From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966472AbcIZLCf (ORCPT ); Mon, 26 Sep 2016 07:02:35 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:36371 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965095AbcIZLCd (ORCPT ); Mon, 26 Sep 2016 07:02:33 -0400 Date: Mon, 26 Sep 2016 13:02:31 +0200 From: Michal Hocko To: Xishi Qiu Cc: 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: <20160926110231.GE28550@dhcp22.suse.cz> References: <57E8E0BD.2070603@huawei.com> <20160926085850.GB28550@dhcp22.suse.cz> <57E8E786.8030703@huawei.com> <20160926094333.GD28550@dhcp22.suse.cz> <57E8F5CE.908@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <57E8F5CE.908@huawei.com> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -- Michal Hocko SUSE Labs