Btrfs: fix fsync after truncate when no_holes feature is enabled

When we have the no_holes feature enabled, if a we truncate a file to a
smaller size, truncate it again but to a size greater than or equals to
its original size and fsync it, the log tree will not have any information
about the hole covering the range [truncate_1_offset, new_file_size[.
Which means if the fsync log is replayed, the file will remain with the
state it had before both truncate operations.

Without the no_holes feature this does not happen, since when the inode
is logged (full sync flag is set) it will find in the fs/subvol tree a
leaf with a generation matching the current transaction id that has an
explicit extent item representing the hole.

Fix this by adding an explicit extent item representing a hole between
the last extent and the inode's i_size if we are doing a full sync.

The issue is easy to reproduce with the following test case for fstests:

  . ./common/rc
  . ./common/filter
  . ./common/dmflakey

  _need_to_be_root
  _supported_fs generic
  _supported_os Linux
  _require_scratch
  _require_dm_flakey

  # This test was motivated by an issue found in btrfs when the btrfs
  # no-holes feature is enabled (introduced in kernel 3.14). So enable
  # the feature if the fs being tested is btrfs.
  if [ $FSTYP == "btrfs" ]; then
      _require_btrfs_fs_feature "no_holes"
      _require_btrfs_mkfs_feature "no-holes"
      MKFS_OPTIONS="$MKFS_OPTIONS -O no-holes"
  fi

  rm -f $seqres.full

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

  # Create our test files and make sure everything is durably persisted.
  $XFS_IO_PROG -f -c "pwrite -S 0xaa 0 64K"         \
                  -c "pwrite -S 0xbb 64K 61K"       \
                  $SCRATCH_MNT/foo | _filter_xfs_io
  $XFS_IO_PROG -f -c "pwrite -S 0xee 0 64K"         \
                  -c "pwrite -S 0xff 64K 61K"       \
                  $SCRATCH_MNT/bar | _filter_xfs_io
  sync

  # Now truncate our file foo to a smaller size (64Kb) and then truncate
  # it to the size it had before the shrinking truncate (125Kb). Then
  # fsync our file. If a power failure happens after the fsync, we expect
  # our file to have a size of 125Kb, with the first 64Kb of data having
  # the value 0xaa and the second 61Kb of data having the value 0x00.
  $XFS_IO_PROG -c "truncate 64K" \
               -c "truncate 125K" \
               -c "fsync" \
               $SCRATCH_MNT/foo

  # Do something similar to our file bar, but the first truncation sets
  # the file size to 0 and the second truncation expands the size to the
  # double of what it was initially.
  $XFS_IO_PROG -c "truncate 0" \
               -c "truncate 253K" \
               -c "fsync" \
               $SCRATCH_MNT/bar

  _load_flakey_table $FLAKEY_DROP_WRITES
  _unmount_flakey

  # Allow writes again, mount to trigger log replay and validate file
  # contents.
  _load_flakey_table $FLAKEY_ALLOW_WRITES
  _mount_flakey

  # We expect foo to have a size of 125Kb, the first 64Kb of data all
  # having the value 0xaa and the remaining 61Kb to be a hole (all bytes
  # with value 0x00).
  echo "File foo content after log replay:"
  od -t x1 $SCRATCH_MNT/foo

  # We expect bar to have a size of 253Kb and no extents (any byte read
  # from bar has the value 0x00).
  echo "File bar content after log replay:"
  od -t x1 $SCRATCH_MNT/bar

  status=0
  exit

The expected file contents in the golden output are:

  File foo content after log replay:
  0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
  *
  0200000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  *
  0372000
  File bar content after log replay:
  0000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  *
  0772000

Without this fix, their contents are:

  File foo content after log replay:
  0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
  *
  0200000 bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb
  *
  0372000
  File bar content after log replay:
  0000000 ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee
  *
  0200000 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
  *
  0372000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  *
  0772000

A test case submission for fstests follows soon.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 108 insertions, 0 deletions

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 66f87156882f..9c45431e69ab 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -4197,6 +4197,107 @@ static int btrfs_log_all_xattrs(struct btrfs_trans_handle *trans,
         return 0;
 }
 
