Btrfs: fix fsync data loss after adding hard link to inode

We have a scenario where after the fsync log replay we can lose file data
that had been previously fsync'ed if we added an hard link for our inode
and after that we sync'ed the fsync log (for example by fsync'ing some
other file or directory).

This is because when adding an hard link we updated the inode item in the
log tree with an i_size value of 0. At that point the new inode item was
in memory only and a subsequent fsync log replay would not make us lose
the file data. However if after adding the hard link we sync the log tree
to disk, by fsync'ing some other file or directory for example, we ended
up losing the file data after log replay, because the inode item in the
persisted log tree had an an i_size of zero.

This is easy to reproduce, and the following excerpt from my test for
xfstests shows this:

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

  # Create one file with data and fsync it.
  # This made the btrfs fsync log persist the data and the inode metadata with
  # a correct inode->i_size (4096 bytes).
  $XFS_IO_PROG -f -c "pwrite -S 0xaa -b 4K 0 4K" -c "fsync" \
       $SCRATCH_MNT/foo | _filter_xfs_io

  # Now add one hard link to our file. This made the btrfs code update the fsync
  # log, in memory only, with an inode metadata having a size of 0.
  ln $SCRATCH_MNT/foo $SCRATCH_MNT/foo_link

  # Now force persistence of the fsync log to disk, for example, by fsyncing some
  # other file.
  touch $SCRATCH_MNT/bar
  $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/bar

  # Before a power loss or crash, we could read the 4Kb of data from our file as
  # expected.
  echo "File content before:"
  od -t x1 $SCRATCH_MNT/foo

  # Simulate a crash/power loss.
  _load_flakey_table $FLAKEY_DROP_WRITES
  _unmount_flakey

  _load_flakey_table $FLAKEY_ALLOW_WRITES
  _mount_flakey

  # After the fsync log replay, because the fsync log had a value of 0 for our
  # inode's i_size, we couldn't read anymore the 4Kb of data that we previously
  # wrote and fsync'ed. The size of the file became 0 after the fsync log replay.
  echo "File content after:"
  od -t x1 $SCRATCH_MNT/foo

Another alternative test, that doesn't need to fsync an inode in the same
transaction it was created, is:

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

  # Create our test file with some data.
  $XFS_IO_PROG -f -c "pwrite -S 0xaa -b 8K 0 8K" \
       $SCRATCH_MNT/foo | _filter_xfs_io

  # Make sure the file is durably persisted.
  sync

  # Append some data to our file, to increase its size.
  $XFS_IO_PROG -f -c "pwrite -S 0xcc -b 4K 8K 4K" \
       $SCRATCH_MNT/foo | _filter_xfs_io

  # Fsync the file, so from this point on if a crash/power failure happens, our
  # new data is guaranteed to be there next time the fs is mounted.
  $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/foo

  # Add one hard link to our file. This made btrfs write into the in memory fsync
  # log a special inode with generation 0 and an i_size of 0 too. Note that this
  # didn't update the inode in the fsync log on disk.
  ln $SCRATCH_MNT/foo $SCRATCH_MNT/foo_link

  # Now make sure the in memory fsync log is durably persisted.
  # Creating and fsync'ing another file will do it.
  touch $SCRATCH_MNT/bar
  $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/bar

  # As expected, before the crash/power failure, we should be able to read the
  # 12Kb of file data.
  echo "File content before:"
  od -t x1 $SCRATCH_MNT/foo

  # Simulate a crash/power loss.
  _load_flakey_table $FLAKEY_DROP_WRITES
  _unmount_flakey

  _load_flakey_table $FLAKEY_ALLOW_WRITES
  _mount_flakey

  # After mounting the fs again, the fsync log was replayed.
  # The btrfs fsync log replay code didn't update the i_size of the persisted
  # inode because the inode item in the log had a special generation with a
  # value of 0 (and it couldn't know the correct i_size, since that inode item
  # had a 0 i_size too). This made the last 4Kb of file data inaccessible and
  # effectively lost.
  echo "File content after:"
  od -t x1 $SCRATCH_MNT/foo

This isn't a new issue/regression. This problem has been around since the
log tree code was added in 2008:

  Btrfs: Add a write ahead tree log to optimize synchronous operations
  (commit e02119d5a7b4396c5a872582fddc8bd6d305a70a)

Test cases for xfstests follow soon.

