Allow increasing the buffer-head per-CPU LRU size

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)

The buffer-head per-CPU LRU size can be changed at config time, and its default value is raised to 16.

---
 fs/Kconfig  |    9 +++++++++
 fs/buffer.c |    3 +--
 2 files changed, 10 insertions(+), 2 deletions(-)

index c229f82..afea808 100644
@@ -268,4 +268,13 @@ endif # NETWORK_FILESYSTEMS
 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.
 endmenu

index 6024877..b83fa63 100644
@@ -1255,8 +1255,7 @@ static struct buffer_head *__bread_slow(struct buffer_head *bh)
  * 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];

1.7.1