaultfd ctx can
> be altered under mmap_lock held for writing. I would think it matters if
> there are writers (which, I presume are blocked by the mmap_lock for
> now?)  Shouldn't we hold the write lock for the entire dup process, I
> mean, if we remove the userfaultfd from the mmap_lock, we cannot let the
> structure being duplicated change half way through the dup process?
>
> I must be missing something with where this is headed?
>
AFAIU, the purpose of mmap_changing is to serialize uffdio operations
with non-cooperative events if and when such events are being
monitored by userspace (in case you missed, in all the cases of writes
to mmap_changing, we only do it if that non-cooperative event has been
requested by the user). As you pointed out there are no correctness
concerns as far as userfaultfd operations are concerned. But these
events are essential for the uffd monitor's functioning.

For example: say the uffd monitor wants to be notified for REMAP
operations while doing uffdio_copy operations. When COPY ioctls start
failing with -EAGAIN and uffdio_copy.copy == 0, then it knows it must
be due to mremap(), in which case it waits for the REMAP event
notification before attempting COPY again.

But there are few things that I didn't get after going through the
history of non-cooperative events. Hopefully Mike (or someone else
familiar) can clarify:

IIUC, the idea behind non-cooperative events was to block uffdio
operations from happening *before* the page tables are manipulated by
the event (like mremap), and that the uffdio ops are resumed after the
event notification is received by the monitor. If so then:

1) Why in the case of REMAP prep() is done after page-tables are
moved? Shouldn't it be done before? All other non-cooperative
operations do the prep() before.
2) UFFD_FEATURE_EVENT_REMOVE only notifies user space. It is not
consistently blocking uffdio operations (as both sides are acquiring
mmap_lock in read-mode) when remove operation is taking place. I can
understand this was intentionally left as is in the interest of not
acquiring mmap_lock in write-mode during madvise. But is only getting
the notification any useful? Can we say this patch fixes it? And in
that case shouldn't I split userfaultfd_remove() into two functions
(like other non-cooperative operations)?
3) Based on [1] I see how mmap_changing helps in eliminating duplicate
work (background copy) by uffd monitor, but didn't get if there is a
correctness aspect too that I'm missing? I concur with Amit's point in
[1] that getting -EEXIST when setting up the pte will avoid memory
corruption, no?

[1] https://lore.kernel.org/lkml/20201206093703.GY123287@linux.ibm.com/
> >
> > > > > You could use the first bit of the atomic_inc as indication of a write.
> > > > > So if the mmap_changing is even, then there are no writers.  If it
> > > > > didn't change and it's even then you know no modification has happened
> > > > > (or it overflowed and hit the same number which would be rare, but
> > > > > maybe okay?).
> > > >
> > > > This is already achievable, right? If mmap_changing is >0 then we know
> > > > there are writers. The problem is that we want writers (like mremap
> > > > operations) to block as long as there is a userfaultfd operation (also
> > > > reader of mmap_changing) going on. Please note that I'm inferring this
> > > > from current implementation.
> > > >
> > > > AFAIU, mmap_changing isn't required for correctness, because all
> > > > operations are happening under the right mode of mmap_lock. It's used
> > > > to ensure that while a non-cooperative operations is happening, if the
> > > > user has asked it to be notified, then no other userfaultfd operations
> > > > should take place until the user gets the event notification.
> > >
> > > I think it is needed, mmap_changing is read before the mmap_lock is
> > > taken, then compared after the mmap_lock is taken (both read mode) to
> > > ensure nothing has changed.
> >
> > mmap_changing is required to ensure that no uffdio operation runs in
> > parallel with operations that modify the memory map, like fork, mremap,
> > munmap and some of madvise calls.
> > And we do need the writers to block if there is an uffdio operation going
> > on, so I think an rwsem is the right way to protect mmap_chaniging.
> >
> > > > > > @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
> > > > > >               return true;
> > > > > >
> > > > > >       userfaultfd_ctx_get(ctx);
> > > > > > +     down_write(&ctx->map_changing_lock);
> > > > > >       atomic_inc(&ctx->mmap_changing);
> > > > > > +     up_write(&ctx->map_changing_lock);
> > > > > >       mmap_read_unlock(mm);
> > > > > >
> > > > > >       msg_init(&ewq.msg);
> > >
> > > If this happens in read mode, then why are you waiting for the readers
> > > to leave?  Can't you just increment the atomic?  It's fine happening in
> > > read mode today, so it should be fine with this new rwsem.
> >
> > It's been a while and the details are blurred now, but if I remember
> > correctly, having this in read mode forced non-cooperative uffd monitor to
> > be single threaded. If a monitor runs, say uffdio_copy, and in parallel a
> > thread in the monitored process does MADV_DONTNEED, the latter will wait
> > for userfaultfd_remove notification to be processed in the monitor and drop
> > the VMA contents only afterwards. If a non-cooperative monitor would
> > process notification in parallel with uffdio ops, MADV_DONTNEED could
> > continue and race with uffdio_copy, so read mode wouldn't be enough.
> >
>
> Right now this function won't stop to wait for readers to exit the
> critical section, but with this change there will be a pause (since the
> down_write() will need to wait for the readers with the read lock).  So
> this is adding a delay in this call path that isn't necessary (?) nor
> existed before.  If you have non-cooperative uffd monitors, then you
> will have to wait for them to finish to mark the uffd as being removed,
> where as before it was a fire & forget, this is now a wait to tell.
>
I think a lot will be clearer once we get a response to my questions
above. IMHO not only this write-lock is needed here, we need to fix
userfaultfd_remove() by splitting it into userfaultfd_remove_prep()
and userfaultfd_remove_complete() (like all other non-cooperative
operations) as well. This patch enables us to do that as we remove
mmap_changing's dependency on mmap_lock for synchronization.
>
> > There was no much sense to make MADV_DONTNEED take mmap_lock in write mode
> > just for this, but now taking the rwsem in write mode here sounds
> > reasonable.
> >
>
> I see why there was no need for a mmap_lock in write mode, but I think
> taking the new rwsem in write mode is unnecessary.
>
> Basically, I see this as a signal to new readers to abort, but we don't
> need to wait for current readers to finish before this one increments
> the atomic.
>
> Unless I missed something, I don't think you want to take the write lock
> here.
What I understood from the history of mmap_changing is that the
intention was to enable informing the uffd monitor about the correct
state of which pages are filled and which aren't. Going through this
thread was very helpful [2]

