linux-kernel - Re: [PATCH v3 3/3] proc: add kpageidle file

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Date:	Wed, 29 Apr 2015 11:31:49 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Minchan Kim <minchan@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	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 v3 3/3] proc: add kpageidle file

On Wed, Apr 29, 2015 at 01:57:59PM +0900, Minchan Kim wrote:
> On Tue, Apr 28, 2015 at 03:24:42PM +0300, Vladimir Davydov wrote:
> > @@ -69,6 +69,14 @@ There are four components to pagemap:
> >     memory cgroup each page is charged to, indexed by PFN. Only available when
> >     CONFIG_MEMCG is set.
> >  
> > + * /proc/kpageidle.  For each page this file contains a 64-bit number, which
> > +   equals 1 if the page is idle or 0 otherwise, indexed by PFN. A page is
> > +   considered idle if it has not been accessed since it was marked idle. To
> > +   mark a page idle one should write 1 to this file at the offset corresponding
> > +   to the page. Only user memory pages can be marked idle, for other page types
> > +   input is silently ignored. Writing to this file beyond max PFN results in
> > +   the ENXIO error. Only available when CONFIG_IDLE_PAGE_TRACKING is set.
> > +
> 
> How about using kpageflags for reading part?
> 
> I mean PG_idle is one of the page flags and we already have a feature to
> parse of each PFN flag so we could reuse existing feature for reading
> idleness.

Reading PG_idle implies clearing all pte references to make sure the
page was not referenced via a pte. This means that exporting it via
/proc/kpageflags would increase the cost of reading this file, even for
users that don't care about PG_idle. I'm not sure all users of
/proc/kpageflags will be fine with it.

Thanks,
Vladimir
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