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

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Message-Id: <20150320151703.dfd116931ddb397ade8bfd8c@linux-foundation.org>
Date:	Fri, 20 Mar 2015 15:17:03 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Eric B Munson <emunson@...mai.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Lameter <cl@...ux.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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, 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?

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