[PATCH] btrfs: reorder struct btrfs_key for better alignment

[David Sterba <dsterba@suse.com>]

We don't use the plain key for any on-disk operations so there's no
requirement for the member order. As the offset is a u64 that should be
on an 8byte aligned address, this can generate ineffective code on
strict alignment architectures and can potentially hurt even on others
(cross-cacheline access).

The resulting asm code on x86_64 only differes in the offset, no significant
change in size of the object size.

The alignment of the structure is unchanged.

Signed-off-by: David Sterba <dsterba@suse.com>
---
 include/uapi/linux/btrfs_tree.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
index aff1356c2bb8..9ca7adcf3b7f 100644
--- a/include/uapi/linux/btrfs_tree.h
+++ b/include/uapi/linux/btrfs_tree.h
@@ -342,10 +342,17 @@ struct btrfs_disk_key {
 	__le64 offset;
 } __attribute__ ((__packed__));
 
+/*
+ * NOTE: this structure does not match the on-disk format of key and must be
+ * converted with the right helpers. The btrfs_key is for in-memory use and the
+ * members are reordered for better alignment. It's still packed as it's never
+ * used in arrays and the extra alignment would consume stack space in
+ * functions.
+ */
 struct btrfs_key {
 	__u64 objectid;
-	__u8 type;
 	__u64 offset;
+	__u8 type;
 } __attribute__ ((__packed__));
 
 struct btrfs_dev_item {
-- 
2.21.0