Re: [PATCH v2 4/6] filemap: do not use folio_contains for swap cache folios

[Kairui Song <ryncsn@gmail.com>]

On Tue, Apr 29, 2025 at 8:22 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 29.04.25 13:49, Kairui Song wrote:
> > From: Kairui Song <kasong@tencent.com>
> >
> > Currently, none of the folio_contains callers should encounter swap
> > cache folios.
> >
> > For fs/ callers, swap cache folios are never part of their workflow.
> >
> > For filemap and truncate, folio_contains is only used for sanity
> > checks to verify the folio index matches the expected
> > lookup / invalidation target.
> >
> > The swap cache does not utilize filemap or truncate helpers in ways
> > that would trigger these checks, as it mostly implements its own
> > cache management.
> >
> > Shmem won't trigger these sanity checks either unless thing went
> > wrong, as it would directly trigger a BUG because swap cache index are
> > unrelated and almost never matches shmem index. Shmem have to handle
> > mixed values of folios, shadows, and swap entries, so it has its own
> > way of handling the mapping.
> >
> > While some filemap helpers works for swap cache space, the swap cache
> > is different from the page cache in many ways. So this particular helper
> > will unlikely to work in a helpful way for swap cache folios.
> >
> > So make it explicit here that folio_contains should not be used for
> > swap cache folios. This helps to avoid misuse, make swap cache less
> > exposed and remove the folio_index usage here.
> >
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > ---
> >   include/linux/pagemap.h | 8 ++++----
> >   1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index af25fb640463..1dc3416a9c0d 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -935,14 +935,14 @@ static inline struct page *folio_file_page(struct folio *folio, pgoff_t index)
> >    * @folio: The folio.
> >    * @index: The page index within the file.
> >    *
> > - * Context: The caller should have the page locked in order to prevent
> > - * (eg) shmem from moving the page between the page cache and swap cache
> > - * and changing its index in the middle of the operation.
> > + * Context: The caller should have the folio locked and ensure
> > + * (e.g.) shmem did not move this folio to swap cache.
>
> The "(e.g.)" looks weird. Maybe "ensure that e.g., shmem ..."
>
> "to the"
>
> >    * Return: true or false.
> >    */
> >   static inline bool folio_contains(struct folio *folio, pgoff_t index)
> >   {
> > -     return index - folio_index(folio) < folio_nr_pages(folio);
> > +     VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
>
> Likely you want VM_WARN_ON_ONCE_FOLIO() here.

All its caller will trigger a bug if it encounters a swap cache, so I
kept that behaviour consistent. Let's keep that unchanged for now.

>
> > +     return index - folio->index < folio_nr_pages(folio);
> >   }
> >
> >   unsigned filemap_get_folios(struct address_space *mapping, pgoff_t *start,
>
>
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks!

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