From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f175.google.com (mail-pd0-f175.google.com [209.85.192.175]) by kanga.kvack.org (Postfix) with ESMTP id 7C5DA6B0036 for ; Thu, 6 Feb 2014 18:30:52 -0500 (EST) Received: by mail-pd0-f175.google.com with SMTP id w10so2351959pde.34 for ; Thu, 06 Feb 2014 15:30:52 -0800 (PST) Received: from mail-pa0-x231.google.com (mail-pa0-x231.google.com [2607:f8b0:400e:c03::231]) by mx.google.com with ESMTPS id q5si2439347pae.172.2014.02.06.13.33.13 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 06 Feb 2014 13:33:43 -0800 (PST) Received: by mail-pa0-f49.google.com with SMTP id hz1so2236969pad.22 for ; Thu, 06 Feb 2014 13:33:13 -0800 (PST) Date: Thu, 6 Feb 2014 13:33:11 -0800 (PST) From: David Rientjes Subject: Re: [patch v2] mm, compaction: avoid isolating pinned pages In-Reply-To: <52F3D912.4020607@suse.cz> Message-ID: References: <20140203095329.GH6732@suse.de> <20140204000237.GA17331@lge.com> <20140204015332.GA14779@lge.com> <20140204021533.GA14924@lge.com> <52F3D912.4020607@suse.cz> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Andrew Morton , Joonsoo Kim , Hugh Dickins , Mel Gorman , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org On Thu, 6 Feb 2014, Vlastimil Babka 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 :) > That's why I sent the fix for page_count(). The "racy check" the comment eludes to above concerns the fact that page_count() and page_mapcount() can change out from under us before isolation and if we had not avoided isolating them that they would have been migratable later. We accept that as a consequence of doing this in a lockless way without page references. -- 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