linux-kernel - Re: [PATCH -mm 1/3] slub: don't fail kmem_cache_shrink if slab placement optimization fails

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Date:	Mon, 26 Jan 2015 09:48:00 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 1/3] slub: don't fail kmem_cache_shrink if slab
 placement optimization fails

On Mon, 26 Jan 2015, Vladimir Davydov wrote:

> SLUB's kmem_cache_shrink not only removes empty slabs from the cache,
> but also sorts slabs by the number of objects in-use to cope with
> fragmentation. To achieve that, it tries to allocate a temporary array.
> If it fails, it will abort the whole procedure.

I do not think its worth optimizing this. If we cannot allocate even a
small object then the system is in an extremely bad state anyways.

> @@ -3400,7 +3407,9 @@ int __kmem_cache_shrink(struct kmem_cache *s)
>  		 * list_lock. page->inuse here is the upper limit.
>  		 */
>  		list_for_each_entry_safe(page, t, &n->partial, lru) {
> -			list_move(&page->lru, slabs_by_inuse + page->inuse);
> +			if (page->inuse < objects)
> +				list_move(&page->lru,
> +					  slabs_by_inuse + page->inuse);
>  			if (!page->inuse)
>  				n->nr_partial--;
>  		}

The condition is always true. A page that has page->inuse == objects
would not be on the partial list.

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