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Subject[PATCH][INODE]: Save some bytes in struct inode
[acme@newtoy net-2.6.20]$ pahole --cacheline 64 fs/inode.o inode
/* /pub/scm/linux/kernel/git/acme/net-2.6.20/include/linux/dcache.h:86 */
struct inode {
struct hlist_node i_hash; /* 0 8 */
struct list_head i_list; /* 8 8 */
struct list_head i_sb_list; /* 16 8 */
struct list_head i_dentry; /* 24 8 */
long unsigned int i_ino; /* 32 4 */
atomic_t i_count; /* 36 4 */
umode_t i_mode; /* 40 2 */

/* XXX 2 bytes hole, try to pack */

unsigned int i_nlink; /* 44 4 */
uid_t i_uid; /* 48 4 */
gid_t i_gid; /* 52 4 */
dev_t i_rdev; /* 56 4 */
loff_t i_size; /* 60 8 */
struct timespec i_atime; /* 68 8 */
struct timespec i_mtime; /* 76 8 */
struct timespec i_ctime; /* 84 8 */
unsigned int i_blkbits; /* 92 4 */
long unsigned int i_version; /* 96 4 */
blkcnt_t i_blocks; /* 100 4 */
short unsigned int i_bytes; /* 104 2 */

/* XXX 2 bytes hole, try to pack */

spinlock_t i_lock; /* 108 40 */
struct mutex i_mutex; /* 148 76 */
struct rw_semaphore i_alloc_sem; /* 224 64 */
struct inode_operations * i_op; /* 288 4 */
const struct file_operations * i_fop; /* 292 4 */
struct super_block * i_sb; /* 296 4 */
struct file_lock * i_flock; /* 300 4 */
struct address_space * i_mapping; /* 304 4 */
struct address_space i_data; /* 308 188 */
struct list_head i_devices; /* 496 8 */
union ; /* 504 4 */
int i_cindex; /* 508 4 */
__u32 i_generation; /* 512 4 */
/* ---------- cacheline 8 boundary ---------- */
long unsigned int i_dnotify_mask; /* 516 4 */
struct dnotify_struct * i_dnotify; /* 520 4 */
struct list_head inotify_watches; /* 524 8 */
struct mutex inotify_mutex; /* 532 76 */
long unsigned int i_state; /* 608 4 */
long unsigned int dirtied_when; /* 612 4 */
unsigned int i_flags; /* 616 4 */
atomic_t i_writecount; /* 620 4 */
void * i_security; /* 624 4 */
void * i_private; /* 628 4 */
}; /* size: 632, sum members: 628, holes: 2, sum holes: 4 */

[acme@newtoy net-2.6.20]$

So just moving i_mode to after i_bytes we save 4 bytes by nuking both holes:

[acme@newtoy net-2.6.20]$ codiff -V /tmp/inode.o.before fs/inode.o
/pub/scm/linux/kernel/git/acme/net-2.6.20/fs/inode.c:
struct inode | -4
i_mode;
from: umode_t /* 40(0) 2(0) */
to: umode_t /* 102(0) 2(0) */
1 struct changed
[acme@newtoy net-2.6.20]$

I've prunned all the other offset changes, only this one is of interest here.

So now we have:

[acme@newtoy net-2.6.20]$ pahole --cacheline 64 ../OUTPUT/qemu/net-2.6.20/fs/inode.o inode
/* /pub/scm/linux/kernel/git/acme/net-2.6.20/include/linux/dcache.h:86 */
struct inode {
struct hlist_node i_hash; /* 0 8 */
struct list_head i_list; /* 8 8 */
struct list_head i_sb_list; /* 16 8 */
struct list_head i_dentry; /* 24 8 */
long unsigned int i_ino; /* 32 4 */
atomic_t i_count; /* 36 4 */
unsigned int i_nlink; /* 40 4 */
uid_t i_uid; /* 44 4 */
gid_t i_gid; /* 48 4 */
dev_t i_rdev; /* 52 4 */
loff_t i_size; /* 56 8 */
/* ---------- cacheline 1 boundary ---------- */
struct timespec i_atime; /* 64 8 */
struct timespec i_mtime; /* 72 8 */
struct timespec i_ctime; /* 80 8 */
unsigned int i_blkbits; /* 88 4 */
long unsigned int i_version; /* 92 4 */
blkcnt_t i_blocks; /* 96 4 */
short unsigned int i_bytes; /* 100 2 */
umode_t i_mode; /* 102 2 */
spinlock_t i_lock; /* 104 40 */
struct mutex i_mutex; /* 144 76 */
struct rw_semaphore i_alloc_sem; /* 220 64 */
struct inode_operations * i_op; /* 284 4 */
const struct file_operations * i_fop; /* 288 4 */
struct super_block * i_sb; /* 292 4 */
struct file_lock * i_flock; /* 296 4 */
struct address_space * i_mapping; /* 300 4 */
struct address_space i_data; /* 304 188 */
struct list_head i_devices; /* 492 8 */
union ; /* 500 4 */
int i_cindex; /* 504 4 */
__u32 i_generation; /* 508 4 */
/* ---------- cacheline 8 boundary ---------- */
long unsigned int i_dnotify_mask; /* 512 4 */
struct dnotify_struct * i_dnotify; /* 516 4 */
struct list_head inotify_watches; /* 520 8 */
struct mutex inotify_mutex; /* 528 76 */
long unsigned int i_state; /* 604 4 */
long unsigned int dirtied_when; /* 608 4 */
unsigned int i_flags; /* 612 4 */
atomic_t i_writecount; /* 616 4 */
void * i_security; /* 620 4 */
void * i_private; /* 624 4 */
}; /* size: 628 */

[acme@newtoy net-2.6.20]$

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
---
fs.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 2fe6e3f..dc7456d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -543,7 +543,6 @@ struct inode {
struct list_head i_dentry;
unsigned long i_ino;
atomic_t i_count;
- umode_t i_mode;
unsigned int i_nlink;
uid_t i_uid;
gid_t i_gid;
@@ -556,6 +555,7 @@ struct inode {
unsigned long i_version;
blkcnt_t i_blocks;
unsigned short i_bytes;
+ umode_t i_mode;
spinlock_t i_lock; /* i_blocks, i_bytes, maybe i_size */
struct mutex i_mutex;
struct rw_semaphore i_alloc_sem;
-
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