From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-we0-f182.google.com (mail-we0-f182.google.com [74.125.82.182]) by kanga.kvack.org (Postfix) with ESMTP id CF63B6B0036 for ; Thu, 6 Feb 2014 18:04:27 -0500 (EST) Received: by mail-we0-f182.google.com with SMTP id u57so1813008wes.13 for ; Thu, 06 Feb 2014 15:04:27 -0800 (PST) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id jc15si180089wic.62.2014.02.06.10.48.45 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Thu, 06 Feb 2014 10:49:15 -0800 (PST) Message-ID: <52F3D912.4020607@suse.cz> Date: Thu, 06 Feb 2014 19:48:50 +0100 From: Vlastimil Babka MIME-Version: 1.0 Subject: Re: [patch v2] mm, compaction: avoid isolating pinned pages References: <20140203095329.GH6732@suse.de> <20140204000237.GA17331@lge.com> <20140204015332.GA14779@lge.com> <20140204021533.GA14924@lge.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: David Rientjes , Andrew Morton Cc: Joonsoo Kim , Hugh Dickins , Mel Gorman , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org On 5.2.2014 3:44, David Rientjes wrote: > Page migration will fail for memory that is pinned in memory with, for > example, get_user_pages(). In this case, it is unnecessary to take > zone->lru_lock or isolating the page and passing it to page migration > which will ultimately fail. > > This is a racy check, the page can still change from under us, but in > that case we'll just fail later when attempting to move the page. > > This avoids very expensive memory compaction when faulting transparent > hugepages after pinning a lot of memory with a Mellanox driver. > > On a 128GB machine and pinning ~120GB of memory, before this patch we > see the enormous disparity in the number of page migration failures > because of the pinning (from /proc/vmstat): > > compact_pages_moved 8450 > compact_pagemigrate_failed 15614415 > > 0.05% of pages isolated are successfully migrated and explicitly > triggering memory compaction takes 102 seconds. After the patch: > > compact_pages_moved 9197 > compact_pagemigrate_failed 7 > > 99.9% of pages isolated are now successfully migrated in this > configuration and memory compaction takes less than one second. > > Signed-off-by: David Rientjes > --- > v2: address page count issue per Joonsoo > > mm/compaction.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/mm/compaction.c b/mm/compaction.c > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -578,6 +578,15 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > continue; > } > > + /* > + * Migration will fail if an anonymous page is pinned in memory, > + * so avoid taking lru_lock and isolating it unnecessarily in an > + * admittedly racy check. > + */ > + if (!page_mapping(page) && > + page_count(page) > page_mapcount(page)) > + continue; > + Hm this page_count() seems it could substantially increase the chance of race with prep_compound_page that your patch "mm, page_alloc: make first_page visible before PageTail" tries to fix :) > /* Check if it is ok to still hold the lock */ > locked = compact_checklock_irqsave(&zone->lru_lock, &flags, > locked, cc); > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org