[PATCH] kernfs: reorder 'struct kernfs_node' to save some memory

[Christophe JAILLET]

'struct kernfs_node' uses a dedicated cache, so shrinking its size is
always a good idea.

On my system, each entry is 128 bytes and their are 32 entries per pages.
After the re-ordering, the struct is 120 bytes and 34 entries are stored
in each page.

Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
---
The numbers below are with a allmodconfig configuration. The delta is
related to CONFIG_DEBUG_LOCK_ALLOC and struct lockdep_map	dep_map.

When I checked on my system, it would have saved 372kb of RAM:
  sudo less /proc/slabinfo | grep kernf
  kernfs_node_cache  49397  49504    128   32    1 : tunables    0    0    0 : slabdata   1547   1547      0

I have left flags close to the union, I *think* that they are related.
I don't if having 'mode' here is logical or not.

Before:
======
struct kernfs_node {
        atomic_t                   count;                /*     0     4 */
        atomic_t                   active;               /*     4     4 */
        struct lockdep_map         dep_map;              /*     8    48 */
        struct kernfs_node *       parent;               /*    56     8 */
        /* --- cacheline 1 boundary (64 bytes) --- */
        const char  *              name;                 /*    64     8 */
        struct rb_node             rb __attribute__((__aligned__(8))); /*    72    24 */
        const void  *              ns;                   /*    96     8 */
        unsigned int               hash;                 /*   104     4 */

        /* XXX 4 bytes hole, try to pack */

        union {
                struct kernfs_elem_dir dir;              /*   112    32 */
                struct kernfs_elem_symlink symlink;      /*   112     8 */
                struct kernfs_elem_attr attr;            /*   112    32 */
        };                                               /*   112    32 */
        /* --- cacheline 2 boundary (128 bytes) was 16 bytes ago --- */
        void *                     priv;                 /*   144     8 */
        u64                        id;                   /*   152     8 */
        short unsigned int         flags;                /*   160     2 */
        umode_t                    mode;                 /*   162     2 */

        /* XXX 4 bytes hole, try to pack */

        struct kernfs_iattrs *     iattr;                /*   168     8 */

        /* size: 176, cachelines: 3, members: 14 */
        /* sum members: 168, holes: 2, sum holes: 8 */
        /* forced alignments: 1 */
        /* last cacheline: 48 bytes */
} __attribute__((__aligned__(8)));

After:
=====
struct kernfs_node {
        atomic_t                   count;                /*     0     4 */
        atomic_t                   active;               /*     4     4 */
        struct lockdep_map         dep_map;              /*     8    48 */
        struct kernfs_node *       parent;               /*    56     8 */
        /* --- cacheline 1 boundary (64 bytes) --- */
        const char  *              name;                 /*    64     8 */
        struct rb_node             rb __attribute__((__aligned__(8))); /*    72    24 */
        const void  *              ns;                   /*    96     8 */
        unsigned int               hash;                 /*   104     4 */
        umode_t                    mode;                 /*   108     2 */
        short unsigned int         flags;                /*   110     2 */
        union {
                struct kernfs_elem_dir dir;              /*   112    32 */
                struct kernfs_elem_symlink symlink;      /*   112     8 */
                struct kernfs_elem_attr attr;            /*   112    32 */
        };                                               /*   112    32 */
        /* --- cacheline 2 boundary (128 bytes) was 16 bytes ago --- */
        void *                     priv;                 /*   144     8 */
        u64                        id;                   /*   152     8 */
        struct kernfs_iattrs *     iattr;                /*   160     8 */

        /* size: 168, cachelines: 3, members: 14 */
        /* forced alignments: 1 */
        /* last cacheline: 40 bytes */
} __attribute__((__aligned__(8)));
---
 include/linux/kernfs.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 99aaa050ccb7..b7cd6e793286 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -206,6 +206,8 @@ struct kernfs_node {
 
 	const void		*ns;	/* namespace tag */
 	unsigned int		hash;	/* ns + name hash */
+	umode_t			mode;
+	unsigned short		flags;
 	union {
 		struct kernfs_elem_dir		dir;
 		struct kernfs_elem_symlink	symlink;
@@ -220,8 +222,6 @@ struct kernfs_node {
 	 */
 	u64			id;
 
-	unsigned short		flags;
-	umode_t			mode;
 	struct kernfs_iattrs	*iattr;
 };
 
-- 
2.34.1