Btrfs: fix stale directory entries after fsync log replay

We have another case where after an fsync log replay we get an inode with
a wrong link count (smaller than it should be) and a number of directory
entries greater than its link count. This happens when we add a new link
hard link to our inode A and then we fsync some other inode B that has
the side effect of logging the parent directory inode too. In this case
at log replay time we add the new hard link to our inode (the item with
key BTRFS_INODE_REF_KEY) when processing the parent directory but we
never adjust the link count of our inode A. As a result we get stale dir
entries for our inode A that can never be deleted and therefore it makes
it impossible to remove the parent directory (as its i_size can never
decrease back to 0).

A simple reproducer for fstests that triggers this issue:

  seq=`basename $0`
  seqres=$RESULT_DIR/$seq
  echo "QA output created by $seq"
  tmp=/tmp/$$
  status=1	# failure is the default!
  trap "_cleanup; exit \$status" 0 1 2 3 15

  _cleanup()
  {
      _cleanup_flakey
      rm -f $tmp.*
  }

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

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

  rm -f $seqres.full

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

  # Create our test directory and files.
  mkdir $SCRATCH_MNT/testdir
  touch $SCRATCH_MNT/testdir/foo
  touch $SCRATCH_MNT/testdir/bar

  # Make sure everything done so far is durably persisted.
  sync

  # Create one hard link for file foo and another one for file bar. After
  # that fsync only the file bar.
  ln $SCRATCH_MNT/testdir/bar $SCRATCH_MNT/testdir/bar_link
  ln $SCRATCH_MNT/testdir/foo $SCRATCH_MNT/testdir/foo_link
  $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/testdir/bar

  # Silently drop all writes on scratch device to simulate power failure.
  _load_flakey_table $FLAKEY_DROP_WRITES
  _unmount_flakey

  # Allow writes again and mount the fs to trigger log/journal replay.
  _load_flakey_table $FLAKEY_ALLOW_WRITES
  _mount_flakey

  # Now verify both our files have a link count of 2.
  echo "Link count for file foo: $(stat --format=%h $SCRATCH_MNT/testdir/foo)"
  echo "Link count for file bar: $(stat --format=%h $SCRATCH_MNT/testdir/bar)"

  # We should be able to remove all the links of our files in testdir, and
  # after that the parent directory should become empty and therefore
  # possible to remove it.
  rm -f $SCRATCH_MNT/testdir/*
  rmdir $SCRATCH_MNT/testdir

  _unmount_flakey

  # The fstests framework will call fsck against our filesystem which will verify
  # that all metadata is in a consistent state.

  status=0
  exit

The test fails with:

 -Link count for file foo: 2
 +Link count for file foo: 1
  Link count for file bar: 2
 +rm: cannot remove '/home/fdmanana/btrfs-tests/scratch_1/testdir/foo_link': Stale file handle
 +rmdir: failed to remove '/home/fdmanana/btrfs-tests/scratch_1/testdir': Directory not empty
 (...)
 _check_btrfs_filesystem: filesystem on /dev/sdc is inconsistent

And fsck's output:

  (...)
  checking fs roots
  root 5 inode 258 errors 2001, no inode item, link count wrong
      unresolved ref dir 257 index 5 namelen 8 name foo_link filetype 1 errors 4, no inode ref
  Checking filesystem on /dev/sdc
  (...)

So fix this by marking inodes for link count fixup at log replay time
whenever a directory entry is replayed if the entry was created in the
transaction where the fsync was made and if it points to a non-directory
inode.

This isn't a new problem/regression, the issue exists for a long time,
possibly since the log tree feature was added (2008).

