Page writeback indirectly handles shared extents via the existence of overlapping COW fork blocks. If COW fork blocks exist, writeback always performs the associated copy-on-write regardless if the underlying blocks are actually shared. If the blocks are shared, then overlapping COW fork blocks must always exist. fstests shared/010 reproduces a case where a buffered write occurs over a shared block without performing the requisite COW fork reservation. This ultimately causes writeback to the shared extent and data corruption that is detected across md5 checks of the filesystem across a mount cycle. The problem occurs when a buffered write lands over a shared extent that crosses an extent size hint boundary and that also happens to have a partial COW reservation that doesn't cover the start and end blocks of the data fork extent. For example, a buffered write occurs across the file offset (in FSB units) range of [29, 57]. A shared extent exists at blocks [29, 35] and COW reservation already exists at blocks [32, 34]. After accommodating a COW extent size hint of 32 blocks and the existing reservation at offset 32, xfs_reflink_reserve_cow() allocates 32 blocks of reservation at offset 0 and returns with COW reservation across the range of [0, 34]. The associated data fork extent is still [29, 35], however, which isn't fully covered by the COW reservation. This leads to a buffered write at file offset 35 over a shared extent without associated COW reservation. Writeback eventually kicks in, performs an overwrite of the underlying shared block and causes the associated data corruption. Update xfs_reflink_reserve_cow() to accommodate the fact that a delalloc allocation request may not fully cover the extent in the data fork. Trim the data fork extent appropriately, just as is done for shared extent boundaries and/or existing COW reservations that happen to overlap the start of the data fork extent. This prevents shared/010 failures due to data corruption on reflink enabled filesystems. Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> --- This is not fully tested yet beyond verification that it solves the problem reproduced by shared/010. I'll be running more tests today, but I'm sending sooner for review and testing due to the nature of the problem and the fact that it's a fairly isolated change. I'll follow up if I discover any resulting regressions.. Brian fs/xfs/xfs_reflink.c | 1 + 1 file changed, 1 insertion(+) diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c index ecdb086bc23e..c56bdbfcf7ae 100644 --- a/fs/xfs/xfs_reflink.c +++ b/fs/xfs/xfs_reflink.c @@ -296,6 +296,7 @@ xfs_reflink_reserve_cow( if (error) return error; + xfs_trim_extent(imap, got.br_startoff, got.br_blockcount); trace_xfs_reflink_cow_alloc(ip, &got); return 0; } -- 2.17.2
