linux-kernel - Re: [PATCH] kernel: fs: drop_caches: add dds drop_caches

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Date:	Tue, 16 Feb 2016 19:35:18 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Nag Avadhanam <nag@...co.com>
Cc:	Theodore Ts'o <tytso@....edu>,
	"Daniel Walker (danielwa)" <danielwa@...co.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Khalid Mughal (khalidm)" <khalidm@...co.com>,
	"xe-kernel@...ernal.cisco.com" <xe-kernel@...ernal.cisco.com>,
	"dave.hansen@...el.com" <dave.hansen@...el.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	"riel@...hat.com" <riel@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] kernel: fs: drop_caches: add dds drop_caches_count

On Mon, Feb 15, 2016 at 11:14:13PM -0800, Nag Avadhanam wrote:
> On Mon, 15 Feb 2016, Dave Chinner wrote:
> 
> >On Tue, Feb 16, 2016 at 02:58:04AM +0000, Nag Avadhanam (nag) wrote:
> >>Its the calculation of the # of bytes of non-reclaimable file system cache
> >>pages that has been troubling us. We do not want to count inactive file
> >>pages (of programs/binaries) that were once mapped by any process in the
> >>system as reclaimable because that might lead to thrashing under memory
> >>pressure (we want to alert admins before system starts dropping text
> >>pages).
> >
> >The code presented does not match your requirements. It only counts
> >pages that are currently mapped into ptes. hence it will tell you
> >that once-used and now unmapped binary pages are reclaimable, and
> >drop caches will reclaim them. hence they'll need to be fetched from
> >disk again if they are faulted in again after a drop_caches run.
> 
> Will the inactive binary pages be automatically unmapped even if the process
> into whose address space they are mapped is still around? I thought they
> are left mapped until such time there is memory pressure.

Right, page reclaim via memory pressure can unmap mapped pages in
order to reclaim them. Drop caches will skip them.

> We only care for binary pages (active and inactive) mapped into the
> address spaces of live processes. Its okay to aggressively reclaim
> inactive
> pages once mapped into processes that are no longer around.

Ok, if you're only concerned about live processes then drop caches
should behave as you want.

Cheers,

Dave.

-- 
Dave Chinner
david@...morbit.com