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

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>

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 6f21786..7d464b0 100644 (file)
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -4242,6 +4242,9 @@ 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;
@@ -4256,9 +4259,43 @@ 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) {
@@ -4296,7 +4333,7 @@ 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)