Re: [PATCH v2] xfs: stop exchanging reflink flags when exchanging mappings

[Dave Chinner <dgc@kernel.org>]

On Wed, Oct 07, 2026 at 04:43:22PM +0200, Norbert Szetei wrote:
> When an exchange covers both files from offset 0 to i_disk_size,
> xmi_can_exchange_reflink_flags() assumes every mapping moved and hands
> the reflink flag over to the other file.
> 
> That assumption is wrong.  A file can still own shared blocks past
> i_disk_size, e.g. after EXCHANGE_RANGE_TO_EOF sets a smaller size
> without unmapping anything.  The flag is cleared anyway, and the next
> write to such a block happens in place instead of CoW, corrupting the
> other file that shares it.
> 
> Commit a23eca88448e ("xfs: fix exchange-range reflink flag clearing
> issue with INO1_WRITTEN") fixed one instance of this.  Rather than add
> another special case, stop moving the flags.
> xfs_exchmaps_ensure_reflink() already sets the flag on both inodes
> before the exchange, so both just keep it.
> 
> Nothing sets CLEAR_INO{1,2}_REFLINK anymore.  Intents logged by older
> kernels may still carry them; recovery already masks them off, so they
> stay in the log format mask and are ignored.
> 
> XFS_IOC_SWAPEXT has the same problem: it moves the reflink flag from
> one inode to the other along with the extents.  Just removing that is
> not enough here: the exchange path flags both inodes up front in
> xfs_exchmaps_ensure_reflink(), but swapext has no such step, so the
> inode receiving the shared extents would be left without the flag.
> Set it on both inodes instead, and make sure both have a CoW fork,
> since an inode with the reflink flag is expected to have one.
> 
> The flag now stays until FALLOC_FL_UNSHARE_RANGE, xfs_scrub, or a
> truncate to zero removes it.
> 
> Fixes: 966ceafc7a43 ("xfs: create deferred log items for file mapping exchanges")
> Fixes: f0bc4d134b46 ("xfs: swap inode reflink flags when swapping inode extents")
> Cc: stable@vger.kernel.org # v6.10
> Assisted-by: LLM
> Signed-off-by: Norbert Szetei <norbert@doyensec.com>
> ---
> v2: also handle XFS_IOC_SWAPEXT
> v1: https://lore.kernel.org/linux-xfs/1E196589-DEBE-40AC-AFEA-D420DAAB067F@doyensec.com/
> Discussion: https://lore.kernel.org/linux-xfs/20261006050914.GU2705364@frogsfrogsfrogs/

Agreed, this seems like the best course of action given the recurring issues
in this area.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
dgc@kernel.org