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 C7A20C43141 for ; Fri, 29 Jun 2018 01:00:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 29B3227ABF for ; Fri, 29 Jun 2018 01:00:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 29B3227ABF 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 S935711AbeF2BAY (ORCPT ); Thu, 28 Jun 2018 21:00:24 -0400 Received: from out30-133.freemail.mail.aliyun.com ([115.124.30.133]:48558 "EHLO out30-133.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933008AbeF2BAW (ORCPT ); Thu, 28 Jun 2018 21:00:22 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R991e4;CH=green;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e01422;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=13;SR=0;TI=SMTPD_---0T3WA7qu_1530233966; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0T3WA7qu_1530233966) by smtp.aliyun-inc.com(127.0.0.1); Fri, 29 Jun 2018 08:59:31 +0800 Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping From: Yang Shi 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: <1529364856-49589-3-git-send-email-yang.shi@linux.alibaba.com> <3DDF2672-FCC4-4387-9624-92F33C309CAE@gmail.com> <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> Message-ID: Date: Thu, 28 Jun 2018 17:59:25 -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: <2ecdb667-f4de-673d-6a5f-ee50df505d0c@linux.alibaba.com> 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 6/28/18 12:10 PM, 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 Promoting to write lock may be a trouble. There might be other users in the critical section with read lock, we have to wait them to finish. > > 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. > > Does it sounds reasonable? It looks we just need care about MAP_FIXED (mmap) and MREMAP_FIXED (mremap), right? How about letting them return -EBUSY or -EAGAIN to notify the application? This changes the behavior a little bit, MAP_FIXED and mremap may fail if they fail the race with munmap (if the mapping is larger than 1GB). I'm not sure if any multi-threaded application uses MAP_FIXED and MREMAP_FIXED very heavily which may run into the race condition. I guess it should be rare to meet all the conditions to trigger the race. The programmer should be very cautious about MAP_FIXED.MREMAP_FIXED since they may corrupt its own address space as the man page noted. Thanks, Yang > > Thanks, > Yang > >> >> looks terrible, no question about that, but we won't drop the mmap sem >> at any time. >