btrfs: add a comment for new_bytenr in backref_cache_node

Add a comment for this field so we know what it is used for.  Previously
we used it to update the backref cache, so people may mistakenly think
it is useless, but in fact exists to make sure the backref cache makes
sense.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
index a810253d7b8a4d1d4f22ece186691e876be068ec..7220bde1fc31c009098cd3c33ae0dbf1a6431196 100644 (file)
--- a/fs/btrfs/backref.h
+++ b/fs/btrfs/backref.h
@@ -318,6 +318,12 @@ struct btrfs_backref_node {
                u64 bytenr;
        }; /* Use rb_simple_node for search/insert */
 
+       /*
+        * This is a sanity check, whenever we COW a block we will update
+        * new_bytenr with it's current location, and we will check this in
+        * various places to validate that the cache makes sense, it shouldn't
+        * be used for anything else.
+        */
        u64 new_bytenr;
        /* Objectid of tree block owner, can be not uptodate */
        u64 owner;