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=-2.5 required=3.0 tests=MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT 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 2405DC4321E for ; Fri, 7 Sep 2018 11:25:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DBB1520844 for ; Fri, 7 Sep 2018 11:25:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DBB1520844 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728974AbeIGQGC (ORCPT ); Fri, 7 Sep 2018 12:06:02 -0400 Received: from mx2.suse.de ([195.135.220.15]:41802 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728000AbeIGQGC (ORCPT ); Fri, 7 Sep 2018 12:06:02 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 63E92AD5A; Fri, 7 Sep 2018 11:25:29 +0000 (UTC) Date: Fri, 7 Sep 2018 13:25:28 +0200 From: Michal Hocko To: "Aneesh Kumar K.V" Cc: akpm@linux-foundation.org, Alexey Kardashevskiy , mpe@ellerman.id.au, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [RFC PATCH V2 4/4] powerpc/mm/iommu: Allow migration of cma allocated pages during mm_iommu_get Message-ID: <20180907112528.GI19621@dhcp22.suse.cz> References: <20180906054342.25094-1-aneesh.kumar@linux.ibm.com> <20180906054342.25094-4-aneesh.kumar@linux.ibm.com> <20180906125356.GX14951@dhcp22.suse.cz> <50d355bf-17d0-ee01-ec35-7f04e79ca277@linux.ibm.com> <20180907090312.GF19621@dhcp22.suse.cz> <8337fdcc-3344-04dd-ddb2-68f86912f333@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8337fdcc-3344-04dd-ddb2-68f86912f333@linux.ibm.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 Fri 07-09-18 16:45:09, Aneesh Kumar K.V wrote: > On 09/07/2018 02:33 PM, Michal Hocko wrote: > > On Thu 06-09-18 19:00:43, Aneesh Kumar K.V wrote: > > > On 09/06/2018 06:23 PM, Michal Hocko wrote: > > > > On Thu 06-09-18 11:13:42, Aneesh Kumar K.V wrote: > > > > > Current code doesn't do page migration if the page allocated is a compound page. > > > > > With HugeTLB migration support, we can end up allocating hugetlb pages from > > > > > CMA region. Also THP pages can be allocated from CMA region. This patch updates > > > > > the code to handle compound pages correctly. > > > > > > > > > > This use the new helper get_user_pages_cma_migrate. It does one get_user_pages > > > > > with right count, instead of doing one get_user_pages per page. That avoids > > > > > reading page table multiple times. > > > > > > > > > > The patch also convert the hpas member of mm_iommu_table_group_mem_t to a union. > > > > > We use the same storage location to store pointers to struct page. We cannot > > > > > update alll the code path use struct page *, because we access hpas in real mode > > > > > and we can't do that struct page * to pfn conversion in real mode. > > > > > > > > I am not fmailiar with this code so bear with me. I am completely > > > > missing the purpose of this patch. The changelog doesn't really explain > > > > that AFAICS. I can only guess that you do not want to establish long > > > > pins on CMA pages, right? So whenever you are about to pin a page that > > > > is in CMA you migrate it away to a different !__GFP_MOVABLE page, right? > > > > > > That is right. > > > > > > > If that is the case then how do you handle pins which are already in > > > > zone_movable? I do not see any specific check for those. > > > > > > > > > > > > > > Btw. why is this a proper thing to do? Problems with longterm pins are > > > > not only for CMA/ZONE_MOVABLE pages. Pinned pages are not reclaimable as > > > > well so there is a risk of OOMs if there are too many of them. We have > > > > discussed approaches that would allow to force pin invalidation/revocation > > > > at LSF/MM. Isn't that a more appropriate solution to the problem you are > > > > seeing? > > > > > > > > > > The CMA area is used on powerpc platforms to allocate guest specific page > > > table (hash page table). If we don't have sufficient free pages we fail to > > > allocate hash page table that result in failure to start guest. > > > > > > Now with vfio, we end up pinning the entire guest RAM. There is a > > > possibility that these guest RAM pages got allocated from CMA region. We > > > already do supporting migrating those pages out except for compound pages. > > > What this patch does is to start supporting compound page migration that got > > > allocated out of CMA region (ie, THP pages and hugetlb pages if platform > > > supported hugetlb migration). > > > > This definitely belongs to the changelog. > > > > > Now to do that I added a helper get_user_pages_cma_migrate(). > > > > > > I agree that long term pinned pages do have other issues. The patchset is > > > not solving that issue. > > > > It would be great to note why a generic approach is not viable. I assume > > the main reason is that those pins are pretty much permanent for the > > guest lifetime so the situation has to be handled in advance. In other > > words, more information please. > > > > That is correct. I will add these details to commit message. And will also > do a cover letter for the patch series. OK, then the early migration makes some sense. Although I suspect this will lead to other issues (OOM in kernel zones) but revocation approach is clearly not usable. An excessive pinning simply sucks. Thanks a lot for the updated information though! -- Michal Hocko SUSE Labs