[PATCH] fs : reorder some 'struct inode' fields to speedup i_size manipulations

[Eric Dumazet]

On 32bits SMP platforms, 64bits i_size is protected by a seqcount 
(i_size_seqcount).

When i_size is read or written, i_size_seqcount is read/written as well, so it 
make sense to group these two fields together in the same cache line.

Before this patch, accessing i_size needed 3 cache lines (2 for i_size, one 
for i_size_seqcount). After, only one cache line is needed/ (dirtied on a 
i_size change).

This patch moves i_size_seqcount next to i_size, and also moves i_version to 
let offsetof(struct inode, i_size) being 0x40 instead of 0x3c (for 32bits 
platforms). 

For 64 bits platforms, i_size_seqcount doesnt exist, and the move of a 'long 
i_version' should not introduce a new hole because of padding.

Signed-off-by: Eric Dumazet <dada1@xxxxxxxxxxxxx>
--- linux-2.6.19-rc6/include/linux/fs.h	2006-11-23 10:47:53.000000000 +0100
+++ linux-2.6.19-rc6-ed/include/linux/fs.h	2006-11-23 11:13:36.000000000 +0100
@@ -548,12 +548,15 @@ struct inode {
 	uid_t			i_uid;
 	gid_t			i_gid;
 	dev_t			i_rdev;
+	unsigned long		i_version;
 	loff_t			i_size;
+#ifdef __NEED_I_SIZE_ORDERED
+	seqcount_t		i_size_seqcount;
+#endif
 	struct timespec		i_atime;
 	struct timespec		i_mtime;
 	struct timespec		i_ctime;
 	unsigned int		i_blkbits;
-	unsigned long		i_version;
 	blkcnt_t		i_blocks;
 	unsigned short          i_bytes;
 	spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */
@@ -598,9 +601,6 @@ struct inode {
 	void			*i_security;
 #endif
 	void			*i_private; /* fs or device private pointer */
-#ifdef __NEED_I_SIZE_ORDERED
-	seqcount_t		i_size_seqcount;
-#endif
 };
 
 /*