[PATCH 4.14 07/26] btrfs: fix partial loss of prealloc extent past i_size after fsync

Greg Kroah-Hartman
2020-05-04T18:14:17Z

From: Filipe Manana <fdmanana@suse.com>

commit f135cea30de5f74d5bfb5116682073841fb4af8f upstream.

When we have an inode with a prealloc extent that starts at an offset
lower than the i_size and there is another prealloc extent that starts at
an offset beyond i_size, we can end up losing part of the first prealloc
extent (the part that starts at i_size) and have an implicit hole if we
fsync the file and then have a power failure.

Consider the following example with comments explaining how and why it
happens.

$ mkfs.btrfs -f /dev/sdb
$ mount /dev/sdb /mnt

# Create our test file with 2 consecutive prealloc extents, each with a
# size of 128Kb, and covering the range from 0 to 256Kb, with a file
# size of 0.
$ xfs_io -f -c "falloc -k 0 128K" /mnt/foo
$ xfs_io -c "falloc -k 128K 128K" /mnt/foo

# Fsync the file to record both extents in the log tree.
$ xfs_io -c "fsync" /mnt/foo

# Now do a redudant extent allocation for the range from 0 to 64Kb.
# This will merely increase the file size from 0 to 64Kb. Instead we
# could also do a truncate to set the file size to 64Kb.
$ xfs_io -c "falloc 0 64K" /mnt/foo

# Fsync the file, so we update the inode item in the log tree with the
# new file size (64Kb). This also ends up setting the number of bytes
# for the first prealloc extent to 64Kb. This is done by the truncation
# at btrfs_log_prealloc_extents().
# This means that if a power failure happens after this, a write into
# the file range 64Kb to 128Kb will not use the prealloc extent and
# will result in allocation of a new extent.
$ xfs_io -c "fsync" /mnt/foo

# Now set the file size to 256K with a truncate and then fsync the file.
# Since no changes happened to the extents, the fsync only updates the
# i_size in the inode item at the log tree. This results in an implicit
# hole for the file range from 64Kb to 128Kb, something which fsck will
# complain when not using the NO_HOLES feature if we replay the log
# after a power failure.
$ xfs_io -c "truncate 256K" -c "fsync" /mnt/foo

So instead of always truncating the log to the inode's current i_size at
btrfs_log_prealloc_extents(), check first if there's a prealloc extent
that starts at an offset lower than the i_size and with a length that
crosses the i_size - if there is one, just make sure we truncate to a
size that corresponds to the end offset of that prealloc extent, so
that we don't lose the part of that extent that starts at i_size if a
power failure happens.

A test case for fstests follows soon.

Fixes: 31d11b83b96f ("Btrfs: fix duplicate extents after fsync of file with prealloc extents")
CC: stable@vger.kernel.org # 4.14+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/tree-log.c | 43 ++++++++++++++++++++++++++++++++++++++++---
1 file changed, 40 insertions(+), 3 deletions(-)

--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -4155,6 +4155,9 @@ static int btrfs_log_prealloc_extents(st
const u64 ino = btrfs_ino(inode);
struct btrfs_path *dst_path = NULL;
bool dropped_extents = false;
+ u64 truncate_offset = i_size;
+ struct extent_buffer *leaf;
+ int slot;
int ins_nr = 0;
int start_slot;
int ret;
@@ -4169,9 +4172,43 @@ static int btrfs_log_prealloc_extents(st
if (ret < 0)
goto out;

+ /*
+ * We must check if there is a prealloc extent that starts before the
+ * i_size and crosses the i_size boundary. This is to ensure later we
+ * truncate down to the end of that extent and not to the i_size, as
+ * otherwise we end up losing part of the prealloc extent after a log
+ * replay and with an implicit hole if there is another prealloc extent
+ * that starts at an offset beyond i_size.
+ */
+ ret = btrfs_previous_item(root, path, ino, BTRFS_EXTENT_DATA_KEY);
+ if (ret < 0)
+ goto out;
+
+ if (ret == 0) {
+ struct btrfs_file_extent_item *ei;
+
+ leaf = path->nodes[0];
+ slot = path->slots[0];
+ ei = btrfs_item_ptr(leaf, slot, struct btrfs_file_extent_item);
+
+ if (btrfs_file_extent_type(leaf, ei) ==
+ BTRFS_FILE_EXTENT_PREALLOC) {
+ u64 extent_end;
+
+ btrfs_item_key_to_cpu(leaf, &key, slot);
+ extent_end = key.offset +
+ btrfs_file_extent_num_bytes(leaf, ei);
+
+ if (extent_end > i_size)
+ truncate_offset = extent_end;
+ }
+ } else {
+ ret = 0;
+ }
+
while (true) {
- struct extent_buffer *leaf = path->nodes[0];
- int slot = path->slots[0];
+ leaf = path->nodes[0];
+ slot = path->slots[0];

if (slot >= btrfs_header_nritems(leaf)) {
if (ins_nr > 0) {
@@ -4209,7 +4246,7 @@ static int btrfs_log_prealloc_extents(st
ret = btrfs_truncate_inode_items(trans,
root->log_root,
&inode->vfs_inode,
- i_size,
+ truncate_offset,
BTRFS_EXTENT_DATA_KEY);
} while (ret == -EAGAIN);
if (ret)