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>

1 files changed, 12 insertions, 16 deletions

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index f9544fda38e9..1502d664c892 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -550,8 +550,7 @@ bool btrfs_finish_block_group_to_copy(struct btrfs_device *srcdev,
 				      u64 physical)
 {
 	struct btrfs_fs_info *fs_info = cache->fs_info;
-	struct extent_map *em;
-	struct map_lookup *map;
+	struct btrfs_chunk_map *map;
 	u64 chunk_offset = cache->start;
 	int num_extents, cur_extent;
 	int i;
@@ -567,9 +566,8 @@ bool btrfs_finish_block_group_to_copy(struct btrfs_device *srcdev,
 	}
 	spin_unlock(&cache->lock);
 
-	em = btrfs_get_chunk_map(fs_info, chunk_offset, 1);
-	ASSERT(!IS_ERR(em));
-	map = em->map_lookup;
+	map = btrfs_get_chunk_map(fs_info, chunk_offset, 1);
+	ASSERT(!IS_ERR(map));
 
 	num_extents = 0;
 	cur_extent = 0;
@@ -583,7 +581,7 @@ bool btrfs_finish_block_group_to_copy(struct btrfs_device *srcdev,
 			cur_extent = i;
 	}
 
-	free_extent_map(em);
+	btrfs_free_chunk_map(map);
 
 	if (num_extents > 1 && cur_extent < num_extents - 1) {
 		/*
@@ -812,25 +810,23 @@ static void btrfs_dev_replace_update_device_in_mapping_tree(
 						struct btrfs_device *srcdev,
 						struct btrfs_device *tgtdev)
 {
-	struct extent_map_tree *em_tree = &fs_info->mapping_tree;
-	struct extent_map *em;
-	struct map_lookup *map;
 	u64 start = 0;
 	int i;
 
-	write_lock(&em_tree->lock);
+	write_lock(&fs_info->mapping_tree_lock);
 	do {
-		em = lookup_extent_mapping(em_tree, start, (u64)-1);
-		if (!em)
+		struct btrfs_chunk_map *map;
+
+		map = btrfs_find_chunk_map_nolock(fs_info, start, U64_MAX);
+		if (!map)
 			break;
-		map = em->map_lookup;
 		for (i = 0; i < map->num_stripes; i++)
 			if (srcdev == map->stripes[i].dev)
 				map->stripes[i].dev = tgtdev;
-		start = em->start + em->len;
-		free_extent_map(em);
+		start = map->start + map->chunk_len;
+		btrfs_free_chunk_map(map);
 	} while (start);
-	write_unlock(&em_tree->lock);
+	write_unlock(&fs_info->mapping_tree_lock);
 }
 
 static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,