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btrfs: fix partial loss of prealloc extent past i_size after fsync

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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: 31d11b83b9 ("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>
This commit is contained in:
Filipe Manana 2020-04-23 16:30:53 +01:00 committed by David Sterba
parent 1402d17dfd
commit f135cea30d
1 changed files with 40 additions and 3 deletions
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43
fs/btrfs/tree-log.c
View File

@ -4226,6 +4226,9 @@ static int btrfs_log_prealloc_extents(struct btrfs_trans_handle *trans,
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;
@ -4240,9 +4243,43 @@ static int btrfs_log_prealloc_extents(struct btrfs_trans_handle *trans,
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) {
@ -4280,7 +4317,7 @@ static int btrfs_log_prealloc_extents(struct btrfs_trans_handle *trans,
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)