[PATCH v3] btrfs: reduce chunk_map lookups in btrfs_map_block

[Johannes Thumshirn <jth@kernel.org>]

From: Johannes Thumshirn <johannes.thumshirn@wdc.com>

Currently we're calling btrfs_num_copies() before btrfs_get_chunk_map() in
btrfs_map_block(). But btrfs_num_copies() itself does a chunk map lookup
to be able to calculate the number of copies.

So split out the code getting the number of copies from btrfs_num_copies()
into a helper called btrfs_chunk_map_num_copies() and directly call it
from btrfs_map_block() and btrfs_num_copies().

This saves us one rbtree lookup per btrfs_map_block() invocation.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
Changes in v2:
- Added Reviewed-bys
- Reflowed comments
- Moved non RAID56 cases to the end without an if
Link to v1:
https://lore.kernel.org/all/20240812165931.9106-1-jth@kernel.org/

Changes in v3:
- constified btrfs_chunk_map_num_copies() parameter
- fixed accidentially changed comment
Link to v2:
https://lore.kernel.org/all/bc73d318c7f24196cdc7305b6a6ce516fb4fc81d.1723546054.git.jth@kernel.org/

 fs/btrfs/volumes.c | 49 +++++++++++++++++++++++++---------------------
 1 file changed, 27 insertions(+), 22 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index e07452207426..4b9b647a7e29 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5781,11 +5781,31 @@ void btrfs_mapping_tree_free(struct btrfs_fs_info *fs_info)
 	write_unlock(&fs_info->mapping_tree_lock);
 }
 
+static int btrfs_chunk_map_num_copies(const struct btrfs_chunk_map *map)
+{
+	enum btrfs_raid_types index = btrfs_bg_flags_to_raid_index(map->type);
+
+	if (map->type & BTRFS_BLOCK_GROUP_RAID5)
+		return 2;
+
+	/*
+	 * There could be two corrupted data stripes, we need to loop
+	 * retry in order to rebuild the correct data.
+	 *
+	 * Fail a stripe at a time on every retry except the stripe
+	 * under reconstruction.
+	 */
+	if (map->type & BTRFS_BLOCK_GROUP_RAID6)
+		return map->num_stripes;
+
+	/* Non-RAID56, use their ncopies from btrfs_raid_array. */
+	return btrfs_raid_array[index].ncopies;
+}
+
 int btrfs_num_copies(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
 {
 	struct btrfs_chunk_map *map;
-	enum btrfs_raid_types index;
-	int ret = 1;
+	int ret;
 
 	map = btrfs_get_chunk_map(fs_info, logical, len);
 	if (IS_ERR(map))
@@ -5797,22 +5817,7 @@ int btrfs_num_copies(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
 		 */
 		return 1;
 
-	index = btrfs_bg_flags_to_raid_index(map->type);
-
-	/* Non-RAID56, use their ncopies from btrfs_raid_array. */
-	if (!(map->type & BTRFS_BLOCK_GROUP_RAID56_MASK))
-		ret = btrfs_raid_array[index].ncopies;
-	else if (map->type & BTRFS_BLOCK_GROUP_RAID5)
-		ret = 2;
-	else if (map->type & BTRFS_BLOCK_GROUP_RAID6)
-		/*
-		 * There could be two corrupted data stripes, we need
-		 * to loop retry in order to rebuild the correct data.
-		 *
-		 * Fail a stripe at a time on every retry except the
-		 * stripe under reconstruction.
-		 */
-		ret = map->num_stripes;
+	ret = btrfs_chunk_map_num_copies(map);
 	btrfs_free_chunk_map(map);
 	return ret;
 }
@@ -6462,14 +6467,14 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	io_geom.stripe_index = 0;
 	io_geom.op = op;
 
-	num_copies = btrfs_num_copies(fs_info, logical, fs_info->sectorsize);
-	if (io_geom.mirror_num > num_copies)
-		return -EINVAL;
-
 	map = btrfs_get_chunk_map(fs_info, logical, *length);
 	if (IS_ERR(map))
 		return PTR_ERR(map);
 
+	num_copies = btrfs_chunk_map_num_copies(map);
+	if (io_geom.mirror_num > num_copies)
+		return -EINVAL;
+
 	map_offset = logical - map->start;
 	io_geom.raid56_full_stripe_start = (u64)-1;
 	max_len = btrfs_max_io_len(map, map_offset, &io_geom);
-- 
2.43.0