linux-kernel - Re: [PATCH] mm: memcontrol: do not bypass slab charge if memcg is offline

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Message-ID: <20160204203328.GC8208@cmpxchg.org>
Date:	Thu, 4 Feb 2016 15:33:28 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memcontrol: do not bypass slab charge if memcg is
 offline

On Thu, Feb 04, 2016 at 03:17:55PM +0300, Vladimir Davydov wrote:
> Slab pages are charged in two steps. First, an appropriate per memcg
> cache is selected (see memcg_kmem_get_cache) basing on the current
> context, then the new slab page is charged to the memory cgroup which
> the selected cache was created for (see memcg_charge_slab ->
> __memcg_kmem_charge_memcg). It is OK to bypass kmemcg charge at step 1,
> but if step 1 succeeded and we successfully allocated a new slab page,
> step 2 must be performed, otherwise we would get a per memcg kmem cache
> which contains a slab that does not hold a reference to the memory
> cgroup owning the cache. Since per memcg kmem caches are destroyed on
> memcg css free, this could result in freeing a cache while there are
> still active objects in it.
> 
> However, currently we will bypass slab page charge if the memory cgroup
> owning the cache is offline (see __memcg_kmem_charge_memcg). This is
> very unlikely to occur in practice, because for this to happen a process
> must be migrated to a different cgroup and the old cgroup must be
> removed while the process is in kmalloc somewhere between steps 1 and 2
> (e.g.  trying to allocate a new page). Nevertheless, it's still better
> to eliminate such a possibility.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>

Acked-by: Johannes Weiner <hannes@...xchg.org>