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

[David Hildenbrand (Arm)]

On 11/9/26 09:57, Jose A. Perez de Azpillaga wrote:
> On 11/7/26 11:23 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.
>>
>> 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@xxxxxxxxxxxxxxx
>> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>> ---
>>  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))) {
>>  				err = fuse_copy_fill(cs);
>>  				if (err)
>>  					return err;
>> diff --git a/fs/splice.c b/fs/splice.c
>> index e6d43329ed3f0..df73a4e65118b 100644
>> --- a/fs/splice.c
>> +++ b/fs/splice.c
>> @@ -956,6 +956,7 @@ static ssize_t do_splice_read(struct file *in, loff_t *ppos,
>>  			      unsigned int flags)
>>  {
>>  	unsigned int p_space;
>> +	ssize_t ret;
>>  
>>  	if (unlikely(!(in->f_mode & FMODE_READ)))
>>  		return -EBADF;
>> @@ -977,7 +978,12 @@ static ssize_t do_splice_read(struct file *in, loff_t *ppos,
>>  	 */
>>  	if ((in->f_flags & O_DIRECT) || IS_DAX(in->f_mapping->host))
>>  		return copy_splice_read(in, ppos, pipe, len, flags);
>> -	return in->f_op->splice_read(in, ppos, pipe, len, flags);
>> +	ret = in->f_op->splice_read(in, ppos, pipe, len, flags);
>> +
>> +	/* Fallback to copying when hitting folio refcount limits. */
>> +	if (unlikely(ret == -EOVERFLOW) && in->f_op->read_iter)
>> +		ret = copy_splice_read(in, ppos, pipe, len, flags);
> 
> here it can't tell where the -EOVERFLOW came from, if another file
> returns -EOVERFLOW (i.e. tty_fops uses copy_splice_read, and
> n_hdlc_tty_read() returns -EOVERFLOW after dropping the frame), it now
> gets a second read.


That's what claude told me during review, and when asked whether that actually
happened in practice it said no ;)

So is that actually a problem?

> also, the first read already lost data. so the data
> read returns different data, the dropped frame is never reported and
> returns the next frame instead.

I don't follow. Can you give an example?

-- 
Cheers,

David