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 <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: fdmanana

fs/btrfs/block-group.c

@@ -168,7 +168,7 @@ void btrfs_put_block_group(struct btrfs_block_group *cache)
 						  cache);
 
 		kfree(cache->free_space_ctl);
-		kfree(cache->physical_map);
+		btrfs_free_chunk_map(cache->physical_map);
 		kfree(cache);
 	}
 }
@@ -1047,7 +1047,7 @@ static int remove_block_group_item(struct btrfs_trans_handle *trans,
 }
 
 int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
-			     u64 group_start, struct extent_map *em)
+			     struct btrfs_chunk_map *map)
 {
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 	struct btrfs_path *path;
@@ -1059,10 +1059,10 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 	int index;
 	int factor;
 	struct btrfs_caching_control *caching_ctl = NULL;
-	bool remove_em;
+	bool remove_map;
 	bool remove_rsv = false;
 
-	block_group = btrfs_lookup_block_group(fs_info, group_start);
+	block_group = btrfs_lookup_block_group(fs_info, map->start);
 	BUG_ON(!block_group);
 	BUG_ON(!block_group->ro);
 
@@ -1252,7 +1252,7 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 	 * entries because we already removed them all when we called
 	 * btrfs_remove_free_space_cache().
 	 *
-	 * And we must not remove the extent map from the fs_info->mapping_tree
+	 * And we must not remove the chunk map from the fs_info->mapping_tree
 	 * to prevent the same logical address range and physical device space
 	 * ranges from being reused for a new block group. This is needed to
 	 * avoid races with trimming and scrub.
@@ -1268,19 +1268,11 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 	 * in place until the extents have been discarded completely when
 	 * the transaction commit has completed.
 	 */
-	remove_em = (atomic_read(&block_group->frozen) == 0);
+	remove_map = (atomic_read(&block_group->frozen) == 0);
 	spin_unlock(&block_group->lock);
 
-	if (remove_em) {
-		struct extent_map_tree *em_tree;
-
-		em_tree = &fs_info->mapping_tree;
-		write_lock(&em_tree->lock);
-		remove_extent_mapping(em_tree, em);
-		write_unlock(&em_tree->lock);
-		/* once for the tree */
-		free_extent_map(em);
-	}
+	if (remove_map)
+		btrfs_remove_chunk_map(fs_info, map);
 
 out:
 	/* Once for the lookup reference */
@@ -1295,16 +1287,12 @@ struct btrfs_trans_handle *btrfs_start_trans_remove_block_group(
 		struct btrfs_fs_info *fs_info, const u64 chunk_offset)
 {
 	struct btrfs_root *root = btrfs_block_group_root(fs_info);
-	struct extent_map_tree *em_tree = &fs_info->mapping_tree;
-	struct extent_map *em;
-	struct map_lookup *map;
+	struct btrfs_chunk_map *map;
 	unsigned int num_items;
 
-	read_lock(&em_tree->lock);
-	em = lookup_extent_mapping(em_tree, chunk_offset, 1);
-	read_unlock(&em_tree->lock);
-	ASSERT(em != NULL);
-	ASSERT(em->start == chunk_offset);
+	map = btrfs_find_chunk_map(fs_info, chunk_offset, 1);
+	ASSERT(map != NULL);
+	ASSERT(map->start == chunk_offset);
 
 	/*
 	 * We need to reserve 3 + N units from the metadata space info in order
@@ -1325,9 +1313,8 @@ struct btrfs_trans_handle *btrfs_start_trans_remove_block_group(
 	 * more device items and remove one chunk item), but this is done at
 	 * btrfs_remove_chunk() through a call to check_system_chunk().
 	 */
-	map = em->map_lookup;
 	num_items = 3 + map->num_stripes;
-	free_extent_map(em);
+	btrfs_free_chunk_map(map);
 
 	return btrfs_start_transaction_fallback_global_rsv(root, num_items);
 }
@@ -1928,8 +1915,7 @@ void btrfs_mark_bg_to_reclaim(struct btrfs_block_group *bg)
 static int read_bg_from_eb(struct btrfs_fs_info *fs_info, struct btrfs_key *key,
 			   struct btrfs_path *path)
 {
-	struct extent_map_tree *em_tree;
-	struct extent_map *em;
+	struct btrfs_chunk_map *map;
 	struct btrfs_block_group_item bg;
 	struct extent_buffer *leaf;
 	int slot;
@@ -1939,40 +1925,37 @@ static int read_bg_from_eb(struct btrfs_fs_info *fs_info, struct btrfs_key *key,
 	slot = path->slots[0];
 	leaf = path->nodes[0];
 
