fallocate unshare mode explicitly breaks extent sharing. When a
command completes, it checks the data fork for any remaining shared
extents to determine whether the reflink inode flag and COW fork
preallocation can be removed. This logic doesn't consider in-core
pagecache and I/O state, however, which means we can unsafely remove
COW fork blocks that are still needed under certain conditions.

For example, consider the following command sequence:

xfs_io -fc "pwrite 0 1k" -c "reflink <file> 0 256k 1k" \
	-c "pwrite 0 32k" -c "funshare 0 1k" <file>

This allocates a data block at offset 0, shares it, and then
overwrites it with a larger buffered write. The overwrite triggers
COW fork preallocation, 32 blocks by default, which maps the entire
32k write to delalloc in the COW fork. All but the shared block at
offset 0 remains hole mapped in the data fork. The unshare command
redirties and flushes the folio at offset 0, removing the only
shared extent from the inode. Since the inode no longer maps shared
extents, unshare purges the COW fork before the remaining 28k may
have written back.

This leaves dirty pagecache backed by holes, which writeback quietly
skips, thus leaving clean, non-zeroed pagecache over holes in the
file. To verify, fiemap shows holes in the first 32k of the file and
reads return different data across a remount:

$ xfs_io -c "fiemap -v" <file>
<file>:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   ...
   1: [8..511]:        hole               504
   ...
$ xfs_io -c "pread -v 4k 8" <file>
00001000:  cd cd cd cd cd cd cd cd  ........
$ umount <mnt>; mount <dev> <mnt>
$ xfs_io -c "pread -v 4k 8" <file>
00001000:  00 00 00 00 00 00 00 00  ........

To avoid this problem, make unshare follow the same rules used for
background cowblock scanning and never purge the COW fork for inodes
with dirty pagecache or in-flight I/O.

Fixes: 46afb062 ("xfs: only flush the unshared range in xfs_reflink_unshare")
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>