linux-kernel - Re: [PATCH] memcg: don't initialize kmem-cache destroying work for root caches

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Date:	Tue, 28 May 2013 15:53:26 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andrey Vagin <avagin@...nvz.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	cgroups@...r.kernel.org,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Glauber Costa <glommer@...allels.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Balbir Singh <bsingharora@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] memcg: don't initialize kmem-cache destroying work for
 root caches

On Tue, 14 May 2013 16:38:38 +0400 Andrey Vagin <avagin@...nvz.org> wrote:

> struct memcg_cache_params has a union. Different parts of this union are
> used for root and non-root caches. A part with destroying work is used only
> for non-root caches.

That union is a bit dangerous.  Perhaps it would be better to do
something like

--- a/include/linux/slab.h~a
+++ a/include/linux/slab.h
@@ -337,15 +337,17 @@ static __always_inline int kmalloc_size(
 struct memcg_cache_params {
 	bool is_root_cache;
 	union {
-		struct kmem_cache *memcg_caches[0];
-		struct {
+		struct memcg_root_cache {
+			struct kmem_cache *caches[0];
+		} memcg_root_cache;
+		struct memcg_child_cache {
 			struct mem_cgroup *memcg;
 			struct list_head list;
 			struct kmem_cache *root_cache;
 			bool dead;
 			atomic_t nr_pages;
 			struct work_struct destroy;
-		};
+		} memcg_child_cache;
 	};
 };

And then adopt the convention of selecting either memcg_root_cache or
memcg_child_cache at the highest level then passing the more strongly
typed pointer to callees.

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