ext4: Don't allow lg prealloc list to be grow large.

Currently, the locality group prealloc list is freed only when there
is a block allocation failure. This can result in large number of
entries in the preallocation list making ext4_mb_use_preallocated()
expensive.

To fix this, we convert the locality group prealloc list to a hash
list. The hash index is the order of number of blocks in the prealloc
space with a max order of 9. When adding prealloc space to the list we
make sure total entries for each order does not exceed 8. If it is
more than 8 we discard few entries and make sure the we have only <= 5
entries.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 8 insertions, 2 deletions

diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
index bfe6add46bcf..c7c9906c2a75 100644
--- a/fs/ext4/mballoc.h
+++ b/fs/ext4/mballoc.h
@@ -164,11 +164,17 @@ struct ext4_free_extent {
  * Locality group:
  *   we try to group all related changes together
  *   so that writeback can flush/allocate them together as well
+ *   Size of lg_prealloc_list hash is determined by MB_DEFAULT_GROUP_PREALLOC
+ *   (512). We store prealloc space into the hash based on the pa_free blocks
+ *   order value.ie, fls(pa_free)-1;
  */
+#define PREALLOC_TB_SIZE 10
 struct ext4_locality_group {
         /* for allocator */
-        struct mutex            lg_mutex;       /* to serialize allocates */
-        struct list_head        lg_prealloc_list;/* list of preallocations */
+        /* to serialize allocates */
+        struct mutex            lg_mutex;
+        /* list of preallocations */
+        struct list_head        lg_prealloc_list[PREALLOC_TB_SIZE];
         spinlock_t              lg_prealloc_lock;
 };