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=-0.8 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 88251C6778A for ; Fri, 29 Jun 2018 16:45:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4883B2150A for ; Fri, 29 Jun 2018 16:45:36 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4883B2150A 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 S1030263AbeF2Qpe (ORCPT ); Fri, 29 Jun 2018 12:45:34 -0400 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:48465 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030235AbeF2Qpd (ORCPT ); Fri, 29 Jun 2018 12:45:33 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R121e4;CH=green;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04400;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=13;SR=0;TI=SMTPD_---0T3ctADR_1530290702; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0T3ctADR_1530290702) by smtp.aliyun-inc.com(127.0.0.1); Sat, 30 Jun 2018 00:45:06 +0800 Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping To: Michal Hocko Cc: Peter Zijlstra , Nadav Amit , Matthew Wilcox , ldufour@linux.vnet.ibm.com, Andrew Morton , Ingo Molnar , acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, "open list:MEMORY MANAGEMENT" , linux-kernel@vger.kernel.org References: <158a4e4c-d290-77c4-a595-71332ede392b@linux.alibaba.com> <20180620071817.GJ13685@dhcp22.suse.cz> <263935d9-d07c-ab3e-9e42-89f73f57be1e@linux.alibaba.com> <20180626074344.GZ2458@hirez.programming.kicks-ass.net> <20180627072432.GC32348@dhcp22.suse.cz> <20180628115101.GE32348@dhcp22.suse.cz> <2ecdb667-f4de-673d-6a5f-ee50df505d0c@linux.alibaba.com> <20180629113447.GA5963@dhcp22.suse.cz> From: Yang Shi Message-ID: Date: Fri, 29 Jun 2018 09:45:01 -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: <20180629113447.GA5963@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/29/18 4:34 AM, Michal Hocko wrote: > On Thu 28-06-18 12:10:10, Yang Shi wrote: >> >> On 6/28/18 4:51 AM, Michal Hocko wrote: >>> On Wed 27-06-18 10:23:39, Yang Shi wrote: >>>> On 6/27/18 12:24 AM, Michal Hocko wrote: >>>>> On Tue 26-06-18 18:03:34, Yang Shi wrote: >>>>>> On 6/26/18 12:43 AM, Peter Zijlstra wrote: >>>>>>> On Mon, Jun 25, 2018 at 05:06:23PM -0700, Yang Shi wrote: >>>>>>>> By looking this deeper, we may not be able to cover all the unmapping range >>>>>>>> for VM_DEAD, for example, if the start addr is in the middle of a vma. We >>>>>>>> can't set VM_DEAD to that vma since that would trigger SIGSEGV for still >>>>>>>> mapped area. >>>>>>>> >>>>>>>> splitting can't be done with read mmap_sem held, so maybe just set VM_DEAD >>>>>>>> to non-overlapped vmas. Access to overlapped vmas (first and last) will >>>>>>>> still have undefined behavior. >>>>>>> Acquire mmap_sem for writing, split, mark VM_DEAD, drop mmap_sem. Acquire >>>>>>> mmap_sem for reading, madv_free drop mmap_sem. Acquire mmap_sem for >>>>>>> writing, free everything left, drop mmap_sem. >>>>>>> >>>>>>> ? >>>>>>> >>>>>>> Sure, you acquire the lock 3 times, but both write instances should be >>>>>>> 'short', and I suppose you can do a demote between 1 and 2 if you care. >>>>>> Thanks, Peter. Yes, by looking the code and trying two different approaches, >>>>>> it looks this approach is the most straight-forward one. >>>>> Yes, you just have to be careful about the max vma count limit. >>>> Yes, we should just need copy what do_munmap does as below: >>>> >>>> if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count) >>>>             return -ENOMEM; >>>> >>>> If the mas map count limit has been reached, it will return failure before >>>> zapping mappings. >>> Yeah, but as soon as you drop the lock and retake it, somebody might >>> have changed the adddress space and we might get inconsistency. >>> >>> So I am wondering whether we really need upgrade_read (to promote read >>> to write lock) and do the >>> down_write >>> split & set up VM_DEAD >>> downgrade_write >>> unmap >>> upgrade_read >>> zap ptes >>> up_write >> I'm supposed address space changing just can be done by mmap, mremap, >> mprotect. If so, we may utilize the new VM_DEAD flag. If the VM_DEAD flag is >> set for the vma, just return failure since it is being unmapped. > I am sorry I do not follow. How does VM_DEAD flag helps for a completely > unrelated vmas? Or maybe it would be better to post the code to see what > you mean exactly. I mean we just care about the vmas which have been found/split by munmap, right? We already set VM_DEAD to them. Even though those other vmas are changed by somebody else, it would not cause any inconsistency to this munmap call.