Allow increasing the buffer-head per-CPU LRU size to allow efficient
filesystem operations that access many blocks for each transaction.
For example, creating a file in a large ext4 directory with quota
enabled will accesses multiple buffer heads and will overflow the LRU
at the default 8-block LRU size:

* parent directory inode table block (ctime, nlinks for subdirs)
* new inode bitmap
* inode table block
* 2 quota blocks
* directory leaf block (not reused, but pollutes one cache entry)
* 2 levels htree blocks (only one is reused, other pollutes cache)
* 2 levels indirect/index blocks (only one is reused)

Signed-off-by: Liang Zhen <liang@whamcoud.com>
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>

--- linux-2.6.32-131.6.1/fs/buffer.c
+++ linux-2.6.32-131.6.1-pdo/fs/buffer.c
@@ -1250,8 +1250,7 @@
  * The LRUs themselves only need locking against invalidate_bh_lrus.  We use
  * a local interrupt disable for that.
  */
-
-#define BH_LRU_SIZE	8
+#define BH_LRU_SIZE	CONFIG_BH_LRU_SIZE
 
 struct bh_lru {
 	struct buffer_head *bhs[BH_LRU_SIZE];
--- linux-2.6.32-131.6.1/fs/Kconfig
+++ linux-2.6.32-131.6.1-pdo/fs/Kconfig
@@ -253,4 +253,13 @@
 source "fs/nls/Kconfig"
 source "fs/dlm/Kconfig"
 
+config BH_LRU_SIZE
+      int "buffer head per-CPU LRU size"
+      range 8 64
+      default "16"
+      help
+        This sets the per-CPU LRU size for buffer heads in memory.
+        More complex filesystems may be modiyfing multiple blocks
+        within a single transaction, so keeping the buffer heads in
+        CPU-local cache speeds up modifations significantly.
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