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

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

[Mateusz Guzik]

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>
---

> 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.

While this is readily apparent with my patch to add dedicated MAY_EXEC
handling, this is already true for the stock kernel.

 include/linux/fs.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index bd0740e3bfcb..314a1349747b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -790,14 +790,13 @@ struct inode_state_flags {
 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;
-- 
2.48.1

Re: [PATCH] fs: move inode fields used during fast path lookup closer together

[Jan Kara]

On Sun 09-11-25 13:19:31, Mateusz Guzik wrote:
> 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>

Looks sensible. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
> 
> > 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.
> 
> While this is readily apparent with my patch to add dedicated MAY_EXEC
> handling, this is already true for the stock kernel.
> 
>  include/linux/fs.h | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index bd0740e3bfcb..314a1349747b 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -790,14 +790,13 @@ struct inode_state_flags {
>  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;
> -- 
> 2.48.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] fs: move inode fields used during fast path lookup closer together

[Christian Brauner]

On Sun, 09 Nov 2025 13:19:31 +0100, Mateusz Guzik wrote:
> 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).
> 
> [...]

Applied to the vfs-6.19.fs_header branch of the vfs/vfs.git tree.
Patches in the vfs-6.19.fs_header branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs-6.19.fs_header

[1/1] fs: move inode fields used during fast path lookup closer together
      https://git.kernel.org/vfs/vfs/c/dca3aa666fbd