linux-kernel - Re: [PATCH -mm v3 7/8] slub: make dead memcg caches discard free slabs immediately

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Date:	Tue, 24 Jun 2014 16:50:11 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	akpm@...ux-foundation.org, cl@...ux.com, rientjes@...gle.com,
	penberg@...nel.org, hannes@...xchg.org, mhocko@...e.cz,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mm v3 7/8] slub: make dead memcg caches discard free
 slabs immediately

On Fri, Jun 13, 2014 at 12:38:21AM +0400, Vladimir Davydov wrote:
> Since a dead memcg cache is destroyed only after the last slab allocated
> to it is freed, we must disable caching of empty slabs for such caches,
> otherwise they will be hanging around forever.
> 
> This patch makes SLUB discard dead memcg caches' slabs as soon as they
> become empty. To achieve that, it disables per cpu partial lists for
> dead caches (see put_cpu_partial) and forbids keeping empty slabs on per
> node partial lists by setting cache's min_partial to 0 on
> kmem_cache_shrink, which is always called on memcg offline (see
> memcg_unregister_all_caches).
> 
> Signed-off-by: Vladimir Davydov <vdavydov@...allels.com>
> Thanks-to: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
>  mm/slub.c |   11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 52565a9426ef..0d2d1978e62c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2064,6 +2064,14 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
>  
>  	} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
>  								!= oldpage);
> +
> +	if (memcg_cache_dead(s)) {
> +		unsigned long flags;
> +
> +		local_irq_save(flags);
> +		unfreeze_partials(s, this_cpu_ptr(s->cpu_slab));
> +		local_irq_restore(flags);
> +	}
>  #endif
>  }
>  
> @@ -3409,6 +3417,9 @@ int __kmem_cache_shrink(struct kmem_cache *s)
>  		kmalloc(sizeof(struct list_head) * objects, GFP_KERNEL);
>  	unsigned long flags;
>  
> +	if (memcg_cache_dead(s))
> +		s->min_partial = 0;
> +
>  	if (!slabs_by_inuse) {
>  		/*
>  		 * Do not fail shrinking empty slabs if allocation of the

I think that you should move down n->nr_partial test after holding the
lock in __kmem_cache_shrink(). Access to n->nr_partial without node lock
is racy and you can see wrong value. It results in skipping to free empty
slab so your destroying logic could fail.

Thanks.
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