[RFC,5/5] Revert "mm/migrate: __unmap_and_move() push good newpage to LRU"

  This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.

That commit made sure we immediately add the new page to the LRU before
remove_migration_ptes() is called in migrate_move_folio() (used to be
__unmap_and_move() back then), such that the rmap walk will rebuild the
correct mlock_count for the page again. This was needed because the
mlock_count was lost when the page is isolated. This is no longer the
case since mlock_count no longer overlays page->lru.

Revert the commit (the code was foliated afterward the commit, so the
revert is updated as such).

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
---
 mm/migrate.c | 24 +++++++++---------------
 1 file changed, 9 insertions(+), 15 deletions(-)
  

Hi, Yosry,

Yosry Ahmed <yosryahmed@google.com> writes:

> This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.
>
> That commit made sure we immediately add the new page to the LRU before
> remove_migration_ptes() is called in migrate_move_folio() (used to be
> __unmap_and_move() back then), such that the rmap walk will rebuild the
> correct mlock_count for the page again. This was needed because the
> mlock_count was lost when the page is isolated. This is no longer the
> case since mlock_count no longer overlays page->lru.
>
> Revert the commit (the code was foliated afterward the commit, so the
> revert is updated as such).
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>  mm/migrate.c | 24 +++++++++---------------
>  1 file changed, 9 insertions(+), 15 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 01cac26a3127..68f693731865 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
>  	if (unlikely(!is_lru))
>  		goto out_unlock_both;

The patch itself looks good to me!  Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

And, it seems that we can remove the above 2 lines and "out_unlock_both"
label now.  That can make the code simpler a little.  Right?

Best Regards,
Huang, Ying

> -	/*
> -	 * When successful, push dst to LRU immediately: so that if it
> -	 * turns out to be an mlocked page, remove_migration_ptes() will
> -	 * automatically build up the correct dst->mlock_count for it.
> -	 *
> -	 * We would like to do something similar for the old page, when
> -	 * unsuccessful, and other cases when a page has been temporarily
> -	 * isolated from the unevictable LRU: but this case is the easiest.
> -	 */
> -	folio_add_lru(dst);
> -	if (page_was_mapped)
> -		lru_add_drain();
> -
>  	if (page_was_mapped)
>  		remove_migration_ptes(src, dst, false);
>  
> @@ -1301,9 +1288,16 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
>  	/*
>  	 * If migration is successful, decrease refcount of dst,
>  	 * which will not free the page because new page owner increased
> -	 * refcounter.
> +	 * refcounter. As well, if it is LRU folio, add the folio to LRU
> +	 * list in here. Use the old state of the isolated source folio to
> +	 * determine if we migrated a LRU folio. dst was already unlocked
> +	 * and possibly modified by its owner - don't rely on the folio
> +	 * state.
>  	 */
> -	folio_put(dst);
> +	if (unlikely(!is_lru))
> +		folio_put(dst);
> +	else
> +		folio_putback_lru(dst);
>  
>  	/*
>  	 * A folio that has been migrated has all references removed
  

On Sun, Jun 18, 2023 at 7:00 PM Huang, Ying <ying.huang@intel.com> wrote:
>
> Hi, Yosry,
>
> Yosry Ahmed <yosryahmed@google.com> writes:
>
> > This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.
> >
> > That commit made sure we immediately add the new page to the LRU before
> > remove_migration_ptes() is called in migrate_move_folio() (used to be
> > __unmap_and_move() back then), such that the rmap walk will rebuild the
> > correct mlock_count for the page again. This was needed because the
> > mlock_count was lost when the page is isolated. This is no longer the
> > case since mlock_count no longer overlays page->lru.
> >
> > Revert the commit (the code was foliated afterward the commit, so the
> > revert is updated as such).
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > ---
> >  mm/migrate.c | 24 +++++++++---------------
> >  1 file changed, 9 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 01cac26a3127..68f693731865 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
> >       if (unlikely(!is_lru))
> >               goto out_unlock_both;
>
> The patch itself looks good to me!  Thanks!
>
> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

Thanks for taking a look!

