btrfs: skip clearing EXTENT_DEFRAG for NOCOW ordered extents

In btrfs_finish_one_ordered(), clear_bits is unconditionally initialized
with EXTENT_DEFRAG.  For NOCOW ordered extents this is always a no-op
because should_nocow() already forces the COW path when EXTENT_DEFRAG is
set, so a NOCOW ordered extent can never have EXTENT_DEFRAG on its range.

Although harmless, the unconditional btrfs_clear_extent_bit() call still
performs a cold rbtree lookup under the io tree spinlock on every NOCOW
write completion.  Avoid this by only adding EXTENT_DEFRAG to clear_bits
for non-NOCOW ordered extents, and skip the call entirely when there are
no bits to clear.

Signed-off-by: Dave Chen <davechen@synology.com>
Signed-off-by: Robbie Ko <robbieko@synology.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fcmaple

fs/btrfs/inode.c

@@ -3197,7 +3197,7 @@ int btrfs_finish_one_ordered(struct btrfs_ordered_extent *ordered_extent)
 	bool freespace_inode;
 	bool truncated = false;
 	bool clear_reserved_extent = true;
-	unsigned int clear_bits = EXTENT_DEFRAG;
+	unsigned int clear_bits = 0;
 
 	start = ordered_extent->file_offset;
 	end = start + ordered_extent->num_bytes - 1;
@@ -3208,6 +3208,9 @@ int btrfs_finish_one_ordered(struct btrfs_ordered_extent *ordered_extent)
 	    !test_bit(BTRFS_ORDERED_ENCODED, &ordered_extent->flags))
 		clear_bits |= EXTENT_DELALLOC_NEW;
 
+	if (!test_bit(BTRFS_ORDERED_NOCOW, &ordered_extent->flags))
+		clear_bits |= EXTENT_DEFRAG;
+
 	freespace_inode = btrfs_is_free_space_inode(inode);
 	if (!freespace_inode)
 		btrfs_lockdep_acquire(fs_info, btrfs_ordered_extent);
@@ -3339,8 +3342,9 @@ int btrfs_finish_one_ordered(struct btrfs_ordered_extent *ordered_extent)
 		goto out;
 	}
 out:
-	btrfs_clear_extent_bit(&inode->io_tree, start, end, clear_bits,
-			       &cached_state);
+	if (clear_bits)
+		btrfs_clear_extent_bit(&inode->io_tree, start, end, clear_bits,
+				       &cached_state);
 
 	if (trans)
 		btrfs_end_transaction(trans);