diff mbox series

[v2,05/32] mm/filemap: allow pte_offset_map_lock() to fail

Message ID 54607cf4-ddb6-7ef3-043-1d2de1a9a71@google.com
State New
Headers
Series mm: allow pte_offset_map[_lock]() to fail |

Commit Message

Hugh Dickins June 9, 2023, 1:11 a.m. UTC 
  filemap_map_pages() allow pte_offset_map_lock() to fail; and remove the
pmd_devmap_trans_unstable() check from filemap_map_pmd(), which can safely
return to filemap_map_pages() and let pte_offset_map_lock() discover that.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/filemap.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

Comments

Zi Yan July 11, 2023, 1:34 a.m. UTC | #1 
On 8 Jun 2023, at 21:11, Hugh Dickins wrote:

> filemap_map_pages() allow pte_offset_map_lock() to fail; and remove the
> pmd_devmap_trans_unstable() check from filemap_map_pmd(), which can safely
> return to filemap_map_pages() and let pte_offset_map_lock() discover that.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>  mm/filemap.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 28b42ee848a4..9e129ad43e0d 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3408,13 +3408,6 @@ static bool filemap_map_pmd(struct vm_fault *vmf, struct folio *folio,
>  	if (pmd_none(*vmf->pmd))
>  		pmd_install(mm, vmf->pmd, &vmf->prealloc_pte);
>
> -	/* See comment in handle_pte_fault() */
> -	if (pmd_devmap_trans_unstable(vmf->pmd)) {
> -		folio_unlock(folio);
> -		folio_put(folio);
> -		return true;
> -	}
> -

There is a pmd_trans_huge() check at the beginning, should it be removed
as well? Since pte_offset_map_lock() is also able to detect it.

>  	return false;
>  }
>
> @@ -3501,6 +3494,11 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
>
>  	addr = vma->vm_start + ((start_pgoff - vma->vm_pgoff) << PAGE_SHIFT);
>  	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
> +	if (!vmf->pte) {
> +		folio_unlock(folio);
> +		folio_put(folio);
> +		goto out;
> +	}
>  	do {
>  again:
>  		page = folio_file_page(folio, xas.xa_index);
> -- 
> 2.35.3

These two changes affect the ret value. Before, pmd_devmap_trans_unstable() == true
made ret = VM_FAULT_NPAGE, but now ret is the default 0 value. So ret should be set
to VM_FAULT_NPAGE before goto out in the second hunk?

--
Best Regards,
Yan, Zi
Hugh Dickins July 11, 2023, 5:21 a.m. UTC | #2 
On Mon, 10 Jul 2023, Zi Yan wrote:
> On 8 Jun 2023, at 21:11, Hugh Dickins wrote:
> 
> > filemap_map_pages() allow pte_offset_map_lock() to fail; and remove the
> > pmd_devmap_trans_unstable() check from filemap_map_pmd(), which can safely
> > return to filemap_map_pages() and let pte_offset_map_lock() discover that.
> >
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> >  mm/filemap.c | 12 +++++-------
> >  1 file changed, 5 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 28b42ee848a4..9e129ad43e0d 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -3408,13 +3408,6 @@ static bool filemap_map_pmd(struct vm_fault *vmf, struct folio *folio,
> >  	if (pmd_none(*vmf->pmd))
> >  		pmd_install(mm, vmf->pmd, &vmf->prealloc_pte);
> >
> > -	/* See comment in handle_pte_fault() */
> > -	if (pmd_devmap_trans_unstable(vmf->pmd)) {
> > -		folio_unlock(folio);
> > -		folio_put(folio);
> > -		return true;
> > -	}
> > -
> 
> There is a pmd_trans_huge() check at the beginning, should it be removed
> as well? Since pte_offset_map_lock() is also able to detect it.

It probably could be removed: but mostly I avoided such cleanups,
in the hope that the patches could be more easily reviewed as safe.
But I was eager to delete that obscure pmd_devmap_trans_unstable().

The whole strategy of dealing with the pmd_trans_huge()-like cases first,
and only finally arriving at the pte_offset_map_lock() when other cases
have been excluded, could be reversed in *many* places.  It had to be that
way before, because pte_offset_map_lock() could only cope with a page
table; but now we could reverse them to do the pte_offset_map_lock()
first, and only try the other cases when it fails.

That would in theory be more efficient; but whether measurably more
efficient I doubt.  And very easy to introduce errors on the way:
my enthusiasm for such cleanups is low!  But maybe there's a few
places where the rearrangement would be worthwhile.

> 
> >  	return false;
> >  }
> >
> > @@ -3501,6 +3494,11 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
> >
> >  	addr = vma->vm_start + ((start_pgoff - vma->vm_pgoff) << PAGE_SHIFT);
> >  	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
> > +	if (!vmf->pte) {
> > +		folio_unlock(folio);
> > +		folio_put(folio);
> > +		goto out;
> > +	}
> >  	do {
> >  again:
> >  		page = folio_file_page(folio, xas.xa_index);
> > -- 
> > 2.35.3
> 
> These two changes affect the ret value. Before, pmd_devmap_trans_unstable() == true
> made ret = VM_FAULT_NPAGE, but now ret is the default 0 value. So ret should be set
> to VM_FAULT_NPAGE before goto out in the second hunk?

Qi Zheng raised a similar question on the original posting, I answered
https://lore.kernel.org/linux-mm/fb9a9d57-dbd7-6a6e-d1cb-8dcd64c829a6@google.com/

It's a rare case to fault here, then find pmd_devmap(*pmd), and it really
doesn't matter whether we return VM_FAULT_NOPAGE or 0 for it - maybe I've
left it inconsistent between THP and devmap, but it doesn't really matter.

I haven't checked Matthew's v5 "new page table range API" posted today,
but I expect this all looks different here anyway.

Thanks a lot for checking these: they are now in 6.5-rc1, so if you find
something that needs fixing, all the more important that we do fix it.

Hugh
diff mbox series

Patch

diff --git a/mm/filemap.c b/mm/filemap.c
index 28b42ee848a4..9e129ad43e0d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3408,13 +3408,6 @@  static bool filemap_map_pmd(struct vm_fault *vmf, struct folio *folio,
 	if (pmd_none(*vmf->pmd))
 		pmd_install(mm, vmf->pmd, &vmf->prealloc_pte);
 
-	/* See comment in handle_pte_fault() */
-	if (pmd_devmap_trans_unstable(vmf->pmd)) {
-		folio_unlock(folio);
-		folio_put(folio);
-		return true;
-	}
-
 	return false;
 }
 
@@ -3501,6 +3494,11 @@  vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 
 	addr = vma->vm_start + ((start_pgoff - vma->vm_pgoff) << PAGE_SHIFT);
 	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
+	if (!vmf->pte) {
+		folio_unlock(folio);
+		folio_put(folio);
+		goto out;
+	}
 	do {
 again:
 		page = folio_file_page(folio, xas.xa_index);