Re: [PATCH] filemap: prefault writes starting beyond EOF

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Nov 04, 2026 at 07:10:02AM +0000, Hungyu Lin wrote:
> Since prefaulting moved to the slow write path, a buffered write can reach
> ->write_begin() and ->write_end() before discovering that its source buffer
> is unreadable. Several filesystems can extend i_size in these callbacks
> even when no data was copied. For example, a one-byte pwrite() at offset
> 4096 into an empty file can fail with EFAULT but leave a 4096-byte file.
> 
> Prefault the source before ->write_begin() when the write position is
> beyond EOF, and return EFAULT if no source bytes can be made readable.
> Overwrites and appends continue to skip this prefault. Keep the existing
> slow-path faulting for forward progress after a failed atomic copy.
> 
> This also leaves intentional zero-length ->write_end() calls used by
> generic_cont_expand_simple() unchanged. As with the previous prefaulting
> behavior, a concurrent userspace unmap can still race with the later copy.
> 
> Fixes: 665575cff098 ("filemap: move prefaulting out of hot write path")
> Reported-by: Kiryl Shutsemau <kas@kernel.org>
> Closes: https://lore.kernel.org/linux-mm/at9xXRhD2xZhszEj@thinkstation/
> Suggested-by: Kiryl Shutsemau <kas@kernel.org>
> Link: https://lore.kernel.org/all/at-XtWBQg3jv3H8Z@thinkstation/
> Cc: stable@vger.kernel.org
> Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>

You found the bug, I only pointed out that it is not tmpfs-specific.
Please drop Reported-by and Closes; Suggested-by and Link are enough.

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov