Subject: Re: [PATCH] mm: memcontrol: MEMCG no longer works with SLOB

From: Johannes Weiner <[email-masked]>
Date: Wed, 9 Dec 2015 15:01:07 -0500

On Wed, Dec 09, 2015 at 05:32:39PM +0100, Arnd Bergmann wrote:
> The change to move the kmem accounting into the normal memcg
> code means we can no longer use memcg with slob, which lacks
> the memcg_params member in its struct kmem_cache:
> 
> ../mm/slab.h: In function 'is_root_cache':
> ../mm/slab.h:187:10: error: 'struct kmem_cache' has no member named 'memcg_params'
> 
> This enforces the new dependency in Kconfig. Alternatively,
> we could change the slob code to allow using MEMCG.

I'm curious, was this a random config or do you actually use
CONFIG_SLOB && CONFIG_MEMCG?

Excluding CONFIG_MEMCG completely for slob seems harsh, but I would
prefer not littering the source with

#if defined(CONFIG_MEMCG) && (defined(CONFIG_SLAB) || defined(CONFIG_SLUB))

or

#if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)

for such a special case. The #ifdefs are already out of hand in there.

Vladimir, what would you think of simply doing this?

diff --git a/mm/slab.h b/mm/slab.h
index 5adec08..0b3ec4b 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -25,6 +25,9 @@ struct kmem_cache {
 	int refcount;		/* Use counter */
 	void (*ctor)(void *);	/* Called on object slot creation */
 	struct list_head list;	/* List of all slab caches on the system */
+#ifdef CONFIG_MEMCG
+	struct memcg_cache_params memcg_params;
+#endif
 };
 
 #endif /* CONFIG_SLOB */