[2] https://lore.kernel.org/lkml/1527061324-19949-1-git-send-email-rppt@linux.vnet.ibm.com/
>
> Thanks,
> Liam
  

On Tue, Jan 30, 2024 at 06:24:24PM -0800, Lokesh Gidra wrote:
> On Tue, Jan 30, 2024 at 9:28 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> >
> > * Mike Rapoport <rppt@kernel.org> [240130 03:55]:
> > > On Mon, Jan 29, 2024 at 10:46:27PM -0500, Liam R. Howlett wrote:
> > > > * Lokesh Gidra <lokeshgidra@google.com> [240129 17:35]:
> > > >
> > > > > > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > > > > > index 58331b83d648..c00a021bcce4 100644
> > > > > > > --- a/fs/userfaultfd.c
> > > > > > > +++ b/fs/userfaultfd.c
> > > > > > > @@ -685,12 +685,15 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> > > > > > >               ctx->flags = octx->flags;
> > > > > > >               ctx->features = octx->features;
> > > > > > >               ctx->released = false;
> > > > > > > +             init_rwsem(&ctx->map_changing_lock);
> > > > > > >               atomic_set(&ctx->mmap_changing, 0);
> > > > > > >               ctx->mm = vma->vm_mm;
> > > > > > >               mmgrab(ctx->mm);
> > > > > > >
> > > > > > >               userfaultfd_ctx_get(octx);
> > > > > > > +             down_write(&octx->map_changing_lock);
> > > > > > >               atomic_inc(&octx->mmap_changing);
> > > > > > > +             up_write(&octx->map_changing_lock);
> > > >
> > > > On init, I don't think taking the lock is strictly necessary - unless
> > > > there is a way to access it before this increment?  Not that it would
> > > > cost much.
> > >
> > > It's fork, the lock is for the context of the parent process and there
> > > could be uffdio ops running in parallel on its VM.
> >
> > Is this necessary then?  We are getting the octx from another mm but the
> > mm is locked for forking.  Why does it matter if there are readers of
> > the octx?
> >
> > I assume, currently, there is no way the userfaultfd ctx can
> > be altered under mmap_lock held for writing. I would think it matters if
> > there are writers (which, I presume are blocked by the mmap_lock for
> > now?)  Shouldn't we hold the write lock for the entire dup process, I
> > mean, if we remove the userfaultfd from the mmap_lock, we cannot let the
> > structure being duplicated change half way through the dup process?
> >
> > I must be missing something with where this is headed?
> >
> AFAIU, the purpose of mmap_changing is to serialize uffdio operations
> with non-cooperative events if and when such events are being
> monitored by userspace (in case you missed, in all the cases of writes
> to mmap_changing, we only do it if that non-cooperative event has been
> requested by the user). As you pointed out there are no correctness
> concerns as far as userfaultfd operations are concerned. But these
> events are essential for the uffd monitor's functioning.
> 
> For example: say the uffd monitor wants to be notified for REMAP
> operations while doing uffdio_copy operations. When COPY ioctls start
> failing with -EAGAIN and uffdio_copy.copy == 0, then it knows it must
> be due to mremap(), in which case it waits for the REMAP event
> notification before attempting COPY again.
> 
> But there are few things that I didn't get after going through the
> history of non-cooperative events. Hopefully Mike (or someone else
> familiar) can clarify:
> 
> IIUC, the idea behind non-cooperative events was to block uffdio
> operations from happening *before* the page tables are manipulated by
> the event (like mremap), and that the uffdio ops are resumed after the
> event notification is received by the monitor.

