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

[Matthew Wilcox <willy@infradead.org>]

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.

> - * 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 ensure folio->index is stable and it's
> + * not added to the swap cache.

I do think we need to keep the part about the folio being locked.