[PATCH] shmem: do not extend i_size when no data was copied

[PATCH] shmem: do not extend i_size when no data was copied

[Hungyu Lin]

shmem_write_end() advances i_size to pos + copied even when copied is
zero. A buffered write past EOF can therefore extend a tmpfs file
without writing any data.

Moving prefaulting to the slow write path makes this easy to reproduce:
a one-byte pwrite() at offset 4096 into an empty memfd, with an unreadable
source buffer, fails with EFAULT but leaves the file size at 4096.
generic_perform_write() calls ->write_end() with copied == 0 before
trying to fault in the source buffer.

Only extend i_size when data was copied. Leave the existing folio
handling unchanged so successful writes, retries and preallocated
pages retain their current behavior.

Fixes: 665575cff098 ("filemap: move prefaulting out of hot write path")
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index d97fc32e7c94..55287c370c73 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3487,7 +3487,7 @@ shmem_write_end(const struct kiocb *iocb, struct address_space *mapping,
 {
 	struct inode *inode = mapping->host;
 
-	if (pos + copied > inode->i_size)
+	if (copied && pos + copied > inode->i_size)
 		i_size_write(inode, pos + copied);
 
 	if (!folio_test_uptodate(folio)) {
-- 
2.43.0

Re: [PATCH] shmem: do not extend i_size when no data was copied

[Kiryl Shutsemau]

On Sun, Oct 25, 2026 at 04:46:19PM +0000, Hungyu Lin wrote:
> shmem_write_end() advances i_size to pos + copied even when copied is
> zero. A buffered write past EOF can therefore extend a tmpfs file
> without writing any data.

+ willy, jack, fsdevel.

This doesn't seem to be tmpfs-specific, right? I see the same with ext4.

simple_write_end() and generic_write_end() check pos + copied the same
way, so filesystems that use them through generic_perform_write() are
affected too.

It makes me think maybe generic_perform_write() is the right place to
fix it. Hm?

> Moving prefaulting to the slow write path makes this easy to reproduce:
> a one-byte pwrite() at offset 4096 into an empty memfd, with an unreadable
> source buffer, fails with EFAULT but leaves the file size at 4096.
> generic_perform_write() calls ->write_end() with copied == 0 before
> trying to fault in the source buffer.
>
> Only extend i_size when data was copied. Leave the existing folio
> handling unchanged so successful writes, retries and preallocated
> pages retain their current behavior.
>
> Fixes: 665575cff098 ("filemap: move prefaulting out of hot write path")
> Cc: stable@vger.kernel.org
> Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
> ---
>  mm/shmem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index d97fc32e7c94..55287c370c73 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3487,7 +3487,7 @@ shmem_write_end(const struct kiocb *iocb, struct address_space *mapping,
>  {
>  	struct inode *inode = mapping->host;
>
> -	if (pos + copied > inode->i_size)
> +	if (copied && pos + copied > inode->i_size)
>  		i_size_write(inode, pos + copied);
>
>  	if (!folio_test_uptodate(folio)) {
> -- 
> 2.43.0
>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

Re: [PATCH] shmem: do not extend i_size when no data was copied

[Jan Kara]

On Mon 26-10-26 15:34:08, Kiryl Shutsemau wrote:
> On Sun, Oct 25, 2026 at 04:46:19PM +0000, Hungyu Lin wrote:
> > shmem_write_end() advances i_size to pos + copied even when copied is
> > zero. A buffered write past EOF can therefore extend a tmpfs file
> > without writing any data.
> 
> + willy, jack, fsdevel.
> 
> This doesn't seem to be tmpfs-specific, right? I see the same with ext4.
> 
> simple_write_end() and generic_write_end() check pos + copied the same
> way, so filesystems that use them through generic_perform_write() are
> affected too.
> 
> It makes me think maybe generic_perform_write() is the right place to
> fix it. Hm?

Well, that would be nice. But how'd you like to do that? i_size update is
the responsibility of ->write_end and we have to call ->write_end if copied
== 0 so that is undoes what ->write_begin has prepared.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] shmem: do not extend i_size when no data was copied

[Kiryl Shutsemau]

On Mon, Oct 26, 2026 at 04:48:58PM +0100, Jan Kara wrote:
> On Mon 26-10-26 15:34:08, Kiryl Shutsemau wrote:
> > On Sun, Oct 25, 2026 at 04:46:19PM +0000, Hungyu Lin wrote:
> > > shmem_write_end() advances i_size to pos + copied even when copied is
> > > zero. A buffered write past EOF can therefore extend a tmpfs file
> > > without writing any data.
> > 
> > + willy, jack, fsdevel.
> > 
> > This doesn't seem to be tmpfs-specific, right? I see the same with ext4.
> > 
> > simple_write_end() and generic_write_end() check pos + copied the same
> > way, so filesystems that use them through generic_perform_write() are
> > affected too.
> > 
> > It makes me think maybe generic_perform_write() is the right place to
> > fix it. Hm?
> 
> Well, that would be nice. But how'd you like to do that? i_size update is
> the responsibility of ->write_end and we have to call ->write_end if copied
> == 0 so that is undoes what ->write_begin has prepared.