The idea was to give userspace some way to serialize processing of
non-cooperative event notifications and uffdio operations running in
parallel. It's not necessary to block uffdio operations from happening
before changes to the memory map, but with the mmap_lock synchronization
that already was there adding mmap_chaning that will prevent uffdio
operations when mmap_lock is taken for write was the simplest thing to do.

When CRIU does post-copy restore of a process, its uffd monitor reacts to
page fault and non-cooperative notifications and also performs a background
copy of the memory contents from the saved state to the address space of
the process being restored.

Since non-cooperative events may happen completely independent from the
uffd monitor, there are cases when the uffd monitor couldn't identify the
order of events, like  e.g. what won the race on mmap_lock, the process
thread doing fork or the uffd monitor's uffdio_copy.

In the fork vs uffdio_copy example, without mmap_changing, if the
uffdio_copy takes the mmap_lock first, the new page will be present in the
parent by the time copy_page_range() is called and the page will appear in
the child's memory mappings by the time uffd monitor gets notification
about the fork event. However, if the fork() is the first to take the
mmap_lock, the new page will appear in the parent address space after
copy_page_range() and it won't be mapped in the child's address space.

With mmap_changing and current locking with mmap_lock, we have a guarantee
that uffdio_copy will bail out if fork already took mmap_lock and the
monitor can act appropriately.
 
> 1) Why in the case of REMAP prep() is done after page-tables are
> moved? Shouldn't it be done before? All other non-cooperative
> operations do the prep() before.

mremap_userfaultfd_prep() is done after page tables are moved because it
initializes uffd context on the new_vma and if the actual remap fails,
there's no point of doing it.
Since mrpemap holds mmap_lock for write it does not matter if mmap_changed
is updated before or after page tables are moved. In the time between
mmap_lock is released and the UFFD_EVENT_REMAP is delivered to the uffd
monitor, mmap_chaging will remain >0 and uffdio operations will bail out.

> 2) UFFD_FEATURE_EVENT_REMOVE only notifies user space. It is not
> consistently blocking uffdio operations (as both sides are acquiring
> mmap_lock in read-mode) when remove operation is taking place. I can
> understand this was intentionally left as is in the interest of not
> acquiring mmap_lock in write-mode during madvise. But is only getting
> the notification any useful? Can we say this patch fixes it? And in
> that case shouldn't I split userfaultfd_remove() into two functions
> (like other non-cooperative operations)?

The notifications are useful because uffd monitor knows what memory should
not be filled with uffdio_copy. Indeed there was no interest in taking
mmap_lock for write in madvise, so there could be race between madvise and
uffdio operations. This race essentially prevents uffd monitor from running
the background copy in a separate thread, and with your change this should
be possible.

> 3) Based on [1] I see how mmap_changing helps in eliminating duplicate
> work (background copy) by uffd monitor, but didn't get if there is a
> correctness aspect too that I'm missing? I concur with Amit's point in
> [1] that getting -EEXIST when setting up the pte will avoid memory
> corruption, no?

In the fork case without mmap_changing the child process may be get data or
zeroes depending on the race for mmap_lock between the fork and
uffdio_copy and -EEXIST is not enough for monitor to detect what was the
ordering between fork and uffdio_copy.
 
