mm: vmscan: protect shrinker idr replace with CONFIG_MEMCG

Since commit 0a432dc ("mm: shrinker: make shrinker not depend on
memcg kmem"), shrinkers' idr is protected by CONFIG_MEMCG instead of
CONFIG_MEMCG_KMEM, so it makes no sense to protect shrinker idr replace
with CONFIG_MEMCG_KMEM.

And in the CONFIG_MEMCG && CONFIG_SLOB case, shrinker_idr contains only
shrinker, and it is deferred_split_shrinker.  But it is never actually
called, since idr_replace() is never compiled due to the wrong #ifdef.
The deferred_split_shrinker all the time is staying in half-registered
state, and it's never called for subordinate mem cgroups.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 0a432dc ("mm: shrinker: make shrinker not depend on memcg kmem")
Signed-off-by: Yang Shi <[email protected]>
Reviewed-by: Kirill Tkhai <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: <[email protected]>	[5.4+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Yang Shi

mm/vmscan.c

@@ -387,7 +387,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
 {
 	down_write(&shrinker_rwsem);
 	list_add_tail(&shrinker->list, &shrinker_list);
-#ifdef CONFIG_MEMCG_KMEM
+#ifdef CONFIG_MEMCG
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
 		idr_replace(&shrinker_idr, shrinker, shrinker->id);
 #endif