From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752293AbcI1Fnt (ORCPT ); Wed, 28 Sep 2016 01:43:49 -0400 Received: from LGEAMRELO12.lge.com ([156.147.23.52]:40232 "EHLO lgeamrelo12.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750970AbcI1Fnj (ORCPT ); Wed, 28 Sep 2016 01:43:39 -0400 X-Original-SENDERIP: 156.147.1.125 X-Original-MAILFROM: iamjoonsoo.kim@lge.com X-Original-SENDERIP: 10.177.222.138 X-Original-MAILFROM: iamjoonsoo.kim@lge.com Date: Wed, 28 Sep 2016 14:52:03 +0900 From: Joonsoo Kim To: Michal Hocko Cc: Xishi Qiu , Mel Gorman , Johannes Weiner , Vlastimil Babka , LKML , Linux MM , Yisheng Xie Subject: Re: [RFC] mm: a question about high-order check in __zone_watermark_ok() Message-ID: <20160928055203.GE22706@js1304-P5Q-DELUXE> References: <57E8E0BD.2070603@huawei.com> <20160926085850.GB28550@dhcp22.suse.cz> <57E8E786.8030703@huawei.com> <20160926094333.GD28550@dhcp22.suse.cz> <57E8F5CE.908@huawei.com> <20160926110231.GE28550@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160926110231.GE28550@dhcp22.suse.cz> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 26, 2016 at 01:02:31PM +0200, Michal Hocko wrote: > On Mon 26-09-16 18:17:50, Xishi Qiu wrote: > > On 2016/9/26 17:43, Michal Hocko wrote: > > > > > On Mon 26-09-16 17:16:54, Xishi Qiu wrote: > > >> On 2016/9/26 16:58, Michal Hocko wrote: > > >> > > >>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote: > > >>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1 > > >>>> (mm, page_alloc: only enforce watermarks for order-0 allocations) > > >>>> rewrite the high-order check in __zone_watermark_ok(), but I think it > > >>>> quietly fix a bug. Please see the following. > > >>>> > > >>>> Before this patch, the high-order check is this: > > >>>> __zone_watermark_ok() > > >>>> ... > > >>>> for (o = 0; o < order; o++) { > > >>>> /* At the next order, this order's pages become unavailable */ > > >>>> free_pages -= z->free_area[o].nr_free << o; > > >>>> > > >>>> /* Require fewer higher order pages to be free */ > > >>>> min >>= 1; > > >>>> > > >>>> if (free_pages <= min) > > >>>> return false; > > >>>> } > > >>>> ... > > >>>> > > >>>> If we have cma memory, and we alloc a high-order movable page, then it's right. > > >>>> > > >>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()), > > >>>> and there are a lot of high-order cma pages, but little high-order unmovable > > >>>> pages, the it is still return *true*, but we will alloc *failed* finally, because > > >>>> we cannot fallback from migrate_unmovable to migrate_cma, right? > > >>> > > >>> AFAIR CMA wmark check was always tricky and the above commit has made > > >>> the situation at least a bit more clear. Anyway IIRC > > >>> > > >>> #ifdef CONFIG_CMA > > >>> /* If allocation can't use CMA areas don't use free CMA pages */ > > >>> if (!(alloc_flags & ALLOC_CMA)) > > >>> free_cma = zone_page_state(z, NR_FREE_CMA_PAGES); > > >>> #endif > > >>> > > >>> if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx]) > > >>> return false; > > >>> > > >>> should reduce the prioblem because a lot of CMA pages should just get us > > >>> below the wmark + reserve boundary. > > >> > > >> Hi Michal, > > >> > > >> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable) > > >> are also enough, but they are fragment, then it will triger the problem. > > >> If we alloc a high-order unmovable page, water mark check return *true*, but we > > >> will alloc *failed*, right? > > > > > > As Vlastimil has written. There were known issues with the wmark checks > > > and high order requests. > > > > Shall we backport to stable? > > I dunno, it was a part of a larger series with high atomic reserves and > changes which sound a bit intrusive for the stable kernel. Considering > that CMA was known to be problematic and there are still some issues > left I do not think this is worth the trouble/risk. CMA problem is known one. I mentioned it on my ZONE_CMA series v1 but removed due to Mel's high atomic reserve series. That series is rather large and has some problems so I think that it is not suitable for stable tree. Thanks.