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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,UNPARSEABLE_RELAY 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 8BE43C4646D for ; Wed, 8 Aug 2018 17:20:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4706F216F4 for ; Wed, 8 Aug 2018 17:20:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4706F216F4 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.alibaba.com 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 S1729338AbeHHTlB (ORCPT ); Wed, 8 Aug 2018 15:41:01 -0400 Received: from out30-133.freemail.mail.aliyun.com ([115.124.30.133]:36647 "EHLO out30-133.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727706AbeHHTlB (ORCPT ); Wed, 8 Aug 2018 15:41:01 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R501e4;CH=green;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04452;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0T6Gr7pI_1533748797; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0T6Gr7pI_1533748797) by smtp.aliyun-inc.com(127.0.0.1); Thu, 09 Aug 2018 01:20:00 +0800 Subject: Re: [RFC v6 PATCH 2/2] mm: mmap: zap pages with read mmap_sem in munmap To: Vlastimil Babka , Michal Hocko Cc: willy@infradead.org, ldufour@linux.vnet.ibm.com, kirill@shutemov.name, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1532628614-111702-1-git-send-email-yang.shi@linux.alibaba.com> <1532628614-111702-3-git-send-email-yang.shi@linux.alibaba.com> <20180803090759.GI27245@dhcp22.suse.cz> <20180806094005.GG19540@dhcp22.suse.cz> <76c0fc2b-fca7-9f22-214a-920ee2537898@linux.alibaba.com> <20180806204119.GL10003@dhcp22.suse.cz> <28de768b-c740-37b3-ea5a-8e2cb07d2bdc@linux.alibaba.com> <20180806205232.GN10003@dhcp22.suse.cz> <0cdff13a-2713-c5be-a33e-28c07e093bcc@linux.alibaba.com> <20180807054524.GQ10003@dhcp22.suse.cz> <04a22c49-fe30-63ac-c1b7-46a405c810e2@linux.alibaba.com> <3f960117-1485-9a61-8468-cb1590494e3c@suse.cz> From: Yang Shi Message-ID: <907e441d-43e2-9a57-a3fd-deb533046b6c@linux.alibaba.com> Date: Wed, 8 Aug 2018 10:19:54 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <3f960117-1485-9a61-8468-cb1590494e3c@suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/8/18 2:22 AM, Vlastimil Babka wrote: > On 08/08/2018 03:51 AM, Yang Shi wrote: >> On 8/6/18 10:45 PM, Michal Hocko wrote: >>> On Mon 06-08-18 15:19:06, Yang Shi wrote: >>>> On 8/6/18 1:52 PM, Michal Hocko wrote: >>>>> On Mon 06-08-18 13:48:35, Yang Shi wrote: >>>>>> On 8/6/18 1:41 PM, Michal Hocko wrote: >>>>>>> On Mon 06-08-18 09:46:30, Yang Shi wrote: >>>>>>>> On 8/6/18 2:40 AM, Michal Hocko wrote: >>>>>>>>> On Fri 03-08-18 14:01:58, Yang Shi wrote: >>>>>>>>>> On 8/3/18 2:07 AM, Michal Hocko wrote: >>>>>>>>>>> On Fri 27-07-18 02:10:14, Yang Shi wrote: >>>>>>> [...] >>>>>>>>>>>> If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, they are >>>>>>>>>>>> considered as special mappings. They will be dealt with before zapping >>>>>>>>>>>> pages with write mmap_sem held. Basically, just update vm_flags. >>>>>>>>>>> Well, I think it would be safer to simply fallback to the current >>>>>>>>>>> implementation with these mappings and deal with them on top. This would >>>>>>>>>>> make potential issues easier to bisect and partial reverts as well. >>>>>>>>>> Do you mean just call do_munmap()? It sounds ok. Although we may waste some >>>>>>>>>> cycles to repeat what has done, it sounds not too bad since those special >>>>>>>>>> mappings should be not very common. >>>>>>>>> VM_HUGETLB is quite spread. Especially for DB workloads. >>>>>>>> Wait a minute. In this way, it sounds we go back to my old implementation >>>>>>>> with special handling for those mappings with write mmap_sem held, right? >>>>>>> Yes, I would really start simple and add further enhacements on top. >>>>>> If updating vm_flags with read lock is safe in this case, we don't have to >>>>>> do this. The only reason for this special handling is about vm_flags update. >>>>> Yes, maybe you are right that this is safe. I would still argue to have >>>>> it in a separate patch for easier review, bisectability etc... >>>> Sorry, I'm a little bit confused. Do you mean I should have the patch >>>> *without* handling the special case (just like to assume it is safe to >>>> update vm_flags with read lock), then have the other patch on top of it, >>>> which simply calls do_munmap() to deal with the special cases? >>> Just skip those special cases in the initial implementation and handle >>> each special case in its own patch on top. >> Thanks. VM_LOCKED area will not be handled specially since it is easy to >> handle it, just follow what do_munmap does. The special cases will just >> handle VM_HUGETLB, VM_PFNMAP and uprobe mappings. > So I think you could maybe structure code like this: instead of > introducing do_munmap_zap_rlock() and all those "bool skip_vm_flags" > additions, add a boolean parameter in do_munmap() to use the new > behavior, with only the first user SYSCALL_DEFINE2(munmap) setting it to > true. If true, do_munmap() will do the > - down_write_killable() itself instead of assuming it's already locked > - munmap_lookup_vma() > - check if any of the vma's in the range is "special", if yes, change > the boolean param to "false", and continue like previously, e.g. no mmap > sem downgrade etc. Thanks for the suggestion. Actually, I did the similar thing in v1 patches, which added a bool parameter in vm_munmap() to tell if releasing mmap_sem is acceptable for some code paths. But, it got pushed back by tglx since vm_munmap() is called by x86 specific code too (and some other architectures). He suggested to define a new function to do the optimization. So, I followed this approach in the later versions. Yang > > That would be a basis for further optimizing the special vma cases in > subsequent patches (maybe it's really ok to touch the vma flags with > mmap sem for read as vma's are detached), and to eventually convert more > do_munmap() callers to the new mode. > > HTH, > Vlastimil > > >