btrfs: add extent allocator hook to decide to allocate chunk or not

Introduce a new hook for an extent allocator policy. With the new
hook, a policy can decide to allocate a new block group or not. If
not, it will return -ENOSPC, so btrfs_reserve_extent() will cut the
allocation size in half and retry the allocation if min_alloc_size is
large enough.

The hook has a place holder and will be replaced with the real
implementation in the next patch.

CC: stable@vger.kernel.org # 5.16
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -3974,6 +3974,19 @@ static void found_extent(struct find_free_extent_ctl *ffe_ctl,
 	}
 }
 
+static bool can_allocate_chunk(struct btrfs_fs_info *fs_info,
+			       struct find_free_extent_ctl *ffe_ctl)
+{
+	switch (ffe_ctl->policy) {
+	case BTRFS_EXTENT_ALLOC_CLUSTERED:
+		return true;
+	case BTRFS_EXTENT_ALLOC_ZONED:
+		return true;
+	default:
+		BUG();
+	}
+}
+
 static int chunk_allocation_failed(struct find_free_extent_ctl *ffe_ctl)
 {
 	switch (ffe_ctl->policy) {
@@ -4061,6 +4074,10 @@ static int find_free_extent_update_loop(struct btrfs_fs_info *fs_info,
 			struct btrfs_trans_handle *trans;
 			int exist = 0;
 
+			/*Check if allocation policy allows to create a new chunk */
+			if (!can_allocate_chunk(fs_info, ffe_ctl))
+				return -ENOSPC;
+
 			trans = current->journal_info;
 			if (trans)
 				exist = 1;