ext4: allocate ->s_blockgroup_lock separately

As spotted by kmemtrace, struct ext4_sb_info is 17664 bytes on 64-bit
which makes it a very bad fit for SLAB allocators.  The culprit of the
wasted memory is ->s_blockgroup_lock which can be as big as 16 KB when
NR_CPUS >= 32.

To fix that, allocate ->s_blockgroup_lock, which fits nicely in a order 2
page in the worst case, separately.  This shinks down struct ext4_sb_info
enough to fit a 2 KB slab cache so now we allocate 16 KB + 2 KB instead of
32 KB saving 14 KB of memory.

Acked-by: Andreas Dilger <adilger@sun.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4_sb.h

@@ -62,7 +62,7 @@ struct ext4_sb_info {
 	struct percpu_counter s_freeinodes_counter;
 	struct percpu_counter s_dirs_counter;
 	struct percpu_counter s_dirtyblocks_counter;
-	struct blockgroup_lock s_blockgroup_lock;
+	struct blockgroup_lock *s_blockgroup_lock;
 	struct proc_dir_entry *s_proc;
 
 	/* Journaling */
@@ -149,7 +149,7 @@ struct ext4_sb_info {
 static inline spinlock_t *
 sb_bgl_lock(struct ext4_sb_info *sbi, unsigned int block_group)
 {
-	return bgl_lock_ptr(&sbi->s_blockgroup_lock, block_group);
+	return bgl_lock_ptr(sbi->s_blockgroup_lock, block_group);
 }
 
 #endif	/* _EXT4_SB */