Hm.

A zero-length copy only moves i_size if the write starts beyond EOF.
So we can bring the prefault back just for that case, before
->write_begin():

        if (unlikely(pos > i_size_read(mapping->host)) &&
            fault_in_iov_iter_readable(i, bytes) == bytes) {
                status = -EFAULT;
                break;
        }

Overwrites and appends keep skipping the prefault. It is racy against
a concurrent munmap(), as it was before 665575cff098, but that is
userspace racing with itself.

The per-filesystem alternative is a 'copied &&' in every ->write_end.

And generic_write_end() cannot take it as is: generic_cont_expand_simple()
relies on a zero-length ->write_end() extending i_size. So we need some
replacement.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

Re: [PATCH] shmem: do not extend i_size when no data was copied

[Hungyu Lin]

On Mon, Oct 26, 2026 at 06:18:39PM +0000, Kiryl Shutsemau wrote:
> The per-filesystem alternative is a 'copied &&' in every ->write_end.

Would it make sense to keep this fix local to shmem for now?
I'll check the folio handling before sending v2.

Thanks,
Hungyu

Re: [PATCH] shmem: do not extend i_size when no data was copied

[Jan Kara]

On Tue 03-11-26 02:34:11, Hungyu Lin wrote:
> On Mon, Oct 26, 2026 at 06:18:39PM +0000, Kiryl Shutsemau wrote:
> > The per-filesystem alternative is a 'copied &&' in every ->write_end.
> 
> Would it make sense to keep this fix local to shmem for now?
> I'll check the folio handling before sending v2.

The trouble is that practically any filesystem is going to have these
issues with 0-length writes starting beyond i_size. So if we can fix it in
generic code, it would be certainly easier in the grand scheme of things.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] shmem: do not extend i_size when no data was copied

[Jan Kara]

On Mon 26-10-26 18:18:39, Kiryl Shutsemau wrote:
> On Mon, Oct 26, 2026 at 04:48:58PM +0100, Jan Kara wrote:
> > On Mon 26-10-26 15:34:08, Kiryl Shutsemau wrote:
> > > On Sun, Oct 25, 2026 at 04:46:19PM +0000, Hungyu Lin wrote:
> > > > shmem_write_end() advances i_size to pos + copied even when copied is
> > > > zero. A buffered write past EOF can therefore extend a tmpfs file
> > > > without writing any data.
> > > 
> > > + willy, jack, fsdevel.
> > > 
> > > This doesn't seem to be tmpfs-specific, right? I see the same with ext4.
> > > 
> > > simple_write_end() and generic_write_end() check pos + copied the same
> > > way, so filesystems that use them through generic_perform_write() are
> > > affected too.
> > > 
> > > It makes me think maybe generic_perform_write() is the right place to
> > > fix it. Hm?
> > 
> > Well, that would be nice. But how'd you like to do that? i_size update is
> > the responsibility of ->write_end and we have to call ->write_end if copied
> > == 0 so that is undoes what ->write_begin has prepared.
> 
> Hm.
> 
> A zero-length copy only moves i_size if the write starts beyond EOF.
> So we can bring the prefault back just for that case, before
> ->write_begin():
> 
>         if (unlikely(pos > i_size_read(mapping->host)) &&
>             fault_in_iov_iter_readable(i, bytes) == bytes) {
>                 status = -EFAULT;
>                 break;
>         }
> 
> Overwrites and appends keep skipping the prefault. It is racy against
> a concurrent munmap(), as it was before 665575cff098, but that is
> userspace racing with itself.
> 
> The per-filesystem alternative is a 'copied &&' in every ->write_end.
> 
> And generic_write_end() cannot take it as is: generic_cont_expand_simple()
> relies on a zero-length ->write_end() extending i_size. So we need some
> replacement.

Yeah, I actually like this. Good idea.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] shmem: do not extend i_size when no data was copied

[Hungyu Lin]

> The trouble is that practically any filesystem is going to have these
> issues with 0-length writes starting beyond i_size. So if we can fix it in
> generic code, it would be certainly easier in the grand scheme of things.

Understood. I'll try Kiryl's approach in generic_perform_write() and test
it across the affected filesystems before sending a patch.

Thanks,
Hungyu