>
> And, it seems that we can remove the above 2 lines and "out_unlock_both"
> label now.  That can make the code simpler a little.  Right?

I am not familiar with this code. If we remove the above condition
then pages that have is_lru == 0 (i.e __PageMovable(src) is true) and
page_was_mapped == 1 will call remove_migration_ptes(). This wouldn't
happen without removing the above 2 lines. If this combination is
impossible (is_lru == 0 && page_was_mapped == 1), then yeah we can
remove the above condition.

It looks like __SetPageMovable() is only called by zsmalloc, z3fold,
and balloon_page_insert(). The former 2 will never have those pages
mapped into userspace. I am not familiar with balloon_page_insert(),
but my gut feeling is that those are pages used by the driver and are
also not mapped into userspace.

So I guess we can just remove the condition, but a confirmation for
the above would be reassuring :)

>
> Best Regards,
> Huang, Ying
>
> > -     /*
> > -      * When successful, push dst to LRU immediately: so that if it
> > -      * turns out to be an mlocked page, remove_migration_ptes() will
> > -      * automatically build up the correct dst->mlock_count for it.
> > -      *
> > -      * We would like to do something similar for the old page, when
> > -      * unsuccessful, and other cases when a page has been temporarily
> > -      * isolated from the unevictable LRU: but this case is the easiest.
> > -      */
> > -     folio_add_lru(dst);
> > -     if (page_was_mapped)
> > -             lru_add_drain();
> > -
> >       if (page_was_mapped)
> >               remove_migration_ptes(src, dst, false);
> >
> > @@ -1301,9 +1288,16 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
> >       /*
> >        * If migration is successful, decrease refcount of dst,
> >        * which will not free the page because new page owner increased
> > -      * refcounter.
> > +      * refcounter. As well, if it is LRU folio, add the folio to LRU
> > +      * list in here. Use the old state of the isolated source folio to
> > +      * determine if we migrated a LRU folio. dst was already unlocked
> > +      * and possibly modified by its owner - don't rely on the folio
> > +      * state.
> >        */
> > -     folio_put(dst);
> > +     if (unlikely(!is_lru))
> > +             folio_put(dst);
> > +     else
> > +             folio_putback_lru(dst);
> >
> >       /*
> >        * A folio that has been migrated has all references removed
>
  

Yosry Ahmed <yosryahmed@google.com> writes:

> On Sun, Jun 18, 2023 at 7:00 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Hi, Yosry,
>>
>> Yosry Ahmed <yosryahmed@google.com> writes:
>>
>> > This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.
>> >
>> > That commit made sure we immediately add the new page to the LRU before
>> > remove_migration_ptes() is called in migrate_move_folio() (used to be
>> > __unmap_and_move() back then), such that the rmap walk will rebuild the
>> > correct mlock_count for the page again. This was needed because the
>> > mlock_count was lost when the page is isolated. This is no longer the
>> > case since mlock_count no longer overlays page->lru.
>> >
>> > Revert the commit (the code was foliated afterward the commit, so the
>> > revert is updated as such).
>> >
>> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>> > ---
>> >  mm/migrate.c | 24 +++++++++---------------
>> >  1 file changed, 9 insertions(+), 15 deletions(-)
>> >
>> > diff --git a/mm/migrate.c b/mm/migrate.c
>> > index 01cac26a3127..68f693731865 100644
>> > --- a/mm/migrate.c
>> > +++ b/mm/migrate.c
>> > @@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
>> >       if (unlikely(!is_lru))
>> >               goto out_unlock_both;
>>
>> The patch itself looks good to me!  Thanks!
>>
>> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
>
> Thanks for taking a look!
>
>>
>> And, it seems that we can remove the above 2 lines and "out_unlock_both"
>> label now.  That can make the code simpler a little.  Right?
>
> I am not familiar with this code. If we remove the above condition
> then pages that have is_lru == 0 (i.e __PageMovable(src) is true) and
> page_was_mapped == 1 will call remove_migration_ptes(). This wouldn't
> happen without removing the above 2 lines. If this combination is
> impossible (is_lru == 0 && page_was_mapped == 1), then yeah we can
> remove the above condition.
>
> It looks like __SetPageMovable() is only called by zsmalloc, z3fold,
> and balloon_page_insert(). The former 2 will never have those pages
> mapped into userspace. I am not familiar with balloon_page_insert(),
> but my gut feeling is that those are pages used by the driver and are
> also not mapped into userspace.

