Btrfs: fix fsync data loss after append write

If we do an append write to a file (which increases its inode's i_size)
that does not have the flag BTRFS_INODE_NEEDS_FULL_SYNC set in its inode,
and the previous transaction added a new hard link to the file, which sets
the flag BTRFS_INODE_COPY_EVERYTHING in the file's inode, and then fsync
the file, the inode's new i_size isn't logged. This has the consequence
that after the fsync log is replayed, the file size remains what it was
before the append write operation, which means users/applications will
not be able to read the data that was successsfully fsync'ed before.

This happens because neither the inode item nor the delayed inode get
their i_size updated when the append write is made - doing so would
require starting a transaction in the buffered write path, something that
we do not do intentionally for performance reasons.

Fix this by making sure that when the flag BTRFS_INODE_COPY_EVERYTHING is
set the inode is logged with its current i_size (log the in-memory inode
into the log tree).

This issue is not a recent regression and is easy to reproduce with the
following test case for fstests:

  seq=`basename $0`
  seqres=$RESULT_DIR/$seq
  echo "QA output created by $seq"

  here=`pwd`
  tmp=/tmp/$$
  status=1	# failure is the default!

  _cleanup()
  {
          _cleanup_flakey
          rm -f $tmp.*
  }
  trap "_cleanup; exit \$status" 0 1 2 3 15

  # get standard environment, filters and checks
  . ./common/rc
  . ./common/filter
  . ./common/dmflakey

  # real QA test starts here
  _supported_fs generic
  _supported_os Linux
  _need_to_be_root
  _require_scratch
  _require_dm_flakey
  _require_metadata_journaling $SCRATCH_DEV

  _crash_and_mount()
  {
          # Simulate a crash/power loss.
          _load_flakey_table $FLAKEY_DROP_WRITES
          _unmount_flakey
          # Allow writes again and mount. This makes the fs replay its fsync log.
          _load_flakey_table $FLAKEY_ALLOW_WRITES
          _mount_flakey
  }

  rm -f $seqres.full

  _scratch_mkfs >> $seqres.full 2>&1
  _init_flakey
  _mount_flakey

  # Create the test file with some initial data and then fsync it.
  # The fsync here is only needed to trigger the issue in btrfs, as it causes the
  # the flag BTRFS_INODE_NEEDS_FULL_SYNC to be removed from the btrfs inode.
  $XFS_IO_PROG -f -c "pwrite -S 0xaa 0 32k" \
                  -c "fsync" \
                  $SCRATCH_MNT/foo | _filter_xfs_io
  sync

  # Add a hard link to our file.
  # On btrfs this sets the flag BTRFS_INODE_COPY_EVERYTHING on the btrfs inode,
  # which is a necessary condition to trigger the issue.
  ln $SCRATCH_MNT/foo $SCRATCH_MNT/bar

  # Sync the filesystem to force a commit of the current btrfs transaction, this
  # is a necessary condition to trigger the bug on btrfs.
  sync

  # Now append more data to our file, increasing its size, and fsync the file.
  # In btrfs because the inode flag BTRFS_INODE_COPY_EVERYTHING was set and the
  # write path did not update the inode item in the btree nor the delayed inode
  # item (in memory struture) in the current transaction (created by the fsync
  # handler), the fsync did not record the inode's new i_size in the fsync
  # log/journal. This made the data unavailable after the fsync log/journal is
  # replayed.
  $XFS_IO_PROG -c "pwrite -S 0xbb 32K 32K" \
               -c "fsync" \
               $SCRATCH_MNT/foo | _filter_xfs_io

  echo "File content after fsync and before crash:"
  od -t x1 $SCRATCH_MNT/foo

  _crash_and_mount

  echo "File content after crash and log replay:"
  od -t x1 $SCRATCH_MNT/foo

  status=0
  exit

The expected file output before and after the crash/power failure expects the
appended data to be available, which is:

  0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
  *
  0100000 bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb
  *
  0200000

Cc: stable@vger.kernel.org
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/tree-log.c

@@ -4155,6 +4155,7 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 	u64 ino = btrfs_ino(inode);
 	struct extent_map_tree *em_tree = &BTRFS_I(inode)->extent_tree;
 	u64 logged_isize = 0;
+	bool need_log_inode_item = true;
 
 	path = btrfs_alloc_path();
 	if (!path)
@@ -4263,11 +4264,6 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 		} else {
 			if (inode_only == LOG_INODE_ALL)
 				fast_search = true;
-			ret = log_inode_item(trans, log, dst_path, inode);
-			if (ret) {
-				err = ret;
-				goto out_unlock;
-			}
 			goto log_extents;
 		}
 
@@ -4290,6 +4286,9 @@ again:
 		if (min_key.type > max_key.type)
 			break;
 
+		if (min_key.type == BTRFS_INODE_ITEM_KEY)
+			need_log_inode_item = false;
+
 		src = path->nodes[0];
 		if (ins_nr && ins_start_slot + ins_nr == path->slots[0]) {
 			ins_nr++;
@@ -4360,6 +4359,11 @@ next_slot:
 log_extents:
 	btrfs_release_path(path);
 	btrfs_release_path(dst_path);
+	if (need_log_inode_item) {
+		err = log_inode_item(trans, log, dst_path, inode);
+		if (err)
+			goto out_unlock;
+	}
 	if (fast_search) {
 		/*
 		 * Some ordered extents started by fsync might have completed