> > > > > > > @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
> > > > > > >               return true;
> > > > > > >
> > > > > > >       userfaultfd_ctx_get(ctx);
> > > > > > > +     down_write(&ctx->map_changing_lock);
> > > > > > >       atomic_inc(&ctx->mmap_changing);
> > > > > > > +     up_write(&ctx->map_changing_lock);
> > > > > > >       mmap_read_unlock(mm);
> > > > > > >
> > > > > > >       msg_init(&ewq.msg);
> > > >
> > > > If this happens in read mode, then why are you waiting for the readers
> > > > to leave?  Can't you just increment the atomic?  It's fine happening in
> > > > read mode today, so it should be fine with this new rwsem.
> > >
> > > It's been a while and the details are blurred now, but if I remember
> > > correctly, having this in read mode forced non-cooperative uffd monitor to
> > > be single threaded. If a monitor runs, say uffdio_copy, and in parallel a
> > > thread in the monitored process does MADV_DONTNEED, the latter will wait
> > > for userfaultfd_remove notification to be processed in the monitor and drop
> > > the VMA contents only afterwards. If a non-cooperative monitor would
> > > process notification in parallel with uffdio ops, MADV_DONTNEED could
> > > continue and race with uffdio_copy, so read mode wouldn't be enough.
> > >
> >
> > Right now this function won't stop to wait for readers to exit the
> > critical section, but with this change there will be a pause (since the
> > down_write() will need to wait for the readers with the read lock).  So
> > this is adding a delay in this call path that isn't necessary (?) nor
> > existed before.  If you have non-cooperative uffd monitors, then you
> > will have to wait for them to finish to mark the uffd as being removed,
> > where as before it was a fire & forget, this is now a wait to tell.
> >
> I think a lot will be clearer once we get a response to my questions
> above. IMHO not only this write-lock is needed here, we need to fix
> userfaultfd_remove() by splitting it into userfaultfd_remove_prep()
> and userfaultfd_remove_complete() (like all other non-cooperative
> operations) as well. This patch enables us to do that as we remove
> mmap_changing's dependency on mmap_lock for synchronization.

The write-lock is not a requirement here for correctness and I don't see
why we would need userfaultfd_remove_prep().

As I've said earlier, having a write-lock here will let CRIU to run
background copy in parallel with processing of uffd events, but I don't
feel strongly about doing it.

> > > There was no much sense to make MADV_DONTNEED take mmap_lock in write mode
> > > just for this, but now taking the rwsem in write mode here sounds
> > > reasonable.
> > >
> >
> > I see why there was no need for a mmap_lock in write mode, but I think
> > taking the new rwsem in write mode is unnecessary.
> >
> > Basically, I see this as a signal to new readers to abort, but we don't
> > need to wait for current readers to finish before this one increments
> > the atomic.
> >
> > Unless I missed something, I don't think you want to take the write lock
> > here.
> What I understood from the history of mmap_changing is that the
> intention was to enable informing the uffd monitor about the correct
> state of which pages are filled and which aren't. Going through this
> thread was very helpful [2]
> 
> [2] https://lore.kernel.org/lkml/1527061324-19949-1-git-send-email-rppt@linux.vnet.ibm.com/
  

