Re: [PATCH v2] xfs: fix exchange-range reflink flag clearing issue with INO1_WRITTEN

On Tue, Jul 28, 2026 at 10:23:03AM +0800, Lin Jiapeng wrote:
> When exchanging two full-file ranges, xmi_can_exchange_reflink_flags()
> can move the reflink inode flag from the file that currently has it to
> the other file, as long as exactly one side is marked.  This assumes
> that the file contents, and therefore all shared extents, are exchanged.
> 
> That assumption is not true when XFS_EXCHMAPS_INO1_WRITTEN is set.
> xfs_exchmaps_can_skip_mapping() can skip hole and unwritten mappings
> from file1, so an exchange can complete without moving every mapping
> that the earlier flag-swap decision accounted for.  In that case the
> post-operation cleanup can clear the reflink flag from an inode that
> still owns shared written extents.  Later writes then take the
> non-reflink write path and may update blocks that should still have
> been protected by CoW, which shows up as data corruption between
> reflink-related files.
> 
> Fix this by disabling the reflink flag exchange whenever
> XFS_EXCHMAPS_INO1_WRITTEN is requested.  The contents exchange can still
> proceed; the conservative outcome is that both inodes keep the reflink
> flag.  The regular reflink flag cleanup path can drop the extra flag
> later once the inode no longer has shared extents.

The fix looks good:

Reviewed-by: Christoph Hellwig <[email protected]>

Can you add a test case to fstests for this?