-	em_tree = &fs_info->mapping_tree;
-	read_lock(&em_tree->lock);
-	em = lookup_extent_mapping(em_tree, key->objectid, key->offset);
-	read_unlock(&em_tree->lock);
-	if (!em) {
+	map = btrfs_find_chunk_map(fs_info, key->objectid, key->offset);
+	if (!map) {
 		btrfs_err(fs_info,
 			  "logical %llu len %llu found bg but no related chunk",
 			  key->objectid, key->offset);
 		return -ENOENT;
 	}
 
-	if (em->start != key->objectid || em->len != key->offset) {
+	if (map->start != key->objectid || map->chunk_len != key->offset) {
 		btrfs_err(fs_info,
 			"block group %llu len %llu mismatch with chunk %llu len %llu",
-			key->objectid, key->offset, em->start, em->len);
+			  key->objectid, key->offset, map->start, map->chunk_len);
 		ret = -EUCLEAN;
-		goto out_free_em;
+		goto out_free_map;
 	}
 
 	read_extent_buffer(leaf, &bg, btrfs_item_ptr_offset(leaf, slot),
 			   sizeof(bg));
 	flags = btrfs_stack_block_group_flags(&bg) &
 		BTRFS_BLOCK_GROUP_TYPE_MASK;
 
-	if (flags != (em->map_lookup->type & BTRFS_BLOCK_GROUP_TYPE_MASK)) {
+	if (flags != (map->type & BTRFS_BLOCK_GROUP_TYPE_MASK)) {
 		btrfs_err(fs_info,
 "block group %llu len %llu type flags 0x%llx mismatch with chunk type flags 0x%llx",
 			  key->objectid, key->offset, flags,
-			  (BTRFS_BLOCK_GROUP_TYPE_MASK & em->map_lookup->type));
+			  (BTRFS_BLOCK_GROUP_TYPE_MASK & map->type));
 		ret = -EUCLEAN;
 	}
 
-out_free_em:
-	free_extent_map(em);
+out_free_map:
+	btrfs_free_chunk_map(map);
 	return ret;
 }
 
@@ -2025,23 +2008,21 @@ static void set_avail_alloc_bits(struct btrfs_fs_info *fs_info, u64 flags)
 int btrfs_rmap_block(struct btrfs_fs_info *fs_info, u64 chunk_start,
 		     u64 physical, u64 **logical, int *naddrs, int *stripe_len)
 {
-	struct extent_map *em;
-	struct map_lookup *map;
+	struct btrfs_chunk_map *map;
 	u64 *buf;
 	u64 bytenr;
 	u64 data_stripe_length;
 	u64 io_stripe_size;
 	int i, nr = 0;
 	int ret = 0;
 
-	em = btrfs_get_chunk_map(fs_info, chunk_start, 1);
-	if (IS_ERR(em))
+	map = btrfs_get_chunk_map(fs_info, chunk_start, 1);
+	if (IS_ERR(map))
 		return -EIO;
 
-	map = em->map_lookup;
-	data_stripe_length = em->orig_block_len;
+	data_stripe_length = map->stripe_size;
 	io_stripe_size = BTRFS_STRIPE_LEN;
-	chunk_start = em->start;
+	chunk_start = map->start;
 
 	/* For RAID5/6 adjust to a full IO stripe length */
 	if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK)
@@ -2095,7 +2076,7 @@ int btrfs_rmap_block(struct btrfs_fs_info *fs_info, u64 chunk_start,
 	*naddrs = nr;
 	*stripe_len = io_stripe_size;
 out:
-	free_extent_map(em);
+	btrfs_free_chunk_map(map);
 	return ret;
 }
 
