memcg: fix memcg_kmem_bypass() for remote memcg charging

While trying to use remote memcg charging in an out-of-tree kernel
module I found it's not working, because the current thread is a
workqueue thread.

As we will probably encounter this issue in the future as the users of
memalloc_use_memcg() grow, and it's nothing wrong for this usage, it's
better we fix it now.

Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Link: http://lkml.kernel.org/r/1d202a12-26fe-0012-ea14-f025ddcd044a@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: lizf-os

mm/memcontrol.c

@@ -2852,7 +2852,12 @@ static void memcg_schedule_kmem_cache_create(struct mem_cgroup *memcg,
 
 static inline bool memcg_kmem_bypass(void)
 {
-	if (in_interrupt() || !current->mm || (current->flags & PF_KTHREAD))
+	if (in_interrupt())
+		return true;
+
+	/* Allow remote memcg charging in kthread contexts. */
+	if ((!current->mm || (current->flags & PF_KTHREAD)) &&
+	     !current->active_memcg)
 		return true;
 	return false;
 }