btrfs: be a bit more careful when setting mirror_num_ret in btrfs_map_block

The mirror_num_ret is allowed to be NULL, although it has to be set when
smap is set.  Unfortunately that is not a well enough specifiable
invariant for static type checkers, so add a NULL check to make sure they
are fine.

Fixes: 03793cbbc80f ("btrfs: add fast path for single device io in __btrfs_map_block")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 30b8744..efa19a5 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6398,7 +6398,8 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
            (op == BTRFS_MAP_READ || !dev_replace_is_ongoing ||
             !dev_replace->tgtdev)) {
                set_io_stripe(smap, map, stripe_index, stripe_offset, stripe_nr);
-               *mirror_num_ret = mirror_num;
+               if (mirror_num_ret)
+                       *mirror_num_ret = mirror_num;
                *bioc_ret = NULL;
                ret = 0;
                goto out;