btrfs: reuse cloned extent buffer during fiemap to avoid re-allocations

During fiemap we may have to visit multiple leaves of the subvolume's
inode tree, and each time we are freeing and allocating an extent buffer
to use as a clone of each visited leaf. Optimize this by reusing cloned
extent buffers, to avoid the freeing and re-allocation both of the extent
buffer structure itself and more importantly of the pages attached to the
extent buffer.

Reviewed-by: Josef Bacik <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Signed-off-by: David Sterba <[email protected]>

fs/btrfs/extent_io.c

@@ -2752,7 +2752,7 @@ static int emit_last_fiemap_cache(struct fiemap_extent_info *fieinfo,
 
 static int fiemap_next_leaf_item(struct btrfs_inode *inode, struct btrfs_path *path)
 {
- struct extent_buffer *clone;
+ struct extent_buffer *clone = path->nodes[0];
  struct btrfs_key key;
  int slot;
  int ret;
@@ -2761,29 +2761,45 @@ static int fiemap_next_leaf_item(struct btrfs_inode *inode, struct btrfs_path *p
  if (path->slots[0] < btrfs_header_nritems(path->nodes[0]))
  return 0;
 
+ /*
+ * Add a temporary extra ref to an already cloned extent buffer to
+ * prevent btrfs_next_leaf() freeing it, we want to reuse it to avoid
+ * the cost of allocating a new one.
+ */
+ ASSERT(test_bit(EXTENT_BUFFER_UNMAPPED, &clone->bflags));
+ atomic_inc(&clone->refs);
+
  ret = btrfs_next_leaf(inode->root, path);
  if (ret != 0)
- return ret;
+ goto out;
 
  /*
  * Don't bother with cloning if there are no more file extent items for
  * our inode.
  */
  btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
- if (key.objectid != btrfs_ino(inode) || key.type != BTRFS_EXTENT_DATA_KEY)
- return 1;
+ if (key.objectid != btrfs_ino(inode) || key.type != BTRFS_EXTENT_DATA_KEY) {
+ ret = 1;
+ goto out;
+ }
 
  /* See the comment at fiemap_search_slot() about why we clone. */
- clone = btrfs_clone_extent_buffer(path->nodes[0]);
- if (!clone)
- return -ENOMEM;
+ copy_extent_buffer_full(clone, path->nodes[0]);
+ /*
+ * Important to preserve the start field, for the optimizations when
+ * checking if extents are shared (see extent_fiemap()).
+ */
+ clone->start = path->nodes[0]->start;
 
  slot = path->slots[0];
  btrfs_release_path(path);
  path->nodes[0] = clone;
  path->slots[0] = slot;
+out:
+ if (ret)
+ free_extent_buffer(clone);
 
- return 0;
+ return ret;
 }
 
 /*