btrfs: document mandatory order of bio in btrfs_io_bio

Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 58b97b6..35327ef 100644 (file)
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -281,6 +281,10 @@ struct btrfs_io_bio {
        u8 *csum_allocated;
        btrfs_io_bio_end_io_t *end_io;
        struct bvec_iter iter;
+       /*
+        * This member must come last, bio_alloc_bioset will allocate enough
+        * bytes for entire btrfs_io_bio but relies on bio being last.
+        */
        struct bio bio;
 };