+/*
+ * If the no holes feature is enabled we need to make sure any hole between the
+ * last extent and the i_size of our inode is explicitly marked in the log. This
+ * is to make sure that doing something like:
+ *
+ *      1) create file with 128Kb of data
+ *      2) truncate file to 64Kb
+ *      3) truncate file to 256Kb
+ *      4) fsync file
+ *      5) <crash/power failure>
+ *      6) mount fs and trigger log replay
+ *
+ * Will give us a file with a size of 256Kb, the first 64Kb of data match what
+ * the file had in its first 64Kb of data at step 1 and the last 192Kb of the
+ * file correspond to a hole. The presence of explicit holes in a log tree is
+ * what guarantees that log replay will remove/adjust file extent items in the
+ * fs/subvol tree.
+ *
+ * Here we do not need to care about holes between extents, that is already done
+ * by copy_items(). We also only need to do this in the full sync path, where we
+ * lookup for extents from the fs/subvol tree only. In the fast path case, we
+ * lookup the list of modified extent maps and if any represents a hole, we
+ * insert a corresponding extent representing a hole in the log tree.
+ */
+static int btrfs_log_trailing_hole(struct btrfs_trans_handle *trans,
+                                   struct btrfs_root *root,
+                                   struct inode *inode,
+                                   struct btrfs_path *path)
+{
+        int ret;
+        struct btrfs_key key;
+        u64 hole_start;
+        u64 hole_size;
+        struct extent_buffer *leaf;
+        struct btrfs_root *log = root->log_root;
+        const u64 ino = btrfs_ino(inode);
+        const u64 i_size = i_size_read(inode);
+
+        if (!btrfs_fs_incompat(root->fs_info, NO_HOLES))
+                return 0;
+
+        key.objectid = ino;
+        key.type = BTRFS_EXTENT_DATA_KEY;
+        key.offset = (u64)-1;
+
+        ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+        ASSERT(ret != 0);
+        if (ret < 0)
+                return ret;
+
+        ASSERT(path->slots[0] > 0);
+        path->slots[0]--;
+        leaf = path->nodes[0];
+        btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
+
+        if (key.objectid != ino || key.type != BTRFS_EXTENT_DATA_KEY) {
+                /* inode does not have any extents */
+                hole_start = 0;
+                hole_size = i_size;
+        } else {
+                struct btrfs_file_extent_item *extent;
+                u64 len;
+
+                /*
+                 * If there's an extent beyond i_size, an explicit hole was
+                 * already inserted by copy_items().
+                 */
+                if (key.offset >= i_size)
+                        return 0;
+
+                extent = btrfs_item_ptr(leaf, path->slots[0],
+                                        struct btrfs_file_extent_item);
+
+                if (btrfs_file_extent_type(leaf, extent) ==
+                    BTRFS_FILE_EXTENT_INLINE) {
+                        len = btrfs_file_extent_inline_len(leaf,
+                                                           path->slots[0],
+                                                           extent);
+                        ASSERT(len == i_size);
+                        return 0;
+                }
+
+                len = btrfs_file_extent_num_bytes(leaf, extent);
+                /* Last extent goes beyond i_size, no need to log a hole. */
+                if (key.offset + len > i_size)
+                        return 0;
+                hole_start = key.offset + len;
+                hole_size = i_size - hole_start;
+        }
+        btrfs_release_path(path);
+
+        /* Last extent ends at i_size. */
+        if (hole_size == 0)
+                return 0;
+
+        hole_size = ALIGN(hole_size, root->sectorsize);
+        ret = btrfs_insert_file_extent(trans, log, ino, hole_start, 0, 0,
+                                       hole_size, 0, hole_size, 0, 0, 0);
+        return ret;
+}
+
 /* 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.
@@ -4460,6 +4561,13 @@ next_slot:
         err = btrfs_log_all_xattrs(trans, root, inode, path, dst_path);
         if (err)
                 goto out_unlock;
+        if (max_key.type >= BTRFS_EXTENT_DATA_KEY && !fast_search) {
+                btrfs_release_path(path);
+                btrfs_release_path(dst_path);
+                err = btrfs_log_trailing_hole(trans, root, inode, path);
+                if (err)
+                        goto out_unlock;
+        }
 log_extents:
         btrfs_release_path(path);
         btrfs_release_path(dst_path);