On Sun, Feb 4, 2024 at 2:27 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Tue, Jan 30, 2024 at 06:24:24PM -0800, Lokesh Gidra wrote:
> > On Tue, Jan 30, 2024 at 9:28 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> > >
> > > * Mike Rapoport <rppt@kernel.org> [240130 03:55]:
> > > > On Mon, Jan 29, 2024 at 10:46:27PM -0500, Liam R. Howlett wrote:
> > > > > * Lokesh Gidra <lokeshgidra@google.com> [240129 17:35]:
> > > > >
> > > > > > > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > > > > > > index 58331b83d648..c00a021bcce4 100644
> > > > > > > > --- a/fs/userfaultfd.c
> > > > > > > > +++ b/fs/userfaultfd.c
> > > > > > > > @@ -685,12 +685,15 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> > > > > > > >               ctx->flags = octx->flags;
> > > > > > > >               ctx->features = octx->features;
> > > > > > > >               ctx->released = false;
> > > > > > > > +             init_rwsem(&ctx->map_changing_lock);
> > > > > > > >               atomic_set(&ctx->mmap_changing, 0);
> > > > > > > >               ctx->mm = vma->vm_mm;
> > > > > > > >               mmgrab(ctx->mm);
> > > > > > > >
> > > > > > > >               userfaultfd_ctx_get(octx);
> > > > > > > > +             down_write(&octx->map_changing_lock);
> > > > > > > >               atomic_inc(&octx->mmap_changing);
> > > > > > > > +             up_write(&octx->map_changing_lock);
> > > > >
> > > > > On init, I don't think taking the lock is strictly necessary - unless
> > > > > there is a way to access it before this increment?  Not that it would
> > > > > cost much.
> > > >
> > > > It's fork, the lock is for the context of the parent process and there
> > > > could be uffdio ops running in parallel on its VM.
> > >
> > > Is this necessary then?  We are getting the octx from another mm but the
> > > mm is locked for forking.  Why does it matter if there are readers of
> > > the octx?
> > >
> > > I assume, currently, there is no way the userfaultfd ctx can
> > > be altered under mmap_lock held for writing. I would think it matters if
> > > there are writers (which, I presume are blocked by the mmap_lock for
> > > now?)  Shouldn't we hold the write lock for the entire dup process, I
> > > mean, if we remove the userfaultfd from the mmap_lock, we cannot let the
> > > structure being duplicated change half way through the dup process?
> > >
> > > I must be missing something with where this is headed?
> > >
> > AFAIU, the purpose of mmap_changing is to serialize uffdio operations
> > with non-cooperative events if and when such events are being
> > monitored by userspace (in case you missed, in all the cases of writes
> > to mmap_changing, we only do it if that non-cooperative event has been
> > requested by the user). As you pointed out there are no correctness
> > concerns as far as userfaultfd operations are concerned. But these
> > events are essential for the uffd monitor's functioning.
> >
> > For example: say the uffd monitor wants to be notified for REMAP
> > operations while doing uffdio_copy operations. When COPY ioctls start
> > failing with -EAGAIN and uffdio_copy.copy == 0, then it knows it must
> > be due to mremap(), in which case it waits for the REMAP event
> > notification before attempting COPY again.
> >
> > But there are few things that I didn't get after going through the
> > history of non-cooperative events. Hopefully Mike (or someone else
> > familiar) can clarify:
> >
> > IIUC, the idea behind non-cooperative events was to block uffdio
> > operations from happening *before* the page tables are manipulated by
> > the event (like mremap), and that the uffdio ops are resumed after the
> > event notification is received by the monitor.
>
> The idea was to give userspace some way to serialize processing of
> non-cooperative event notifications and uffdio operations running in
> parallel. It's not necessary to block uffdio operations from happening
> before changes to the memory map, but with the mmap_lock synchronization
> that already was there adding mmap_chaning that will prevent uffdio
> operations when mmap_lock is taken for write was the simplest thing to do.
>
> When CRIU does post-copy restore of a process, its uffd monitor reacts to
> page fault and non-cooperative notifications and also performs a background
> copy of the memory contents from the saved state to the address space of
> the process being restored.
>
> Since non-cooperative events may happen completely independent from the
> uffd monitor, there are cases when the uffd monitor couldn't identify the
> order of events, like  e.g. what won the race on mmap_lock, the process
> thread doing fork or the uffd monitor's uffdio_copy.
>
> In the fork vs uffdio_copy example, without mmap_changing, if the
> uffdio_copy takes the mmap_lock first, the new page will be present in the
> parent by the time copy_page_range() is called and the page will appear in
> the child's memory mappings by the time uffd monitor gets notification
> about the fork event. However, if the fork() is the first to take the
> mmap_lock, the new page will appear in the parent address space after
> copy_page_range() and it won't be mapped in the child's address space.
>
> With mmap_changing and current locking with mmap_lock, we have a guarantee
> that uffdio_copy will bail out if fork already took mmap_lock and the
> monitor can act appropriately.
>
Thanks for the explanation. Really helpful!

> > 1) Why in the case of REMAP prep() is done after page-tables are
> > moved? Shouldn't it be done before? All other non-cooperative
> > operations do the prep() before.
>
> mremap_userfaultfd_prep() is done after page tables are moved because it
> initializes uffd context on the new_vma and if the actual remap fails,
> there's no point of doing it.
> Since mrpemap holds mmap_lock for write it does not matter if mmap_changed
> is updated before or after page tables are moved. In the time between
> mmap_lock is released and the UFFD_EVENT_REMAP is delivered to the uffd
> monitor, mmap_chaging will remain >0 and uffdio operations will bail out.
>
Yes this makes sense. Even with per-vma locks, I see that the new_vma
is write-locked (vma_start_write()) in vma_link() guaranteeing the
same.

> > 2) UFFD_FEATURE_EVENT_REMOVE only notifies user space. It is not
> > consistently blocking uffdio operations (as both sides are acquiring
> > mmap_lock in read-mode) when remove operation is taking place. I can
> > understand this was intentionally left as is in the interest of not
> > acquiring mmap_lock in write-mode during madvise. But is only getting
> > the notification any useful? Can we say this patch fixes it? And in
> > that case shouldn't I split userfaultfd_remove() into two functions
> > (like other non-cooperative operations)?
>
> The notifications are useful because uffd monitor knows what memory should
> not be filled with uffdio_copy. Indeed there was no interest in taking
> mmap_lock for write in madvise, so there could be race between madvise and
> uffdio operations. This race essentially prevents uffd monitor from running
> the background copy in a separate thread, and with your change this should
> be possible.
>
Makes sense. Thanks!

> > 3) Based on [1] I see how mmap_changing helps in eliminating duplicate
> > work (background copy) by uffd monitor, but didn't get if there is a
> > correctness aspect too that I'm missing? I concur with Amit's point in
> > [1] that getting -EEXIST when setting up the pte will avoid memory
> > corruption, no?
>
> In the fork case without mmap_changing the child process may be get data or
> zeroes depending on the race for mmap_lock between the fork and
> uffdio_copy and -EEXIST is not enough for monitor to detect what was the
> ordering between fork and uffdio_copy.

