linux-kernel - Re: [PATCH -mm v9 7/8] proc: export idle flag via kpageflags

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Message-Id: <20150722124421.3313e8f007d76b386e1d61ec@linux-foundation.org>
Date:	Wed, 22 Jul 2015 12:44:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	"Greg Thelen" <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	"David Rientjes" <rientjes@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Jonathan Corbet <corbet@....net>, <linux-api@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm v9 7/8] proc: export idle flag via kpageflags

On Wed, 22 Jul 2015 19:25:28 +0300 Vladimir Davydov <vdavydov@...allels.com> wrote:

> On Tue, Jul 21, 2015 at 04:35:00PM -0700, Andrew Morton wrote:
> > On Sun, 19 Jul 2015 15:31:16 +0300 Vladimir Davydov <vdavydov@...allels.com> wrote:
> > 
> > > As noted by Minchan, a benefit of reading idle flag from
> > > /proc/kpageflags is that one can easily filter dirty and/or unevictable
> > > pages while estimating the size of unused memory.
> > > 
> > > Note that idle flag read from /proc/kpageflags may be stale in case the
> > > page was accessed via a PTE, because it would be too costly to iterate
> > > over all page mappings on each /proc/kpageflags read to provide an
> > > up-to-date value. To make sure the flag is up-to-date one has to read
> > > /proc/kpageidle first.
> > 
> > Is there any value in teaching the regular old page scanner to update
> > these flags?  If it's doing an rmap scan anyway...
> 
> I don't understand what you mean by "regular old page scanner". Could
> you please elaborate?

Whenever kswapd or direct reclaim perform an rmap scan, take that as an
opportunity to also update PageIdle().

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