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, 5 insertions, 6 deletions

diff --git a/include/trace/events/btrfs.h b/include/trace/events/btrfs.h
index 279a7a0c90c09..4a95097ab5902 100644
--- a/include/trace/events/btrfs.h
+++ b/include/trace/events/btrfs.h
@@ -21,7 +21,7 @@ struct btrfs_delayed_data_ref;
 struct btrfs_delayed_ref_head;
 struct btrfs_block_group;
 struct btrfs_free_cluster;
-struct map_lookup;
+struct btrfs_chunk_map;
 struct extent_buffer;
 struct btrfs_work;
 struct btrfs_workqueue;
@@ -277,8 +277,7 @@ DEFINE_EVENT(btrfs__inode, btrfs_inode_evict,
 		{ (1 << EXTENT_FLAG_COMPRESSED), 	"COMPRESSED" 	},\
 		{ (1 << EXTENT_FLAG_PREALLOC), 		"PREALLOC" 	},\
 		{ (1 << EXTENT_FLAG_LOGGING),	 	"LOGGING" 	},\
-		{ (1 << EXTENT_FLAG_FILLING),	 	"FILLING" 	},\
-		{ (1 << EXTENT_FLAG_FS_MAPPING),	"FS_MAPPING"	})
+		{ (1 << EXTENT_FLAG_FILLING),		"FILLING"	})
 
 TRACE_EVENT_CONDITION(btrfs_get_extent,
 
@@ -1061,7 +1060,7 @@ DEFINE_EVENT(btrfs_delayed_ref_head,  run_delayed_ref_head,
 DECLARE_EVENT_CLASS(btrfs__chunk,
 
 	TP_PROTO(const struct btrfs_fs_info *fs_info,
-		 const struct map_lookup *map, u64 offset, u64 size),
+		 const struct btrfs_chunk_map *map, u64 offset, u64 size),
 
 	TP_ARGS(fs_info, map, offset, size),
 
@@ -1095,7 +1094,7 @@ DECLARE_EVENT_CLASS(btrfs__chunk,
 DEFINE_EVENT(btrfs__chunk,  btrfs_chunk_alloc,
 
 	TP_PROTO(const struct btrfs_fs_info *fs_info,
-		 const struct map_lookup *map, u64 offset, u64 size),
+		 const struct btrfs_chunk_map *map, u64 offset, u64 size),
 
 	TP_ARGS(fs_info, map, offset, size)
 );
@@ -1103,7 +1102,7 @@ DEFINE_EVENT(btrfs__chunk,  btrfs_chunk_alloc,
 DEFINE_EVENT(btrfs__chunk,  btrfs_chunk_free,
 
 	TP_PROTO(const struct btrfs_fs_info *fs_info,
-		 const struct map_lookup *map, u64 offset, u64 size),
+		 const struct btrfs_chunk_map *map, u64 offset, u64 size),
 
 	TP_ARGS(fs_info, map, offset, size)
 );