You can take a look at migrate_folio_unmap(), where "page_was_mapped"
will not be set to 1 if !is_lru.

Best Regards,
Huang, Ying

> So I guess we can just remove the condition, but a confirmation for
> the above would be reassuring :)
>
>>
>> Best Regards,
>> Huang, Ying
>>
>> > -     /*
>> > -      * When successful, push dst to LRU immediately: so that if it
>> > -      * turns out to be an mlocked page, remove_migration_ptes() will
>> > -      * automatically build up the correct dst->mlock_count for it.
>> > -      *
>> > -      * We would like to do something similar for the old page, when
>> > -      * unsuccessful, and other cases when a page has been temporarily
>> > -      * isolated from the unevictable LRU: but this case is the easiest.
>> > -      */
>> > -     folio_add_lru(dst);
>> > -     if (page_was_mapped)
>> > -             lru_add_drain();
>> > -
>> >       if (page_was_mapped)
>> >               remove_migration_ptes(src, dst, false);
>> >
>> > @@ -1301,9 +1288,16 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
>> >       /*
>> >        * If migration is successful, decrease refcount of dst,
>> >        * which will not free the page because new page owner increased
>> > -      * refcounter.
>> > +      * refcounter. As well, if it is LRU folio, add the folio to LRU
>> > +      * list in here. Use the old state of the isolated source folio to
>> > +      * determine if we migrated a LRU folio. dst was already unlocked
>> > +      * and possibly modified by its owner - don't rely on the folio
>> > +      * state.
>> >        */
>> > -     folio_put(dst);
>> > +     if (unlikely(!is_lru))
>> > +             folio_put(dst);
>> > +     else
>> > +             folio_putback_lru(dst);
>> >
>> >       /*
>> >        * A folio that has been migrated has all references removed
>>
  

On Sun, Jun 18, 2023 at 9:29 PM Huang, Ying <ying.huang@intel.com> wrote:
>
> Yosry Ahmed <yosryahmed@google.com> writes:
>
> > On Sun, Jun 18, 2023 at 7:00 PM Huang, Ying <ying.huang@intel.com> wrote:
> >>
> >> Hi, Yosry,
> >>
> >> Yosry Ahmed <yosryahmed@google.com> writes:
> >>
> >> > This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.
> >> >
> >> > That commit made sure we immediately add the new page to the LRU before
> >> > remove_migration_ptes() is called in migrate_move_folio() (used to be
> >> > __unmap_and_move() back then), such that the rmap walk will rebuild the
> >> > correct mlock_count for the page again. This was needed because the
> >> > mlock_count was lost when the page is isolated. This is no longer the
> >> > case since mlock_count no longer overlays page->lru.
> >> >
> >> > Revert the commit (the code was foliated afterward the commit, so the
> >> > revert is updated as such).
> >> >
> >> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> >> > ---
> >> >  mm/migrate.c | 24 +++++++++---------------
> >> >  1 file changed, 9 insertions(+), 15 deletions(-)
> >> >
> >> > diff --git a/mm/migrate.c b/mm/migrate.c
> >> > index 01cac26a3127..68f693731865 100644
> >> > --- a/mm/migrate.c
> >> > +++ b/mm/migrate.c
> >> > @@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
> >> >       if (unlikely(!is_lru))
> >> >               goto out_unlock_both;
> >>
> >> The patch itself looks good to me!  Thanks!
> >>
> >> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
> >
> > Thanks for taking a look!
> >
> >>
> >> And, it seems that we can remove the above 2 lines and "out_unlock_both"
> >> label now.  That can make the code simpler a little.  Right?
> >
> > I am not familiar with this code. If we remove the above condition
> > then pages that have is_lru == 0 (i.e __PageMovable(src) is true) and
> > page_was_mapped == 1 will call remove_migration_ptes(). This wouldn't
> > happen without removing the above 2 lines. If this combination is
> > impossible (is_lru == 0 && page_was_mapped == 1), then yeah we can
> > remove the above condition.
> >
> > It looks like __SetPageMovable() is only called by zsmalloc, z3fold,
> > and balloon_page_insert(). The former 2 will never have those pages
> > mapped into userspace. I am not familiar with balloon_page_insert(),
> > but my gut feeling is that those are pages used by the driver and are
> > also not mapped into userspace.
>
> You can take a look at migrate_folio_unmap(), where "page_was_mapped"
> will not be set to 1 if !is_lru.