CC: <stable@vger.kernel.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/tree-log.c

@@ -490,8 +490,20 @@ insert:
 		src_item = (struct btrfs_inode_item *)src_ptr;
 		dst_item = (struct btrfs_inode_item *)dst_ptr;
 
-		if (btrfs_inode_generation(eb, src_item) == 0)
+		if (btrfs_inode_generation(eb, src_item) == 0) {
+			struct extent_buffer *dst_eb = path->nodes[0];
+
+			if (S_ISREG(btrfs_inode_mode(eb, src_item)) &&
+			    S_ISREG(btrfs_inode_mode(dst_eb, dst_item))) {
+				struct btrfs_map_token token;
+				u64 ino_size = btrfs_inode_size(eb, src_item);
+
+				btrfs_init_map_token(&token);
+				btrfs_set_token_inode_size(dst_eb, dst_item,
+							   ino_size, &token);
+			}
 			goto no_copy;
+		}
 
 		if (overwrite_root &&
 		    S_ISDIR(btrfs_inode_mode(eb, src_item)) &&
@@ -3250,7 +3262,8 @@ static int drop_objectid_items(struct btrfs_trans_handle *trans,
 static void fill_inode_item(struct btrfs_trans_handle *trans,
 			    struct extent_buffer *leaf,
 			    struct btrfs_inode_item *item,
-			    struct inode *inode, int log_inode_only)
+			    struct inode *inode, int log_inode_only,
+			    u64 logged_isize)
 {
 	struct btrfs_map_token token;
 
@@ -3263,7 +3276,7 @@ static void fill_inode_item(struct btrfs_trans_handle *trans,
 		 * to say 'update this inode with these values'
 		 */
 		btrfs_set_token_inode_generation(leaf, item, 0, &token);
-		btrfs_set_token_inode_size(leaf, item, 0, &token);
+		btrfs_set_token_inode_size(leaf, item, logged_isize, &token);
 	} else {
 		btrfs_set_token_inode_generation(leaf, item,
 						 BTRFS_I(inode)->generation,
@@ -3315,7 +3328,7 @@ static int log_inode_item(struct btrfs_trans_handle *trans,
 		return ret;
 	inode_item = btrfs_item_ptr(path->nodes[0], path->slots[0],
 				    struct btrfs_inode_item);
-	fill_inode_item(trans, path->nodes[0], inode_item, inode, 0);
+	fill_inode_item(trans, path->nodes[0], inode_item, inode, 0, 0);
 	btrfs_release_path(path);
 	return 0;
 }
@@ -3324,7 +3337,8 @@ static noinline int copy_items(struct btrfs_trans_handle *trans,
 			       struct inode *inode,
 			       struct btrfs_path *dst_path,
 			       struct btrfs_path *src_path, u64 *last_extent,
-			       int start_slot, int nr, int inode_only)
+			       int start_slot, int nr, int inode_only,
+			       u64 logged_isize)
 {
 	unsigned long src_offset;
 	unsigned long dst_offset;
@@ -3381,7 +3395,8 @@ static noinline int copy_items(struct btrfs_trans_handle *trans,
 						    dst_path->slots[0],
 						    struct btrfs_inode_item);
 			fill_inode_item(trans, dst_path->nodes[0], inode_item,
-					inode, inode_only == LOG_INODE_EXISTS);
+					inode, inode_only == LOG_INODE_EXISTS,
+					logged_isize);
 		} else {
 			copy_extent_buffer(dst_path->nodes[0], src, dst_offset,
 					   src_offset, ins_sizes[i]);
@@ -3933,6 +3948,33 @@ process:
 	return ret;
 }
 
