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>

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c0c0095b2b60397a1f17038e8f7c0542d9ed7c16..64dc2e2c281f19421e6aaf51d566c50200eddda6 100644 (file)
--- a/include/linux/fs.h
+++ b/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;