linux-kernel - Re: [PATCH V7] Allow compaction of unevictable pages

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Date:	Sat, 21 Mar 2015 14:33:11 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Eric B Munson <emunson@...mai.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Lameter <cl@...ux.com>, Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Michal Hocko <mhocko@...e.cz>, linux-doc@...r.kernel.org,
	linux-rt-users@...r.kernel.org, linux-mm@...ck.org,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V7] Allow compaction of unevictable pages

On Fri, Mar 20, 2015 at 03:17:03PM -0700, Andrew Morton wrote:
> On Fri, 20 Mar 2015 09:49:50 -0400 Eric B Munson <emunson@...mai.com> wrote:
> 
> > Currently, pages which are marked as unevictable are protected from
> > compaction, but not from other types of migration.  The POSIX real time
> > extension explicitly states that mlock() will prevent a major page
> > fault, but the spirit of this is that mlock() should give a process the
> > ability to control sources of latency, including minor page faults.
> > However, the mlock manpage only explicitly says that a locked page will
> > not be written to swap and this can cause some confusion.  The
> > compaction code today does not give a developer who wants to avoid swap
> > but wants to have large contiguous areas available any method to achieve
> > this state.  This patch introduces a sysctl for controlling compaction
> > behavior with respect to the unevictable lru.  Users that demand no page
> > faults after a page is present can set compact_unevictable_allowed to 0
> > and users who need the large contiguous areas can enable compaction on
> > locked memory by leaving the default value of 1.
> 
> Do we really really really need the /proc knob?  We're already
> migrating these pages so users of mlock will occasionally see some
> latency - how likely is it that this patch will significantly damage
> anyone?

-rt disables everything that causes those migrations (with exception of
sys_migrate_pages).

And like I've argued before; mlock() is part of the posix real-time
extensions and the real-time people really do not want those migrations.
And while the letter of the posix spec allows migrations, the spirit
clearly does not.


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