fs: move inode fields used during fast path lookup closer together

This should avoid *some* cache misses.

Successful path lookup is guaranteed to load at least ->i_mode,
->i_opflags and ->i_acl. At the same time the common case will avoid
looking at more fields.

struct inode is not guaranteed to have any particular alignment, notably
ext4 has it only aligned to 8 bytes meaning nearby fields might happen
to be on the same or only adjacent cache lines depending on luck (or no
luck).

According to pahole:
        umode_t                    i_mode;               /*     0     2 */
        short unsigned int         i_opflags;            /*     2     2 */
        kuid_t                     i_uid;                /*     4     4 */
        kgid_t                     i_gid;                /*     8     4 */
        unsigned int               i_flags;              /*    12     4 */
        struct posix_acl *         i_acl;                /*    16     8 */
        struct posix_acl *         i_default_acl;        /*    24     8 */

->i_acl is unnecessarily separated by 8 bytes from the other fields.
With struct inode being offset 48 bytes into the cacheline this means an
avoidable miss. Note it will still be there for the 56 byte case.

New layout:
        umode_t                    i_mode;               /*     0     2 */
        short unsigned int         i_opflags;            /*     2     2 */
        unsigned int               i_flags;              /*     4     4 */
        struct posix_acl *         i_acl;                /*     8     8 */
        struct posix_acl *         i_default_acl;        /*    16     8 */
        kuid_t                     i_uid;                /*    24     4 */
        kgid_t                     i_gid;                /*    28     4 */

I verified with pahole there are no size or hole changes.

This is stopgap until someone(tm) sanitizes the layout in the first
place, allocation methods aside.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251109121931.1285366-1-mjguzik@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mjguzik

include/linux/fs.h

@@ -781,14 +781,13 @@ enum inode_state_flags_t {
 struct inode {
 	umode_t			i_mode;
 	unsigned short		i_opflags;
-	kuid_t			i_uid;
-	kgid_t			i_gid;
 	unsigned int		i_flags;
-
 #ifdef CONFIG_FS_POSIX_ACL
 	struct posix_acl	*i_acl;
 	struct posix_acl	*i_default_acl;
 #endif
+	kuid_t			i_uid;
+	kgid_t			i_gid;
 
 	const struct inode_operations	*i_op;
 	struct super_block	*i_sb;