Re: [PATCH v2 2/5] mm: limit pagecache folio refcount when splicing into pipes

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On 11/10/26 11:38 AM, David Hildenbrand (Arm) wrote:
> On 11/7/26 23:23, 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.
>>
>> Let's keep it simple and check in the two callers of
>> splice_folio_into_pipe(). Return -EOVERFLOW if the limit prevents any
>> progress, so do_splice_read() can fall back to copy_splice_read() when
>> read_iter is available. This keeps sendfile() and other splice users
>> working when ordinary reads would still succeed.
>>
>> Also fall back to copying in FUSE when taking another reference through
>> fuse_ref_folio() would exceed the limit.
>>
>> Also check in try_get_page() to cover pipe-buffer duplication by tee()
>> and partial pipe-to-pipe splicing.
>>
>> 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@vger.kernel.org
>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>> ---
>>  fs/fuse/dev.c      |  5 +++--
>>  fs/splice.c        |  8 +++++++-
>>  include/linux/mm.h |  9 ++++++---
>>  mm/filemap.c       | 11 ++++++++---
>>  mm/shmem.c         |  4 ++++
>>  5 files changed, 28 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
>> index 3641abb90d6bb..4f6b7b42bfd3e 100644
>> --- a/fs/fuse/dev.c
>> +++ b/fs/fuse/dev.c
>> @@ -1265,9 +1265,10 @@ int fuse_copy_folio(struct fuse_copy_state *cs, struct folio **foliop,
>>  		if (cs->write && cs->pipebufs && folio) {
>>  			/*
>>  			 * Can't control lifetime of pipe buffers, so always
>> -			 * copy user pages.
>> +			 * copy user pages. Also copy folios at their refcount limit.
>>  			 */
>> -			if (cs->req->args->user_pages) {
>> +			if (cs->req->args->user_pages ||
>> +			    unlikely(folio_ref_count_over_limit(folio))) {
> 
> Linus points out off-list (thanks!), that fuse_ref_folio() should have
> really used try_get_page() right from the start.
> 
> And that we should just not fallback to a copy, in particular,
> if the refcount is invalid (<= 0).

now that is said out loud, makes sense.

> So the following is what I have for now. Once we remove the limit
> handling again, that code won't change, which is good.
> 
> I went back and forth with folio_get_checked() naming, but decided to
> not mix in the "try" as that just reminds of our speculative refcount
> handling. (folio_try_get_checked / folio_try_get_referenced)

agreed with that.

[snip]

> -			if (pipe_is_full(pipe))
> +			if (pipe_is_full(pipe) || *ppos < folio_next_pos(folio))
>  				goto out;

nice!

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

-- 
cheers,
jose a. p-a