linux-kernel - Re: [PATCH 2/2] mm, vmscan: do not overestimate anonymous reclaimable pages

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Date:	Tue, 24 Nov 2015 14:37:10 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm, vmscan: do not overestimate anonymous
 reclaimable pages

On Tue 24-11-15 16:07:40, Vladimir Davydov wrote:
> On Tue, Nov 24, 2015 at 12:55:00PM +0100, Michal Hocko wrote:
> > zone_reclaimable_pages considers all anonymous pages on LRUs reclaimable
> > if there is at least one entry on the swap storage left. This can be
> > really misleading when the swap is short on space and skew reclaim
> > decisions based on zone_reclaimable_pages. Fix this by clamping the
> > number to the minimum of the available swap space and anon LRU pages.
> 
> Suppose there's 100M of swap and 1G of anon pages. This patch makes
> zone_reclaimable_pages return 100M instead of 1G in this case. If you
> rotate 600M of oldest anon pages, which is quite possible,
> zone_reclaimable will start returning false, which is wrong, because
> there are still 400M pages that were not even scanned, besides those
> 600M of rotated pages could have become reclaimable after their ref bits
> got cleared.

Uhm, OK, I guess you are right. Making zone_reclaimable less
conservative can lead to hard to expect results. Scratch this patch
please.
 
> I think it is the name of zone_reclaimable_pages which is misleading. It
> should be called something like "zone_scannable_pages" judging by how it
> is used in zone_reclaimable.

Thanks!
-- 
Michal Hocko
SUSE Labs
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