linux-kernel - Re: [patch 10/10] mm: workingset: keep shadow entries in check

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Message-ID: <20130603152032.GF15576@cmpxchg.org>
Date:	Mon, 3 Jun 2013 11:20:32 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-mm@...ck.org, Andi Kleen <andi@...stfloor.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	metin d <metdos@...oo.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [patch 10/10] mm: workingset: keep shadow entries in check

On Mon, Jun 03, 2013 at 10:25:33AM +0200, Peter Zijlstra wrote:
> On Thu, May 30, 2013 at 02:04:06PM -0400, Johannes Weiner wrote:
> > Previously, page cache radix tree nodes were freed after reclaim
> > emptied out their page pointers.  But now reclaim stores shadow
> > entries in their place, which are only reclaimed when the inodes
> > themselves are reclaimed.  This is problematic for bigger files that
> > are still in use after they have a significant amount of their cache
> > reclaimed, without any of those pages actually refaulting.  The shadow
> > entries will just sit there and waste memory.  In the worst case, the
> > shadow entries will accumulate until the machine runs out of memory.
> > 
> 
> Can't we simply prune all refault entries that have a distance larger
> than the memory size? Then we must assume that no refault entry means
> its too old, which I think is a fair assumption.

Two workloads bound to two nodes might not push pages through the LRUs
at the same pace, so a distance might be bigger than memory due to the
faster moving node, yet still be a hit in the slower moving one.  We
can't really know until we evaluate it on a per-zone basis.
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