Oh I was looking in the wrong place huh, thanks. Will remove it when I respin!

>
> Best Regards,
> Huang, Ying
>
> > So I guess we can just remove the condition, but a confirmation for
> > the above would be reassuring :)
> >
> >>
> >> Best Regards,
> >> Huang, Ying
> >>
> >> > -     /*
> >> > -      * When successful, push dst to LRU immediately: so that if it
> >> > -      * turns out to be an mlocked page, remove_migration_ptes() will
> >> > -      * automatically build up the correct dst->mlock_count for it.
> >> > -      *
> >> > -      * We would like to do something similar for the old page, when
> >> > -      * unsuccessful, and other cases when a page has been temporarily
> >> > -      * isolated from the unevictable LRU: but this case is the easiest.
> >> > -      */
> >> > -     folio_add_lru(dst);
> >> > -     if (page_was_mapped)
> >> > -             lru_add_drain();
> >> > -
> >> >       if (page_was_mapped)
> >> >               remove_migration_ptes(src, dst, false);
> >> >
> >> > @@ -1301,9 +1288,16 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
> >> >       /*
> >> >        * If migration is successful, decrease refcount of dst,
> >> >        * which will not free the page because new page owner increased
> >> > -      * refcounter.
> >> > +      * refcounter. As well, if it is LRU folio, add the folio to LRU
> >> > +      * list in here. Use the old state of the isolated source folio to
> >> > +      * determine if we migrated a LRU folio. dst was already unlocked
> >> > +      * and possibly modified by its owner - don't rely on the folio
> >> > +      * state.
> >> >        */
> >> > -     folio_put(dst);
> >> > +     if (unlikely(!is_lru))
> >> > +             folio_put(dst);
> >> > +     else
> >> > +             folio_putback_lru(dst);
> >> >
> >> >       /*
> >> >        * A folio that has been migrated has all references removed
> >>
>
  

