mm: check shrinker is memcg-aware in register_shrinker_prepared()

There is a sad BUG introduced in patch adding SHRINKER_REGISTERING.
shrinker_idr business is only for memcg-aware shrinkers.  Only such type
of shrinkers have id and they must be finaly installed via idr_replace()
in this function.  For !memcg-aware shrinkers we never initialize
shrinker->id field.

But there are all types of shrinkers passed to idr_replace(), and every
!memcg-aware shrinker with random ID (most probably, its id is 0)
replaces memcg-aware shrinker pointed by the ID in IDR.

This patch fixes the problem.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 7e010df "mm: use special value SHRINKER_REGISTERING instead of list_empty() check"
Signed-off-by: Kirill Tkhai <[email protected]>
Reported-by: <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Josef Bacik <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: <[email protected]>
Cc: Huang Ying <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/vmscan.c

@@ -408,7 +408,8 @@ void register_shrinker_prepared(struct shrinker *shrinker)
 	down_write(&shrinker_rwsem);
 	list_add_tail(&shrinker->list, &shrinker_list);
 #ifdef CONFIG_MEMCG_KMEM
-	idr_replace(&shrinker_idr, shrinker, shrinker->id);
+	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
+		idr_replace(&shrinker_idr, shrinker, shrinker->id);
 #endif
 	up_write(&shrinker_rwsem);
 }