+static int logged_inode_size(struct btrfs_root *log, struct inode *inode,
+			     struct btrfs_path *path, u64 *size_ret)
+{
+	struct btrfs_key key;
+	int ret;
+
+	key.objectid = btrfs_ino(inode);
+	key.type = BTRFS_INODE_ITEM_KEY;
+	key.offset = 0;
+
+	ret = btrfs_search_slot(NULL, log, &key, path, 0, 0);
+	if (ret < 0) {
+		return ret;
+	} else if (ret > 0) {
+		*size_ret = i_size_read(inode);
+	} else {
+		struct btrfs_inode_item *item;
+
+		item = btrfs_item_ptr(path->nodes[0], path->slots[0],
+				      struct btrfs_inode_item);
+		*size_ret = btrfs_inode_size(path->nodes[0], item);
+	}
+
+	btrfs_release_path(path);
+	return 0;
+}
+
 /* log a single inode in the tree log.
  * At least one parent directory for this inode must exist in the tree
  * or be logged already.
@@ -3970,6 +4012,7 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 	bool fast_search = false;
 	u64 ino = btrfs_ino(inode);
 	struct extent_map_tree *em_tree = &BTRFS_I(inode)->extent_tree;
+	u64 logged_isize = 0;
 
 	path = btrfs_alloc_path();
 	if (!path)
@@ -4030,6 +4073,25 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 			max_key_type = BTRFS_XATTR_ITEM_KEY;
 		ret = drop_objectid_items(trans, log, path, ino, max_key_type);
 	} else {
+		if (inode_only == LOG_INODE_EXISTS) {
+			/*
+			 * Make sure the new inode item we write to the log has
+			 * the same isize as the current one (if it exists).
+			 * This is necessary to prevent data loss after log
+			 * replay, and also to prevent doing a wrong expanding
+			 * truncate - for e.g. create file, write 4K into offset
+			 * 0, fsync, write 4K into offset 4096, add hard link,
+			 * fsync some other file (to sync log), power fail - if
+			 * we use the inode's current i_size, after log replay
+			 * we get a 8Kb file, with the last 4Kb extent as a hole
+			 * (zeroes), as if an expanding truncate happened,
+			 * instead of getting a file of 4Kb only.
+			 */
+			err = logged_inode_size(log, inode, path,
+						&logged_isize);
+			if (err)
+				goto out_unlock;
+		}
 		if (test_and_clear_bit(BTRFS_INODE_NEEDS_FULL_SYNC,
 				       &BTRFS_I(inode)->runtime_flags)) {
 			clear_bit(BTRFS_INODE_COPY_EVERYTHING,
@@ -4085,7 +4147,8 @@ again:
 		}
 
 		ret = copy_items(trans, inode, dst_path, path, &last_extent,
-				 ins_start_slot, ins_nr, inode_only);
+				 ins_start_slot, ins_nr, inode_only,
+				 logged_isize);
 		if (ret < 0) {
 			err = ret;
 			goto out_unlock;
@@ -4109,7 +4172,7 @@ next_slot:
 		if (ins_nr) {
 			ret = copy_items(trans, inode, dst_path, path,
 					 &last_extent, ins_start_slot,
-					 ins_nr, inode_only);
+					 ins_nr, inode_only, logged_isize);
 			if (ret < 0) {
 				err = ret;
 				goto out_unlock;
@@ -4130,7 +4193,8 @@ next_slot:
 	}
 	if (ins_nr) {
 		ret = copy_items(trans, inode, dst_path, path, &last_extent,
-				 ins_start_slot, ins_nr, inode_only);
+				 ins_start_slot, ins_nr, inode_only,
+				 logged_isize);
 		if (ret < 0) {
 			err = ret;
 			goto out_unlock;