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/tree-log.c

@@ -1613,6 +1613,9 @@ static bool name_in_log_ref(struct btrfs_root *log_root,
  * not exist in the FS, it is skipped.  fsyncs on directories
  * do not force down inodes inside that directory, just changes to the
  * names or unlinks in a directory.
+ *
+ * Returns < 0 on error, 0 if the name wasn't replayed (dentry points to a
+ * non-existing inode) and 1 if the name was replayed.
  */
 static noinline int replay_one_name(struct btrfs_trans_handle *trans,
 				    struct btrfs_root *root,
@@ -1631,6 +1634,7 @@ static noinline int replay_one_name(struct btrfs_trans_handle *trans,
 	int exists;
 	int ret = 0;
 	bool update_size = (key->type == BTRFS_DIR_INDEX_KEY);
+	bool name_added = false;
 
 	dir = read_one_inode(root, key->objectid);
 	if (!dir)
@@ -1708,6 +1712,8 @@ out:
 	}
 	kfree(name);
 	iput(dir);
+	if (!ret && name_added)
+		ret = 1;
 	return ret;
 
 insert:
@@ -1723,6 +1729,8 @@ insert:
 			      name, name_len, log_type, &log_key);
 	if (ret && ret != -ENOENT && ret != -EEXIST)
 		goto out;
+	if (!ret)
+		name_added = true;
 	update_size = false;
 	ret = 0;
 	goto out;
@@ -1740,12 +1748,13 @@ static noinline int replay_one_dir_item(struct btrfs_trans_handle *trans,
 					struct extent_buffer *eb, int slot,
 					struct btrfs_key *key)
 {
-	int ret;
+	int ret = 0;
 	u32 item_size = btrfs_item_size_nr(eb, slot);
 	struct btrfs_dir_item *di;
 	int name_len;
 	unsigned long ptr;
 	unsigned long ptr_end;
+	struct btrfs_path *fixup_path = NULL;
 
 	ptr = btrfs_item_ptr_offset(eb, slot);
 	ptr_end = ptr + item_size;
@@ -1755,12 +1764,59 @@ static noinline int replay_one_dir_item(struct btrfs_trans_handle *trans,
 			return -EIO;
 		name_len = btrfs_dir_name_len(eb, di);
 		ret = replay_one_name(trans, root, path, eb, di, key);
-		if (ret)
-			return ret;
+		if (ret < 0)
+			break;
 		ptr = (unsigned long)(di + 1);
 		ptr += name_len;
+
+		/*
+		 * If this entry refers to a non-directory (directories can not
+		 * have a link count > 1) and it was added in the transaction
+		 * that was not committed, make sure we fixup the link count of
+		 * the inode it the entry points to. Otherwise something like
+		 * the following would result in a directory pointing to an
+		 * inode with a wrong link that does not account for this dir
+		 * entry:
+		 *
+		 * mkdir testdir
+		 * touch testdir/foo
+		 * touch testdir/bar
+		 * sync
+		 *
+		 * ln testdir/bar testdir/bar_link
+		 * ln testdir/foo testdir/foo_link
+		 * xfs_io -c "fsync" testdir/bar
+		 *
+		 * <power failure>
+		 *
+		 * mount fs, log replay happens
+		 *
+		 * File foo would remain with a link count of 1 when it has two
+		 * entries pointing to it in the directory testdir. This would
+		 * make it impossible to ever delete the parent directory has
+		 * it would result in stale dentries that can never be deleted.
+		 */
+		if (ret == 1 && btrfs_dir_type(eb, di) != BTRFS_FT_DIR) {
+			struct btrfs_key di_key;
+
+			if (!fixup_path) {
+				fixup_path = btrfs_alloc_path();
+				if (!fixup_path) {
+					ret = -ENOMEM;
+					break;
+				}
+			}
+
+			btrfs_dir_item_key_to_cpu(eb, di, &di_key);
+			ret = link_to_fixup_dir(trans, root, fixup_path,
+						di_key.objectid);
+			if (ret)
+				break;
+		}
+		ret = 0;
 	}
-	return 0;
+	btrfs_free_path(fixup_path);
+	return ret;
 }
 
 /*