@@ -2200,49 +2181,47 @@ static struct btrfs_block_group *btrfs_create_block_group_cache(
  */
 static int check_chunk_block_group_mappings(struct btrfs_fs_info *fs_info)
 {
-	struct extent_map_tree *map_tree = &fs_info->mapping_tree;
-	struct extent_map *em;
-	struct btrfs_block_group *bg;
 	u64 start = 0;
 	int ret = 0;
 
 	while (1) {
-		read_lock(&map_tree->lock);
+		struct btrfs_chunk_map *map;
+		struct btrfs_block_group *bg;
+
 		/*
-		 * lookup_extent_mapping will return the first extent map
-		 * intersecting the range, so setting @len to 1 is enough to
+		 * btrfs_find_chunk_map() will return the first chunk map
+		 * intersecting the range, so setting @length to 1 is enough to
 		 * get the first chunk.
 		 */
-		em = lookup_extent_mapping(map_tree, start, 1);
-		read_unlock(&map_tree->lock);
-		if (!em)
+		map = btrfs_find_chunk_map(fs_info, start, 1);
+		if (!map)
 			break;
 
-		bg = btrfs_lookup_block_group(fs_info, em->start);
+		bg = btrfs_lookup_block_group(fs_info, map->start);
 		if (!bg) {
 			btrfs_err(fs_info,
 	"chunk start=%llu len=%llu doesn't have corresponding block group",
-				     em->start, em->len);
+				     map->start, map->chunk_len);
 			ret = -EUCLEAN;
-			free_extent_map(em);
+			btrfs_free_chunk_map(map);
 			break;
 		}
-		if (bg->start != em->start || bg->length != em->len ||
+		if (bg->start != map->start || bg->length != map->chunk_len ||
 		    (bg->flags & BTRFS_BLOCK_GROUP_TYPE_MASK) !=
-		    (em->map_lookup->type & BTRFS_BLOCK_GROUP_TYPE_MASK)) {
+		    (map->type & BTRFS_BLOCK_GROUP_TYPE_MASK)) {
 			btrfs_err(fs_info,
 "chunk start=%llu len=%llu flags=0x%llx doesn't match block group start=%llu len=%llu flags=0x%llx",
-				em->start, em->len,
-				em->map_lookup->type & BTRFS_BLOCK_GROUP_TYPE_MASK,
+				map->start, map->chunk_len,
+				map->type & BTRFS_BLOCK_GROUP_TYPE_MASK,
 				bg->start, bg->length,
 				bg->flags & BTRFS_BLOCK_GROUP_TYPE_MASK);
 			ret = -EUCLEAN;
-			free_extent_map(em);
+			btrfs_free_chunk_map(map);
 			btrfs_put_block_group(bg);
 			break;
 		}
-		start = em->start + em->len;
-		free_extent_map(em);
+		start = map->start + map->chunk_len;
+		btrfs_free_chunk_map(map);
 		btrfs_put_block_group(bg);
 	}
 	return ret;
@@ -2370,28 +2349,25 @@ static int read_one_block_group(struct btrfs_fs_info *info,
 
 static int fill_dummy_bgs(struct btrfs_fs_info *fs_info)
 {
-	struct extent_map_tree *em_tree = &fs_info->mapping_tree;
 	struct rb_node *node;
 	int ret = 0;
 
-	for (node = rb_first_cached(&em_tree->map); node; node = rb_next(node)) {
-		struct extent_map *em;
-		struct map_lookup *map;
+	for (node = rb_first_cached(&fs_info->mapping_tree); node; node = rb_next(node)) {
+		struct btrfs_chunk_map *map;
 		struct btrfs_block_group *bg;
 
-		em = rb_entry(node, struct extent_map, rb_node);
-		map = em->map_lookup;
-		bg = btrfs_create_block_group_cache(fs_info, em->start);
+		map = rb_entry(node, struct btrfs_chunk_map, rb_node);
+		bg = btrfs_create_block_group_cache(fs_info, map->start);
 		if (!bg) {
 			ret = -ENOMEM;
 			break;
 		}
 
