From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A322AC43331 for ; Tue, 12 Nov 2019 08:04:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7837C21872 for ; Tue, 12 Nov 2019 08:04:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573545845; bh=6SnOfqdXhqaOls6U29IR2NKAD/SMfAUwGfpxBSUmbL8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=tK7eFWfxBdpDByl5EaYqb6ciQgyvzaweUAcJzLTGQUxiIQLTMX5x+2phXpvrQ9Mal h68LU5PglOfIrHA3+Z8I9tHIJ9BaLi3n8oGhD31+kzREOC5XpB/t1qa2jrg6kkiRAm I73qbVkUy/C/MSf46s73CYYDsl7WhdNH6Mz3wqRg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726939AbfKLIEE (ORCPT ); Tue, 12 Nov 2019 03:04:04 -0500 Received: from mx2.suse.de ([195.135.220.15]:34054 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725283AbfKLIEE (ORCPT ); Tue, 12 Nov 2019 03:04:04 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 33614B156; Tue, 12 Nov 2019 08:04:02 +0000 (UTC) Date: Tue, 12 Nov 2019 09:04:01 +0100 From: Michal Hocko To: Yang Shi Cc: mgorman@techsingularity.net, vbabka@suse.cz, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: migrate: handle freed page at the first place Message-ID: <20191112080401.GA2763@dhcp22.suse.cz> References: <1573510165-113395-1-git-send-email-yang.shi@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1573510165-113395-1-git-send-email-yang.shi@linux.alibaba.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 12-11-19 06:09:25, Yang Shi wrote: > When doing migration if the freed page is met, we just return without > migrating it since it is pointless to migrate a freed page. But, the > current code did two things before handling freed page: > > 1. Return -ENOMEM if the page is THP and THP migration is not supported. > 2. Allocate target page unconditionally. > > Both makes not too much sense. If we handle freed page at the first place > we don't have to worry about allocating/freeing target page and split > THP at all. > > For example (worst case) if we are trying to migrate a freed THP without > THP migration supported, the migrate_pages() would just split the THP then > retry to migrate base pages one by one by pointless allocating and freeing > pages, this is just waste of time. > > I didn't run into any actual problem with the current code (or I may > just not notice it yet), it was found by visual inspection. It would be preferable to accompany a change like this with some actual numbers. A race with page freeing should be a very rare situation. Maybe it is not under some workloads but that would better be checked and documented. I also do not like to do page state changes for THP migration without a support. I cannot really say this is 100% correct from top of my head and I do not see a sufficient justification to go and chase all those tiny details because that is time consuming. > Cc: Michal Hocko > Cc: Mel Gorman > Cc: Vlastimil Babka > Signed-off-by: Yang Shi > --- > mm/migrate.c | 18 +++++++----------- > 1 file changed, 7 insertions(+), 11 deletions(-) > > diff --git a/mm/migrate.c b/mm/migrate.c > index 4fe45d1..ef96997 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -1170,13 +1170,6 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page, > int rc = MIGRATEPAGE_SUCCESS; > struct page *newpage; > > - if (!thp_migration_supported() && PageTransHuge(page)) > - return -ENOMEM; > - > - newpage = get_new_page(page, private); > - if (!newpage) > - return -ENOMEM; > - > if (page_count(page) == 1) { > /* page was freed from under us. So we are done. */ > ClearPageActive(page); > @@ -1187,13 +1180,16 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page, > __ClearPageIsolated(page); > unlock_page(page); > } > - if (put_new_page) > - put_new_page(newpage, private); > - else > - put_page(newpage); > goto out; > } > > + if (!thp_migration_supported() && PageTransHuge(page)) > + return -ENOMEM; > + > + newpage = get_new_page(page, private); > + if (!newpage) > + return -ENOMEM; > + > rc = __unmap_and_move(page, newpage, force, mode); > if (rc == MIGRATEPAGE_SUCCESS) > set_page_owner_migrate_reason(newpage, reason); > -- > 1.8.3.1 > -- Michal Hocko SUSE Labs