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

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

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@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))) {
>  				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. 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.


-- 
cheers,
jose a. p-a