linux-kernel - Re: [PATCH -mm v2] vmscan: move reclaim_state handling to shrink

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Message-ID: <20150115170726.GH11264@esperanza>
Date:	Thu, 15 Jan 2015 20:07:26 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm v2] vmscan: move reclaim_state handling to shrink_slab

On Thu, Jan 15, 2015 at 03:48:38PM +0100, Michal Hocko wrote:
> On Thu 15-01-15 16:25:16, Vladimir Davydov wrote:
> > 		memcg = mem_cgroup_iter(root, NULL, &reclaim);
> > 		do {
> > 			[...]
> > 			if (memcg && is_classzone)
> > 				shrink_slab(sc->gfp_mask, zone_to_nid(zone),
> > 					    memcg, sc->nr_scanned - scanned,
> > 					    lru_pages);
> > 
> > 			/*
> > 			 * Direct reclaim and kswapd have to scan all memory
> > 			 * cgroups to fulfill the overall scan target for the
> > 			 * zone.
> > 			 *
> > 			 * Limit reclaim, on the other hand, only cares about
> > 			 * nr_to_reclaim pages to be reclaimed and it will
> > 			 * retry with decreasing priority if one round over the
> > 			 * whole hierarchy is not sufficient.
> > 			 */
> > 			if (!global_reclaim(sc) &&
> > 					sc->nr_reclaimed >= sc->nr_to_reclaim) {
> > 				mem_cgroup_iter_break(root, memcg);
> > 				break;
> > 			}
> > 			memcg = mem_cgroup_iter(root, memcg, &reclaim);
> > 		} while (memcg);
> > 
> > 
> > If we can ignore reclaimed slab pages here (?), let's drop this patch.
> 
> I see what you are trying to achieve but can this lead to a serious
> over-reclaim?

I think it can, but only if we shrink an inode with lots of pages
attached to its address space (they also count to reclaim_state). In
this case, we overreclaim anyway though.

I agree that this is a high risk for a vague benefit. Let's drop it
until we see this problem in real life.

Thanks,
Vladimir
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