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

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Message-ID: <20130718084235.GA9761@lge.com>
Date:	Thu, 18 Jul 2013 17:42:35 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Davidlohr Bueso <davidlohr.bueso@...com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Gibson <david@...son.dropbear.id.au>,
	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 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);
 */

Thanks.
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