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=-1.4 required=3.0 tests=DATE_IN_PAST_06_12, 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 0CE5BC43144 for ; Thu, 28 Jun 2018 19:03:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C634D22456 for ; Thu, 28 Jun 2018 19:03:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C634D22456 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 S1030608AbeF1TDy (ORCPT ); Thu, 28 Jun 2018 15:03:54 -0400 Received: from mx2.suse.de ([195.135.220.15]:40332 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S968017AbeF1TDt (ORCPT ); Thu, 28 Jun 2018 15:03:49 -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 359F1AD8B; Thu, 28 Jun 2018 19:03:46 +0000 (UTC) Date: Thu, 28 Jun 2018 13:51:01 +0200 From: Michal Hocko To: Yang Shi 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 Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping Message-ID: <20180628115101.GE32348@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 looks terrible, no question about that, but we won't drop the mmap sem at any time. -- Michal Hocko SUSE Labs