Re: [PATCH v3 2/9] mm: limit pagecache folio refcount when splicing into pipes

[Jose A. Perez de Azpillaga]

On 11/10/26 9:35 PM, David Hildenbrand (Arm) wrote:
> Similar to mapping pagecache folios into page tables, let's reject
> additional references when splicing folios into a pipe if the refcount
> is at its limit, similar to how we handle it during GUP.
> 
> This will prevent more cases of possible folio refcount overflows with
> large pagecache folios.
> 
> Introduce folio_get_checked() for checked reference acquisition when
> the caller already holds a reference. It warns and fails on a non-positive
> refcount and rejects additional references at the limit.
> 
> Use folio_get_checked() in splice_folio_into_pipe() and
> fuse_ref_folio(), returning -EFAULT on failure, like we already do for
> pipe-buffer duplication. Similarly, make try_get_page() use it instead.
> 
> Likely try_get_page() should be renamed (or removed) later, as it doesn't
> match the folio_try_get() semantics and is confusing.
> 
> If we hit the limit after splicing some data, return the number of bytes
> already spliced. Teach the callers to handle errors, and stop
> filemap_splice_read() if we only spliced part of a folio, so we don't
> skip ahead to the next folio.
> 
> The check in try_get_page() also covers pipe-buffer duplication by tee()
> and partial pipe-to-pipe splicing.
> 
> Note that we deliberately will not fall back to the slow path of copying
> the folio, particularly as the folio might have an invalid refcount. If
> it is ever really required, we can revisit.
> 
> Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

LGTM :)

Reviewed-by: Jose A. Perez de Azpillaga <azpijr@xxxxxxxxx>

-- 
cheers,
jose a. p-a