linux-kernel - Re: [PATCH] hugepage: allow parallelization of the hugepage fault path

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Message-ID: <20130719071432.GB19634@voom.fritz.box>
Date:	Fri, 19 Jul 2013 17:14:32 +1000
From:	David Gibson <david@...son.dropbear.id.au>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Michal Hocko <mhocko@...e.cz>,
	"AneeshKumarK.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hillf Danton <dhillf@...il.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Eric B Munson <emunson@...bm.net>,
	Anton Blanchard <anton@...ba.org>
Subject: Re: [PATCH] hugepage: allow parallelization of the hugepage fault
 path

On Thu, Jul 18, 2013 at 05:42:35PM +0900, Joonsoo Kim wrote:
> On Wed, Jul 17, 2013 at 12:50:25PM -0700, Davidlohr Bueso wrote:
> > From: David Gibson <david@...son.dropbear.id.au>
> > 
> > At present, the page fault path for hugepages is serialized by a
> > single mutex. This is used to avoid spurious out-of-memory conditions
> > when the hugepage pool is fully utilized (two processes or threads can
> > race to instantiate the same mapping with the last hugepage from the
> > pool, the race loser returning VM_FAULT_OOM).  This problem is
> > specific to hugepages, because it is normal to want to use every
> > single hugepage in the system - with normal pages we simply assume
> > there will always be a few spare pages which can be used temporarily
> > until the race is resolved.
> > 
> > Unfortunately this serialization also means that clearing of hugepages
> > cannot be parallelized across multiple CPUs, which can lead to very
> > long process startup times when using large numbers of hugepages.
> > 
> > This patch improves the situation by replacing the single mutex with a
> > table of mutexes, selected based on a hash, which allows us to know
> > which page in the file we're instantiating. For shared mappings, the
> > hash key is selected based on the address space and file offset being faulted.
> > Similarly, for private mappings, the mm and virtual address are used.
> > 
> 
> Hello.
> 
> With this table mutex, we cannot protect region tracking structure.
> See below comment.
> 
> /*
>  * Region tracking -- allows tracking of reservations and instantiated pages
>  *                    across the pages in a mapping.
>  *
>  * The region data structures are protected by a combination of the mmap_sem
>  * and the hugetlb_instantion_mutex.  To access or modify a region the caller
>  * must either hold the mmap_sem for write, or the mmap_sem for read and
>  * the hugetlb_instantiation mutex:
>  *
>  *      down_write(&mm->mmap_sem);
>  * or
>  *      down_read(&mm->mmap_sem);
>  *      mutex_lock(&hugetlb_instantiation_mutex);
>  */

Ugh.  Who the hell added that.  I guess you'll need to split of
another mutex for that purpose, afaict there should be no interaction
with the actual, intended purpose of the instantiation mutex.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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