On 19.06.23 05:59, Yosry Ahmed wrote:
> On Sun, Jun 18, 2023 at 7:00 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Hi, Yosry,
>>
>> Yosry Ahmed <yosryahmed@google.com> writes:
>>
>>> This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.
>>>
>>> That commit made sure we immediately add the new page to the LRU before
>>> remove_migration_ptes() is called in migrate_move_folio() (used to be
>>> __unmap_and_move() back then), such that the rmap walk will rebuild the
>>> correct mlock_count for the page again. This was needed because the
>>> mlock_count was lost when the page is isolated. This is no longer the
>>> case since mlock_count no longer overlays page->lru.
>>>
>>> Revert the commit (the code was foliated afterward the commit, so the
>>> revert is updated as such).
>>>
>>> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>>> ---
>>>   mm/migrate.c | 24 +++++++++---------------
>>>   1 file changed, 9 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 01cac26a3127..68f693731865 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
>>>        if (unlikely(!is_lru))
>>>                goto out_unlock_both;
>>
>> The patch itself looks good to me!  Thanks!
>>
>> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
> 
> Thanks for taking a look!
> 
>>
>> And, it seems that we can remove the above 2 lines and "out_unlock_both"
>> label now.  That can make the code simpler a little.  Right?
> 
> I am not familiar with this code. If we remove the above condition
> then pages that have is_lru == 0 (i.e __PageMovable(src) is true) and
> page_was_mapped == 1 will call remove_migration_ptes(). This wouldn't
> happen without removing the above 2 lines. If this combination is
> impossible (is_lru == 0 && page_was_mapped == 1), then yeah we can
> remove the above condition.
> 
> It looks like __SetPageMovable() is only called by zsmalloc, z3fold,
> and balloon_page_insert(). The former 2 will never have those pages
> mapped into userspace. I am not familiar with balloon_page_insert(),
> but my gut feeling is that those are pages used by the driver and are
> also not mapped into userspace.

On XEN, there is xen_alloc_ballooned_pages(), which ends up mapping 
balloon-inflated pages into user space (for something like MMIO IIRC). 
But the XEN balloon does not use the balloon compaction framework, so 
__SetPageMovable() does not apply.

The other balloon_page_insert() users (VMware balloon, CMM, 
virtio-balloon) shouldn't be doing something like that.
  

On 19.06.23 09:56, David Hildenbrand wrote:
> On 19.06.23 05:59, Yosry Ahmed wrote:
>> On Sun, Jun 18, 2023 at 7:00 PM Huang, Ying <ying.huang@intel.com> wrote:
>>>
>>> Hi, Yosry,
>>>
>>> Yosry Ahmed <yosryahmed@google.com> writes:
>>>
>>>> This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.
>>>>
>>>> That commit made sure we immediately add the new page to the LRU before
>>>> remove_migration_ptes() is called in migrate_move_folio() (used to be
>>>> __unmap_and_move() back then), such that the rmap walk will rebuild the
>>>> correct mlock_count for the page again. This was needed because the
>>>> mlock_count was lost when the page is isolated. This is no longer the
>>>> case since mlock_count no longer overlays page->lru.
>>>>
>>>> Revert the commit (the code was foliated afterward the commit, so the
>>>> revert is updated as such).
>>>>
>>>> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>>>> ---
>>>>    mm/migrate.c | 24 +++++++++---------------
>>>>    1 file changed, 9 insertions(+), 15 deletions(-)
>>>>
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index 01cac26a3127..68f693731865 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
>>>>         if (unlikely(!is_lru))
>>>>                 goto out_unlock_both;
>>>
>>> The patch itself looks good to me!  Thanks!
>>>
>>> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
>>
>> Thanks for taking a look!
>>
>>>
>>> And, it seems that we can remove the above 2 lines and "out_unlock_both"
>>> label now.  That can make the code simpler a little.  Right?
>>
>> I am not familiar with this code. If we remove the above condition
>> then pages that have is_lru == 0 (i.e __PageMovable(src) is true) and
>> page_was_mapped == 1 will call remove_migration_ptes(). This wouldn't
>> happen without removing the above 2 lines. If this combination is
>> impossible (is_lru == 0 && page_was_mapped == 1), then yeah we can
>> remove the above condition.
>>
>> It looks like __SetPageMovable() is only called by zsmalloc, z3fold,
>> and balloon_page_insert(). The former 2 will never have those pages
>> mapped into userspace. I am not familiar with balloon_page_insert(),
>> but my gut feeling is that those are pages used by the driver and are
>> also not mapped into userspace.
> 
> On XEN, there is xen_alloc_ballooned_pages(), which ends up mapping
> balloon-inflated pages into user space (for something like MMIO IIRC).
> But the XEN balloon does not use the balloon compaction framework, so
> __SetPageMovable() does not apply.
> 
> The other balloon_page_insert() users (VMware balloon, CMM,
> virtio-balloon) shouldn't be doing something like that.

