From 74121f7cbb2f60b41c48184ad5b889c0704e7b90 Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Thu, 7 Aug 2014 12:00:44 +0100 Subject: [PATCH] Btrfs: fix hole detection during file fsync The file hole detection logic during a file fsync wasn't correct, because it didn't look back (in a previous leaf) for the last file extent item that can be in a leaf to the left of our leaf and that has a generation lower than the current transaction id. This made it assume that a hole exists when it really doesn't exist in the file. Such false positive hole detection happens in the following scenario: * We have a file that has many file extent items, covering 3 or more btree leafs (the first leaf must contain non file extent items too). * Two ranges of the file are modified, with their extent items being located at 2 different leafs and those leafs aren't consecutive. * When processing the second modified leaf, we weren't checking if some file extent item exists that is located in some leaf that is between our 2 modified leafs, and therefore assumed the range defined between the last file extent item in the first leaf and the first file extent item in the second leaf matched a hole. Fortunately this didn't result in overriding the log with wrong data, instead it made the last loop in copy_items() attempt to insert a duplicated key (for a hole file extent item), which makes the file fsync code return with -EEXIST to file.c:btrfs_sync_file() which in turn ends up doing a full transaction commit, which is much more expensive then writing only to the log tree and wait for it to be durably persisted (as well as the file's modified extents/pages). Therefore fix the hole detection logic, so that we don't pay the cost of doing full transaction commits. I could trigger this issue with the following test for xfstests (which never fails, either without or with this patch). The last fsync call results in a full transaction commit, due to the -EEXIST error mentioned above. I could also observe this behaviour happening frequently when running xfstests/generic/075 in a loop. Test: _cleanup() { _cleanup_flakey rm -fr $tmp } # get standard environment, filters and checks . ./common/rc . ./common/filter . ./common/dmflakey # real QA test starts here _supported_fs btrfs _supported_os Linux _require_scratch _require_dm_flakey _need_to_be_root rm -f $seqres.full # Create a file with many file extent items, each representing a 4Kb extent. # These items span 3 btree leaves, of 16Kb each (default mkfs.btrfs leaf size # as of btrfs-progs 3.12). _scratch_mkfs -l 16384 >/dev/null 2>&1 _init_flakey SAVE_MOUNT_OPTIONS="$MOUNT_OPTIONS" MOUNT_OPTIONS="$MOUNT_OPTIONS -o commit=999" _mount_flakey # First fsync, inode has BTRFS_INODE_NEEDS_FULL_SYNC flag set. $XFS_IO_PROG -f -c "pwrite -S 0x01 -b 4096 0 4096" -c "fsync" \ $SCRATCH_MNT/foo | _filter_xfs_io # For any of the following fsync calls, inode doesn't have the flag # BTRFS_INODE_NEEDS_FULL_SYNC set. for ((i = 1; i <= 500; i++)); do OFFSET=$((4096 * i)) LEN=4096 $XFS_IO_PROG -c "pwrite -S 0x01 $OFFSET $LEN" -c "fsync" \ $SCRATCH_MNT/foo | _filter_xfs_io done # Commit transaction and bump next transaction's id (to 7). sync # Truncate will set the BTRFS_INODE_NEEDS_FULL_SYNC flag in the btrfs's # inode runtime flags. $XFS_IO_PROG -c "truncate 2048000" $SCRATCH_MNT/foo # Commit transaction and bump next transaction's id (to 8). sync # Touch 1 extent item from the first leaf and 1 from the last leaf. The leaf # in the middle, containing only file extent items, isn't touched. So the # next fsync, when calling btrfs_search_forward(), won't visit that middle # leaf. First and 3rd leaf have now a generation with value 8, while the # middle leaf remains with a generation with value 6. $XFS_IO_PROG \ -c "pwrite -S 0xee -b 4096 0 4096" \ -c "pwrite -S 0xff -b 4096 2043904 4096" \ -c "fsync" \ $SCRATCH_MNT/foo | _filter_xfs_io _load_flakey_table $FLAKEY_DROP_WRITES md5sum $SCRATCH_MNT/foo | _filter_scratch _unmount_flakey _load_flakey_table $FLAKEY_ALLOW_WRITES # During mount, we'll replay the log created by the fsync above, and the file's # md5 digest should be the same we got before the unmount. _mount_flakey md5sum $SCRATCH_MNT/foo | _filter_scratch _unmount_flakey MOUNT_OPTIONS="$SAVE_MOUNT_OPTIONS" status=0 exit Signed-off-by: Filipe Manana Signed-off-by: Chris Mason --- fs/btrfs/tree-log.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-) diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c index 9e1f2cd5e67a..7e0e6e3029dd 100644 --- a/fs/btrfs/tree-log.c +++ b/fs/btrfs/tree-log.c @@ -3298,7 +3298,7 @@ static noinline int copy_items(struct btrfs_trans_handle *trans, struct list_head ordered_sums; int skip_csum = BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM; bool has_extents = false; - bool need_find_last_extent = (*last_extent == 0); + bool need_find_last_extent = true; bool done = false; INIT_LIST_HEAD(&ordered_sums); @@ -3352,8 +3352,7 @@ static noinline int copy_items(struct btrfs_trans_handle *trans, */ if (ins_keys[i].type == BTRFS_EXTENT_DATA_KEY) { has_extents = true; - if (need_find_last_extent && - first_key.objectid == (u64)-1) + if (first_key.objectid == (u64)-1) first_key = ins_keys[i]; } else { need_find_last_extent = false; @@ -3427,6 +3426,16 @@ static noinline int copy_items(struct btrfs_trans_handle *trans, if (!has_extents) return ret; + if (need_find_last_extent && *last_extent == first_key.offset) { + /* + * We don't have any leafs between our current one and the one + * we processed before that can have file extent items for our + * inode (and have a generation number smaller than our current + * transaction id). + */ + need_find_last_extent = false; + } + /* * Because we use btrfs_search_forward we could skip leaves that were * not modified and then assume *last_extent is valid when it really @@ -3537,7 +3546,7 @@ fill_holes: 0, 0); if (ret) break; - *last_extent = offset + len; + *last_extent = extent_end; } /* * Need to let the callers know we dropped the path so they should