This is extremely helpful. IIUC, there is a window after mmap_lock
(write-mode) is released and before the uffd monitor thread is
notified of fork. In that window, the monitor doesn't know that fork
has already happened. So, without mmap_changing it would have done
background copy only in the parent, thereby causing data inconsistency
between parent and child processes.

It seems to me that the correctness argument for mmap_changing is
there in case of FORK event and REMAP when mremap is called with
MREMAP_DONTUNMAP. In all other cases its only benefit is by avoiding
unnecessary background copies, right?

>
> > > > > > > > @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
> > > > > > > >               return true;
> > > > > > > >
> > > > > > > >       userfaultfd_ctx_get(ctx);
> > > > > > > > +     down_write(&ctx->map_changing_lock);
> > > > > > > >       atomic_inc(&ctx->mmap_changing);
> > > > > > > > +     up_write(&ctx->map_changing_lock);
> > > > > > > >       mmap_read_unlock(mm);
> > > > > > > >
> > > > > > > >       msg_init(&ewq.msg);
> > > > >
> > > > > If this happens in read mode, then why are you waiting for the readers
> > > > > to leave?  Can't you just increment the atomic?  It's fine happening in
> > > > > read mode today, so it should be fine with this new rwsem.
> > > >
> > > > It's been a while and the details are blurred now, but if I remember
> > > > correctly, having this in read mode forced non-cooperative uffd monitor to
> > > > be single threaded. If a monitor runs, say uffdio_copy, and in parallel a
> > > > thread in the monitored process does MADV_DONTNEED, the latter will wait
> > > > for userfaultfd_remove notification to be processed in the monitor and drop
> > > > the VMA contents only afterwards. If a non-cooperative monitor would
> > > > process notification in parallel with uffdio ops, MADV_DONTNEED could
> > > > continue and race with uffdio_copy, so read mode wouldn't be enough.
> > > >
> > >
> > > Right now this function won't stop to wait for readers to exit the
> > > critical section, but with this change there will be a pause (since the
> > > down_write() will need to wait for the readers with the read lock).  So
> > > this is adding a delay in this call path that isn't necessary (?) nor
> > > existed before.  If you have non-cooperative uffd monitors, then you
> > > will have to wait for them to finish to mark the uffd as being removed,
> > > where as before it was a fire & forget, this is now a wait to tell.
> > >
> > I think a lot will be clearer once we get a response to my questions
> > above. IMHO not only this write-lock is needed here, we need to fix
> > userfaultfd_remove() by splitting it into userfaultfd_remove_prep()
> > and userfaultfd_remove_complete() (like all other non-cooperative
> > operations) as well. This patch enables us to do that as we remove
> > mmap_changing's dependency on mmap_lock for synchronization.
>
> The write-lock is not a requirement here for correctness and I don't see
> why we would need userfaultfd_remove_prep().
>
> As I've said earlier, having a write-lock here will let CRIU to run
> background copy in parallel with processing of uffd events, but I don't
> feel strongly about doing it.
>
Got it. Anyways, such a change needn't be part of this patch, so I'm
going to keep it unchanged.

> > > > There was no much sense to make MADV_DONTNEED take mmap_lock in write mode
> > > > just for this, but now taking the rwsem in write mode here sounds
> > > > reasonable.
> > > >
> > >
> > > I see why there was no need for a mmap_lock in write mode, but I think
> > > taking the new rwsem in write mode is unnecessary.
> > >
> > > Basically, I see this as a signal to new readers to abort, but we don't
> > > need to wait for current readers to finish before this one increments
> > > the atomic.
> > >
> > > Unless I missed something, I don't think you want to take the write lock
> > > here.
> > What I understood from the history of mmap_changing is that the
> > intention was to enable informing the uffd monitor about the correct
> > state of which pages are filled and which aren't. Going through this
> > thread was very helpful [2]
> >
> > [2] https://lore.kernel.org/lkml/1527061324-19949-1-git-send-email-rppt@linux.vnet.ibm.com/
>
> --
> Sincerely yours,
> Mike.
  