Ah, and I remember they even can't, because in balloon_page_insert() we 
also do a __SetPageOffline(). And such typed pages cannot be mapped into 
user space (because the type overlays the mapcount).
  

On Mon, Jun 19, 2023 at 12:58 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 19.06.23 09:56, David Hildenbrand wrote:
> > On 19.06.23 05:59, Yosry Ahmed wrote:
> >> On Sun, Jun 18, 2023 at 7:00 PM Huang, Ying <ying.huang@intel.com> wrote:
> >>>
> >>> Hi, Yosry,
> >>>
> >>> Yosry Ahmed <yosryahmed@google.com> writes:
> >>>
> >>>> This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.
> >>>>
> >>>> That commit made sure we immediately add the new page to the LRU before
> >>>> remove_migration_ptes() is called in migrate_move_folio() (used to be
> >>>> __unmap_and_move() back then), such that the rmap walk will rebuild the
> >>>> correct mlock_count for the page again. This was needed because the
> >>>> mlock_count was lost when the page is isolated. This is no longer the
> >>>> case since mlock_count no longer overlays page->lru.
> >>>>
> >>>> Revert the commit (the code was foliated afterward the commit, so the
> >>>> revert is updated as such).
> >>>>
> >>>> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> >>>> ---
> >>>>    mm/migrate.c | 24 +++++++++---------------
> >>>>    1 file changed, 9 insertions(+), 15 deletions(-)
> >>>>
> >>>> diff --git a/mm/migrate.c b/mm/migrate.c
> >>>> index 01cac26a3127..68f693731865 100644
> >>>> --- a/mm/migrate.c
> >>>> +++ b/mm/migrate.c
> >>>> @@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
> >>>>         if (unlikely(!is_lru))
> >>>>                 goto out_unlock_both;
> >>>
> >>> The patch itself looks good to me!  Thanks!
> >>>
> >>> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
> >>
> >> Thanks for taking a look!
> >>
> >>>
> >>> And, it seems that we can remove the above 2 lines and "out_unlock_both"
> >>> label now.  That can make the code simpler a little.  Right?
> >>
> >> I am not familiar with this code. If we remove the above condition
> >> then pages that have is_lru == 0 (i.e __PageMovable(src) is true) and
> >> page_was_mapped == 1 will call remove_migration_ptes(). This wouldn't
> >> happen without removing the above 2 lines. If this combination is
> >> impossible (is_lru == 0 && page_was_mapped == 1), then yeah we can
> >> remove the above condition.
> >>
> >> It looks like __SetPageMovable() is only called by zsmalloc, z3fold,
> >> and balloon_page_insert(). The former 2 will never have those pages
> >> mapped into userspace. I am not familiar with balloon_page_insert(),
> >> but my gut feeling is that those are pages used by the driver and are
> >> also not mapped into userspace.
> >
> > On XEN, there is xen_alloc_ballooned_pages(), which ends up mapping
> > balloon-inflated pages into user space (for something like MMIO IIRC).
> > But the XEN balloon does not use the balloon compaction framework, so
> > __SetPageMovable() does not apply.
> >
> > The other balloon_page_insert() users (VMware balloon, CMM,
> > virtio-balloon) shouldn't be doing something like that.
>
> Ah, and I remember they even can't, because in balloon_page_insert() we
> also do a __SetPageOffline(). And such typed pages cannot be mapped into
> user space (because the type overlays the mapcount).

Thanks David, good to know! I will remove the condition as Ying
suggested in the next version then!

>
> --
> Cheers,
>
> David / dhildenb
>