btrfs: use a kmem_cache for block groups

We are currently allocating block groups using the generic slabs, and
given that the size of btrfs_block_group structure is 672 bytes (on a
release kernel), we end up using the kmalloc-1024 slab and therefore
waste quite some memory since on a 4K page system we can only fit 4
block groups per page. The block groups are also allocated and
delallocated with some frequency, specially if we have auto reclaim
enabled.

So use a kmem_cache for block groups, this way on a 4K page system we
can fit 6 block groups per page instead of 4.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -22,6 +22,23 @@
 #include "accessors.h"
 #include "extent-tree.h"
 
+static struct kmem_cache *block_group_cache;
+
+int __init btrfs_init_block_group(void)
+{
+	block_group_cache = kmem_cache_create("btrfs_block_group",
+					      sizeof(struct btrfs_block_group),
+					      0, 0, NULL);
+	if (!block_group_cache)
+		return -ENOMEM;
+	return 0;
+}
+
+void __cold btrfs_exit_block_group(void)
+{
+	kmem_cache_destroy(block_group_cache);
+}
+
 #ifdef CONFIG_BTRFS_DEBUG
 int btrfs_should_fragment_free_space(const struct btrfs_block_group *block_group)
 {
@@ -182,7 +199,7 @@ void btrfs_put_block_group(struct btrfs_block_group *cache)
 
 		kfree(cache->free_space_ctl);
 		btrfs_free_chunk_map(cache->physical_map);
-		kfree(cache);
+		kmem_cache_free(block_group_cache, cache);
 	}
 }
 
@@ -2371,13 +2388,13 @@ static struct btrfs_block_group *btrfs_create_block_group(
 {
 	struct btrfs_block_group *cache;
 
-	cache = kzalloc_obj(*cache, GFP_NOFS);
+	cache = kmem_cache_zalloc(block_group_cache, GFP_NOFS);
 	if (!cache)
 		return NULL;
 
 	cache->free_space_ctl = kzalloc_obj(*cache->free_space_ctl, GFP_NOFS);
 	if (!cache->free_space_ctl) {
-		kfree(cache);
+		kmem_cache_free(block_group_cache, cache);
 		return NULL;
 	}
 

fs/btrfs/block-group.h

@@ -320,6 +320,9 @@ static inline u64 btrfs_block_group_available_space(const struct btrfs_block_gro
 int btrfs_should_fragment_free_space(const struct btrfs_block_group *block_group);
 #endif
 
+int __init btrfs_init_block_group(void);
+void __cold btrfs_exit_block_group(void);
+
 struct btrfs_block_group *btrfs_lookup_first_block_group(
 		struct btrfs_fs_info *info, u64 bytenr);
 struct btrfs_block_group *btrfs_lookup_block_group(

fs/btrfs/super.c

@@ -2603,6 +2603,9 @@ static const struct init_sequence mod_init_seq[] = {
 	}, {
 		.init_func = btrfs_init_compress,
 		.exit_func = btrfs_exit_compress,
+	}, {
+		.init_func = btrfs_init_block_group,
+		.exit_func = btrfs_exit_block_group,
 	}, {
 		.init_func = btrfs_init_cachep,
 		.exit_func = btrfs_destroy_cachep,