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Brian Foster authored
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>
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