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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 11/9/26 11:25, Jose A. Perez de Azpillaga wrote:
> On 11/9/26 11:08 AM, David Hildenbrand (Arm) wrote:
>> On 11/9/26 10:49, David Hildenbrand (Arm) wrote:
>>>
>>> Double checked, claude managed to confuse itself.
>>>
>>> n_hdlc_tty_read() can indeed return -EOVERFLOW.
>>>
>>> I should just have let splice keep failing, dealing with copy fallback starts to
>>> get annoying.
>>>
>>> I guess switching to -ETOOMANYREFS should work (and might be a better fitting error)
>>
>> Yeah, the following should do I think, please double check:
>>
>> From 7ce030711737e8c26fb5500f2705940e93da0d44 Mon Sep 17 00:00:00 2001
>> From: "David Hildenbrand (Arm)" <david@kernel.org>
>> Date: Wed, 4 Nov 2026 19:02:48 +0100
>> Subject: [PATCH] mm: limit pagecache folio refcount when splicing into pipes
>>
>> 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 -ETOOMANYREFS 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..2fc1279628af5 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 == -ETOOMANYREFS) && in->f_op->read_iter)
>> +		ret = copy_splice_read(in, ppos, pipe, len, flags);
>> +	return ret;
>>  }
>>  
>>  /**
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 3356f7a670d1e..dd50262a70758 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2288,10 +2288,13 @@ static inline void get_page(struct page *page)
>>  
>>  static inline __must_check bool try_get_page(struct page *page)
>>  {
>> -	page = compound_head(page);
>> -	if (WARN_ON_ONCE(page_ref_count(page) <= 0))
>> +	struct folio *folio = page_folio(page);
>> +
>> +	if (WARN_ON_ONCE(folio_ref_count(folio) <= 0))
>>  		return false;
>> -	page_ref_inc(page);
>> +	if (unlikely(folio_ref_count_over_limit(folio)))
>> +		return false;
>> +	folio_ref_inc(folio);
>>  	return true;
>>  }
>>  
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 920bbf86dfdff..44b23c51ea45f 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -3094,9 +3094,10 @@ size_t splice_folio_into_pipe(struct pipe_inode_info *pipe,
>>   * Return: On success, the number of bytes read will be returned and *@ppos
>>   * will be updated if appropriate; 0 will be returned if there is no more data
>>   * to be read; -EAGAIN will be returned if the pipe had no space, and some
>> - * other negative error code will be returned on error.  A short read may occur
>> - * if the pipe has insufficient space, we reach the end of the data or we hit a
>> - * hole.
>> + * other negative error code will be returned on error; -ETOOMANYREFS will be
>> + * returned if the folio's refcount is too high and the caller can fall back to
>> + * copying.  A short read may occur if the pipe has insufficient space, we reach
>> + * the end of the data or we hit a hole.
>>   */
>>  ssize_t filemap_splice_read(struct file *in, loff_t *ppos,
>>  			    struct pipe_inode_info *pipe,
>> @@ -3168,6 +3169,10 @@ ssize_t filemap_splice_read(struct file *in, loff_t *ppos,
>>  			if (writably_mapped)
>>  				flush_dcache_folio(folio);
>>  
>> +			if (unlikely(folio_ref_count_over_limit(folio))) {
>> +				error = -ETOOMANYREFS;
>> +				goto out;
>> +			}
>>  			n = min_t(loff_t, len, isize - *ppos);
>>  			n = splice_folio_into_pipe(pipe, folio, *ppos, n);
>>  			if (!n)
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index be4ab228cf67c..0b30b1de81a7c 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -3830,6 +3830,10 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
>>  			 * Ok, we have the page, and it's up-to-date, so we can
>>  			 * now splice it into the pipe.
>>  			 */
>> +			if (unlikely(folio_ref_count_over_limit(folio))) {
>> +				error = -ETOOMANYREFS;
>> +				break;
>> +			}
>>  			n = splice_folio_into_pipe(pipe, folio, *ppos, part);
>>  			folio_put(folio);
>>  			folio = NULL;
> 
> this LGTM :)

Thanks!

-- 
Cheers,

David