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@xxxxxxxx>
---
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
