btrfs: use a dedicated data structure for chunk maps

Currently we abuse the extent_map structure for two purposes:

1) To actually represent extents for inodes;
2) To represent chunk mappings.

This is odd and has several disadvantages:

1) To create a chunk map, we need to do two memory allocations: one for
   an extent_map structure and another one for a map_lookup structure, so
   more potential for an allocation failure and more complicated code to
   manage and link two structures;

2) For a chunk map we actually only use 3 fields (24 bytes) of the
   respective extent map structure: the 'start' field to have the logical
   start address of the chunk, the 'len' field to have the chunk's size,
   and the 'orig_block_len' field to contain the chunk's stripe size.

   Besides wasting a memory, it's also odd and not intuitive at all to
   have the stripe size in a field named 'orig_block_len'.

   We are also using 'block_len' of the extent_map structure to contain
   the chunk size, so we have 2 fields for the same value, 'len' and
   'block_len', which is pointless;

3) When an extent map is associated to a chunk mapping, we set the bit
   EXTENT_FLAG_FS_MAPPING on its flags and then make its member named
   'map_lookup' point to the associated map_lookup structure. This means
   that for an extent map associated to an inode extent, we are not using
   this 'map_lookup' pointer, so wasting 8 bytes (on a 64 bits platform);

4) Extent maps associated to a chunk mapping are never merged or split so
   it's pointless to use the existing extent map infrastructure.

So add a dedicated data structure named 'btrfs_chunk_map' to represent
chunk mappings, this is basically the existing map_lookup structure with
some extra fields:

1) 'start' to contain the chunk logical address;
2) 'chunk_len' to contain the chunk's length;
3) 'stripe_size' for the stripe size;
4) 'rb_node' for insertion into a rb tree;
5) 'refs' for reference counting.

This way we do a single memory allocation for chunk mappings and we don't
waste memory for them with unused/unnecessary fields from an extent_map.

We also save 8 bytes from the extent_map structure by removing the
'map_lookup' pointer, so the size of struct extent_map is reduced from
144 bytes down to 136 bytes, and we can now have 30 extents map per 4K
page instead of 28.

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

3 files changed, 13 insertions, 31 deletions

diff --git a/fs/btrfs/tests/btrfs-tests.c b/fs/btrfs/tests/btrfs-tests.c
index ca09cf9afce8..b50cfac7ad4e 100644
--- a/fs/btrfs/tests/btrfs-tests.c
+++ b/fs/btrfs/tests/btrfs-tests.c
@@ -28,6 +28,7 @@ const char *test_error[] = {
 	[TEST_ALLOC_INODE]	     = "cannot allocate inode",
 	[TEST_ALLOC_BLOCK_GROUP]     = "cannot allocate block group",
 	[TEST_ALLOC_EXTENT_MAP]      = "cannot allocate extent map",
+	[TEST_ALLOC_CHUNK_MAP]       = "cannot allocate chunk map",
 };
 
 static const struct super_operations btrfs_test_super_ops = {
@@ -185,7 +186,7 @@ void btrfs_free_dummy_fs_info(struct btrfs_fs_info *fs_info)
 	}
 	spin_unlock(&fs_info->buffer_lock);
 
-	btrfs_mapping_tree_free(&fs_info->mapping_tree);
+	btrfs_mapping_tree_free(fs_info);
 	list_for_each_entry_safe(dev, tmp, &fs_info->fs_devices->devices,
 				 dev_list) {
 		btrfs_free_dummy_device(dev);
diff --git a/fs/btrfs/tests/btrfs-tests.h b/fs/btrfs/tests/btrfs-tests.h
index 7a2d7ffbe30e..dc2f2ab15fa5 100644
--- a/fs/btrfs/tests/btrfs-tests.h
+++ b/fs/btrfs/tests/btrfs-tests.h
@@ -23,6 +23,7 @@ enum {
 	TEST_ALLOC_INODE,
 	TEST_ALLOC_BLOCK_GROUP,
 	TEST_ALLOC_EXTENT_MAP,
+	TEST_ALLOC_CHUNK_MAP,
 };
 
 extern const char *test_error[];
diff --git a/fs/btrfs/tests/extent-map-tests.c b/fs/btrfs/tests/extent-map-tests.c
index 29bdd08b241f..8602f94cc29d 100644
--- a/fs/btrfs/tests/extent-map-tests.c
+++ b/fs/btrfs/tests/extent-map-tests.c
@@ -859,33 +859,21 @@ struct rmap_test_vector {
 static int test_rmap_block(struct btrfs_fs_info *fs_info,
 			   struct rmap_test_vector *test)
 {
-	struct extent_map *em;
-	struct map_lookup *map = NULL;
+	struct btrfs_chunk_map *map;
 	u64 *logical = NULL;
 	int i, out_ndaddrs, out_stripe_len;
 	int ret;
 
-	em = alloc_extent_map();
-	if (!em) {
-		test_std_err(TEST_ALLOC_EXTENT_MAP);
-		return -ENOMEM;
-	}
-
-	map = kmalloc(map_lookup_size(test->num_stripes), GFP_KERNEL);
+	map = btrfs_alloc_chunk_map(test->num_stripes, GFP_KERNEL);
 	if (!map) {
-		kfree(em);
-		test_std_err(TEST_ALLOC_EXTENT_MAP);
+		test_std_err(TEST_ALLOC_CHUNK_MAP);
 		return -ENOMEM;
 	}
 
-	set_bit(EXTENT_FLAG_FS_MAPPING, &em->flags);
 	/* Start at 4GiB logical address */
-	em->start = SZ_4G;
-	em->len = test->data_stripe_size * test->num_data_stripes;
-	em->block_len = em->len;
-	em->orig_block_len = test->data_stripe_size;
-	em->map_lookup = map;
-
+	map->start = SZ_4G;
+	map->chunk_len = test->data_stripe_size * test->num_data_stripes;
+	map->stripe_size = test->data_stripe_size;
 	map->num_stripes = test->num_stripes;
 	map->type = test->raid_type;
 
@@ -901,15 +889,13 @@ static int test_rmap_block(struct btrfs_fs_info *fs_info,
 		map->stripes[i].physical = test->data_stripe_phys_start[i];
 	}
 
-	write_lock(&fs_info->mapping_tree.lock);
-	ret = add_extent_mapping(&fs_info->mapping_tree, em, 0);
-	write_unlock(&fs_info->mapping_tree.lock);
+	ret = btrfs_add_chunk_map(fs_info, map);
 	if (ret) {
-		test_err("error adding block group mapping to mapping tree");
+		test_err("error adding chunk map to mapping tree");
 		goto out_free;
 	}
 
-	ret = btrfs_rmap_block(fs_info, em->start, btrfs_sb_offset(1),
+	ret = btrfs_rmap_block(fs_info, map->start, btrfs_sb_offset(1),
 			       &logical, &out_ndaddrs, &out_stripe_len);
 	if (ret || (out_ndaddrs == 0 && test->expected_mapped_addr)) {
 		test_err("didn't rmap anything but expected %d",
@@ -938,14 +924,8 @@ static int test_rmap_block(struct btrfs_fs_info *fs_info,
 
 	ret = 0;
 out:
-	write_lock(&fs_info->mapping_tree.lock);
-	remove_extent_mapping(&fs_info->mapping_tree, em);
-	write_unlock(&fs_info->mapping_tree.lock);
-	/* For us */
-	free_extent_map(em);
+	btrfs_remove_chunk_map(fs_info, map);
 out_free:
-	/* For the tree */
-	free_extent_map(em);
 	kfree(logical);
 	return ret;
 }