On Mon, Feb 05, 2024 at 12:53:33PM -0800, Lokesh Gidra wrote:
> On Sun, Feb 4, 2024 at 2:27 AM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > > 3) Based on [1] I see how mmap_changing helps in eliminating duplicate
> > > work (background copy) by uffd monitor, but didn't get if there is a
> > > correctness aspect too that I'm missing? I concur with Amit's point in
> > > [1] that getting -EEXIST when setting up the pte will avoid memory
> > > corruption, no?
> >
> > In the fork case without mmap_changing the child process may be get data or
> > zeroes depending on the race for mmap_lock between the fork and
> > uffdio_copy and -EEXIST is not enough for monitor to detect what was the
> > ordering between fork and uffdio_copy.
> 
> This is extremely helpful. IIUC, there is a window after mmap_lock
> (write-mode) is released and before the uffd monitor thread is
> notified of fork. In that window, the monitor doesn't know that fork
> has already happened. So, without mmap_changing it would have done
> background copy only in the parent, thereby causing data inconsistency
> between parent and child processes.

Yes.
 
> It seems to me that the correctness argument for mmap_changing is
> there in case of FORK event and REMAP when mremap is called with
> MREMAP_DONTUNMAP. In all other cases its only benefit is by avoiding
> unnecessary background copies, right?

Yes, I think you are right, but it's possible I've forgot some nasty race
that will need mmap_changing for other events.

> > > > > > > > > @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
> > > > > > > > >               return true;
> > > > > > > > >
> > > > > > > > >       userfaultfd_ctx_get(ctx);
> > > > > > > > > +     down_write(&ctx->map_changing_lock);
> > > > > > > > >       atomic_inc(&ctx->mmap_changing);
> > > > > > > > > +     up_write(&ctx->map_changing_lock);
> > > > > > > > >       mmap_read_unlock(mm);
> > > > > > > > >
> > > > > > > > >       msg_init(&ewq.msg);
> > > > > >
> > > > > > If this happens in read mode, then why are you waiting for the readers
> > > > > > to leave?  Can't you just increment the atomic?  It's fine happening in
> > > > > > read mode today, so it should be fine with this new rwsem.
> > > > >
> > > > > It's been a while and the details are blurred now, but if I remember
> > > > > correctly, having this in read mode forced non-cooperative uffd monitor to
> > > > > be single threaded. If a monitor runs, say uffdio_copy, and in parallel a
> > > > > thread in the monitored process does MADV_DONTNEED, the latter will wait
> > > > > for userfaultfd_remove notification to be processed in the monitor and drop
> > > > > the VMA contents only afterwards. If a non-cooperative monitor would
> > > > > process notification in parallel with uffdio ops, MADV_DONTNEED could
> > > > > continue and race with uffdio_copy, so read mode wouldn't be enough.
> > > > >
> > > >
> > > > Right now this function won't stop to wait for readers to exit the
> > > > critical section, but with this change there will be a pause (since the
> > > > down_write() will need to wait for the readers with the read lock).  So
> > > > this is adding a delay in this call path that isn't necessary (?) nor
> > > > existed before.  If you have non-cooperative uffd monitors, then you
> > > > will have to wait for them to finish to mark the uffd as being removed,
> > > > where as before it was a fire & forget, this is now a wait to tell.
> > > >
> > > I think a lot will be clearer once we get a response to my questions
> > > above. IMHO not only this write-lock is needed here, we need to fix
> > > userfaultfd_remove() by splitting it into userfaultfd_remove_prep()
> > > and userfaultfd_remove_complete() (like all other non-cooperative
> > > operations) as well. This patch enables us to do that as we remove
> > > mmap_changing's dependency on mmap_lock for synchronization.
> >
> > The write-lock is not a requirement here for correctness and I don't see
> > why we would need userfaultfd_remove_prep().
> >
> > As I've said earlier, having a write-lock here will let CRIU to run
> > background copy in parallel with processing of uffd events, but I don't
> > feel strongly about doing it.
> >
> Got it. Anyways, such a change needn't be part of this patch, so I'm
> going to keep it unchanged.

You mean with a read lock?
  