 		/* Fill dummy cache as FULL */
-		bg->length = em->len;
+		bg->length = map->chunk_len;
 		bg->flags = map->type;
 		bg->cached = BTRFS_CACHE_FINISHED;
-		bg->used = em->len;
+		bg->used = map->chunk_len;
 		bg->flags = map->type;
 		ret = btrfs_add_block_group_cache(fs_info, bg);
 		/*
@@ -2619,19 +2595,17 @@ static int insert_dev_extents(struct btrfs_trans_handle *trans,
 {
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 	struct btrfs_device *device;
-	struct extent_map *em;
-	struct map_lookup *map;
+	struct btrfs_chunk_map *map;
 	u64 dev_offset;
 	u64 stripe_size;
 	int i;
 	int ret = 0;
 
-	em = btrfs_get_chunk_map(fs_info, chunk_offset, chunk_size);
-	if (IS_ERR(em))
-		return PTR_ERR(em);
+	map = btrfs_get_chunk_map(fs_info, chunk_offset, chunk_size);
+	if (IS_ERR(map))
+		return PTR_ERR(map);
 
-	map = em->map_lookup;
-	stripe_size = em->orig_block_len;
+	stripe_size = map->stripe_size;
 
 	/*
 	 * Take the device list mutex to prevent races with the final phase of
@@ -2654,7 +2628,7 @@ static int insert_dev_extents(struct btrfs_trans_handle *trans,
 	}
 	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
 
-	free_extent_map(em);
+	btrfs_free_chunk_map(map);
 	return ret;
 }
 
@@ -4407,8 +4381,6 @@ void btrfs_freeze_block_group(struct btrfs_block_group *cache)
 void btrfs_unfreeze_block_group(struct btrfs_block_group *block_group)
 {
 	struct btrfs_fs_info *fs_info = block_group->fs_info;
-	struct extent_map_tree *em_tree;
-	struct extent_map *em;
 	bool cleanup;
 
 	spin_lock(&block_group->lock);
@@ -4417,17 +4389,16 @@ void btrfs_unfreeze_block_group(struct btrfs_block_group *block_group)
 	spin_unlock(&block_group->lock);
 
 	if (cleanup) {
-		em_tree = &fs_info->mapping_tree;
-		write_lock(&em_tree->lock);
-		em = lookup_extent_mapping(em_tree, block_group->start,
-					   1);
-		BUG_ON(!em); /* logic error, can't happen */
-		remove_extent_mapping(em_tree, em);
-		write_unlock(&em_tree->lock);
-
-		/* once for us and once for the tree */
-		free_extent_map(em);
-		free_extent_map(em);
+		struct btrfs_chunk_map *map;
+
+		map = btrfs_find_chunk_map(fs_info, block_group->start, 1);
+		/* Logic error, can't happen. */
+		ASSERT(map);
+
+		btrfs_remove_chunk_map(fs_info, map);
+
+		/* Once for our lookup reference. */
+		btrfs_free_chunk_map(map);
 
 		/*
 		 * We may have left one free space entry and other possible

fs/btrfs/block-group.h

@@ -5,6 +5,8 @@
 
 #include "free-space-cache.h"
 
+struct btrfs_chunk_map;
+
 enum btrfs_disk_cache_state {
 	BTRFS_DC_WRITTEN,
 	BTRFS_DC_ERROR,
@@ -243,7 +245,7 @@ struct btrfs_block_group {
 	u64 zone_unusable;
 	u64 zone_capacity;
 	u64 meta_write_pointer;
-	struct map_lookup *physical_map;
+	struct btrfs_chunk_map *physical_map;
 	struct list_head active_bg_list;
 	struct work_struct zone_finish_work;
 	struct extent_buffer *last_eb;
@@ -297,7 +299,7 @@ struct btrfs_trans_handle *btrfs_start_trans_remove_block_group(
 				struct btrfs_fs_info *fs_info,
 				const u64 chunk_offset);
 int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
-			     u64 group_start, struct extent_map *em);
+			     struct btrfs_chunk_map *map);
 void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info);
 void btrfs_mark_bg_unused(struct btrfs_block_group *bg);
 void btrfs_reclaim_bgs_work(struct work_struct *work);