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

[Kairui Song <ryncsn@gmail.com>]

On Mon, Apr 28, 2025 at 8:44 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Apr 28, 2025 at 02:59:06AM +0800, Kairui Song wrote:
> > 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 interact with these sanity checks either unless thing went
> > wrong, it would directly trigger a BUG, because swap cache index are
> > unrelated to shmem index, and would almost certainly mismatch (unless
> > on collide).
>
> It does happen though.  If shmem is writing the folio to swap at the
> same time that the file containing the folio is being truncated, we
> can hit this.

Thanks for the info! I didn't check it in detail because that would
likley trigger a BUG_ON but so far I didn't see any BUG_ON commit from
there.

Just checked there are two users in truncate:

One will lock the folio and check if `folio->mapping != mapping`
first, on swapout shmem removes the folio from shmem mapping so this
check will skip the folio_contains check.

Another seems might hit the check, the time window is extremely tiny
though, only if the truncate's `xa_is_value(folio)` check passed while
another CPU is running between `folio_alloc_swap` and
`shmem_delete_from_page_cache` in shmem_writepage, then the next
VM_BUG_ON_FOLIO(!folio_contains) will fail as folio is now  a
swap cache, not shmem folio anymore. Let me double check if this needs
another fix.