On Wed, Feb 7, 2024 at 7:27 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Mon, Feb 05, 2024 at 12:53:33PM -0800, Lokesh Gidra wrote:
> > On Sun, Feb 4, 2024 at 2:27 AM Mike Rapoport <rppt@kernel.org> wrote:
> > >
> > > > 3) Based on [1] I see how mmap_changing helps in eliminating duplicate
> > > > work (background copy) by uffd monitor, but didn't get if there is a
> > > > correctness aspect too that I'm missing? I concur with Amit's point in
> > > > [1] that getting -EEXIST when setting up the pte will avoid memory
> > > > corruption, no?
> > >
> > > In the fork case without mmap_changing the child process may be get data or
> > > zeroes depending on the race for mmap_lock between the fork and
> > > uffdio_copy and -EEXIST is not enough for monitor to detect what was the
> > > ordering between fork and uffdio_copy.
> >
> > This is extremely helpful. IIUC, there is a window after mmap_lock
> > (write-mode) is released and before the uffd monitor thread is
> > notified of fork. In that window, the monitor doesn't know that fork
> > has already happened. So, without mmap_changing it would have done
> > background copy only in the parent, thereby causing data inconsistency
> > between parent and child processes.
>
> Yes.
>
> > It seems to me that the correctness argument for mmap_changing is
> > there in case of FORK event and REMAP when mremap is called with
> > MREMAP_DONTUNMAP. In all other cases its only benefit is by avoiding
> > unnecessary background copies, right?
>
> Yes, I think you are right, but it's possible I've forgot some nasty race
> that will need mmap_changing for other events.
>
> > > > > > > > > > @@ -783,7 +788,9 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
> > > > > > > > > >               return true;
> > > > > > > > > >
> > > > > > > > > >       userfaultfd_ctx_get(ctx);
> > > > > > > > > > +     down_write(&ctx->map_changing_lock);
> > > > > > > > > >       atomic_inc(&ctx->mmap_changing);
> > > > > > > > > > +     up_write(&ctx->map_changing_lock);
> > > > > > > > > >       mmap_read_unlock(mm);
> > > > > > > > > >
> > > > > > > > > >       msg_init(&ewq.msg);
> > > > > > >
> > > > > > > If this happens in read mode, then why are you waiting for the readers
> > > > > > > to leave?  Can't you just increment the atomic?  It's fine happening in
> > > > > > > read mode today, so it should be fine with this new rwsem.
> > > > > >
> > > > > > It's been a while and the details are blurred now, but if I remember
> > > > > > correctly, having this in read mode forced non-cooperative uffd monitor to
> > > > > > be single threaded. If a monitor runs, say uffdio_copy, and in parallel a
> > > > > > thread in the monitored process does MADV_DONTNEED, the latter will wait
> > > > > > for userfaultfd_remove notification to be processed in the monitor and drop
> > > > > > the VMA contents only afterwards. If a non-cooperative monitor would
> > > > > > process notification in parallel with uffdio ops, MADV_DONTNEED could
> > > > > > continue and race with uffdio_copy, so read mode wouldn't be enough.
> > > > > >
> > > > >
> > > > > Right now this function won't stop to wait for readers to exit the
> > > > > critical section, but with this change there will be a pause (since the
> > > > > down_write() will need to wait for the readers with the read lock).  So
> > > > > this is adding a delay in this call path that isn't necessary (?) nor
> > > > > existed before.  If you have non-cooperative uffd monitors, then you
> > > > > will have to wait for them to finish to mark the uffd as being removed,
> > > > > where as before it was a fire & forget, this is now a wait to tell.
> > > > >
> > > > I think a lot will be clearer once we get a response to my questions
> > > > above. IMHO not only this write-lock is needed here, we need to fix
> > > > userfaultfd_remove() by splitting it into userfaultfd_remove_prep()
> > > > and userfaultfd_remove_complete() (like all other non-cooperative
> > > > operations) as well. This patch enables us to do that as we remove
> > > > mmap_changing's dependency on mmap_lock for synchronization.
> > >
> > > The write-lock is not a requirement here for correctness and I don't see
> > > why we would need userfaultfd_remove_prep().
> > >
> > > As I've said earlier, having a write-lock here will let CRIU to run
> > > background copy in parallel with processing of uffd events, but I don't
> > > feel strongly about doing it.
> > >
> > Got it. Anyways, such a change needn't be part of this patch, so I'm
> > going to keep it unchanged.
>
> You mean with a read lock?

No, I think write lock is good as it enables parallel background copy.
Also because it brings consistency in blocking userfaultfd operations.

I meant encapsulating remove operations within
userfaultfd_remove_prep() and userfaultfd_remove_complete(). I
couldn't figure out any need for that.


>
> --
> Sincerely yours,
> Mike.
  

On Wed, Feb 07, 2024 at 12:24:52PM -0800, Lokesh Gidra wrote:
> On Wed, Feb 7, 2024 at 7:27 AM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > > > The write-lock is not a requirement here for correctness and I don't see
> > > > why we would need userfaultfd_remove_prep().
> > > >
> > > > As I've said earlier, having a write-lock here will let CRIU to run
> > > > background copy in parallel with processing of uffd events, but I don't
> > > > feel strongly about doing it.
> > > >
> > > Got it. Anyways, such a change needn't be part of this patch, so I'm
> > > going to keep it unchanged.
> >
> > You mean with a read lock?
> 
> No, I think write lock is good as it enables parallel background copy.
> Also because it brings consistency in blocking userfaultfd operations.
> 
> I meant encapsulating remove operations within
> userfaultfd_remove_prep() and userfaultfd_remove_complete(). I
> couldn't figure out any need for that.

I don't think there is a need for that. With fork/mremap prep is required
to ensure there's uffd context for new vmas.