btrfs: don't use size classes for zoned file systems

When a file system has ZNS devices which are constrained by a maximum
number of active block groups, then not being able to use all the block
groups for every allocation is not ideal, and could cause us to loop a
ton with mixed size allocations.

In general, since zoned doesn't write into gaps behind where block
groups are writing, it is not susceptible to the same sort of
fragmentation that size classes are designed to solve, so we can skip
size classes for zoned file systems in general, even though there would
probably be no harm for SMR devices.
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -665,7 +665,7 @@ static int load_block_group_size_class(struct btrfs_caching_control *caching_ctl
 	enum btrfs_block_group_size_class size_class = BTRFS_BG_SZ_NONE;
 	int ret;
 
-	if (!btrfs_is_block_group_data_only(block_group))
+	if (!btrfs_block_group_should_use_size_class(block_group))
 		return 0;
 
 	for (i = 0; i < 5; ++i) {
@@ -3597,7 +3597,7 @@ int btrfs_add_reserved_bytes(struct btrfs_block_group *cache,
 		goto out;
 	}
 
-	if (btrfs_is_block_group_data_only(cache)) {
+	if (btrfs_block_group_should_use_size_class(cache)) {
 		size_class = btrfs_calc_block_group_size_class(num_bytes);
 		ret = btrfs_use_block_group_size_class(cache, size_class, force_wrong_size_class);
 		if (ret)
@@ -4439,3 +4439,12 @@ int btrfs_use_block_group_size_class(struct btrfs_block_group *bg,
 
 	return 0;
 }
+
+bool btrfs_block_group_should_use_size_class(struct btrfs_block_group *bg)
+{
+	if (btrfs_is_zoned(bg->fs_info))
+		return false;
+	if (!btrfs_is_block_group_data_only(bg))
+		return false;
+	return true;
+}

fs/btrfs/block-group.h

@@ -363,5 +363,6 @@ enum btrfs_block_group_size_class btrfs_calc_block_group_size_class(u64 size);
 int btrfs_use_block_group_size_class(struct btrfs_block_group *bg,
 				     enum btrfs_block_group_size_class size_class,
 				     bool force_wrong_size_class);
+bool btrfs_block_group_should_use_size_class(struct btrfs_block_group *bg);
 
 #endif /* BTRFS_BLOCK_GROUP_H */

fs/btrfs/extent-tree.c

@@ -4064,7 +4064,7 @@ static bool find_free_extent_check_size_class(struct find_free_extent_ctl *ffe_c
 {
 	if (ffe_ctl->policy == BTRFS_EXTENT_ALLOC_ZONED)
 		return true;
-	if (!btrfs_is_block_group_data_only(bg))
+	if (!btrfs_block_group_should_use_size_class(bg))
 		return true;
 	if (ffe_ctl->loop >= LOOP_WRONG_SIZE_CLASS)
 		return true;