From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f71.google.com (mail-ed1-f71.google.com [209.85.208.71]) by kanga.kvack.org (Postfix) with ESMTP id B1E2C6B026A for ; Thu, 28 Jun 2018 15:03:52 -0400 (EDT) Received: by mail-ed1-f71.google.com with SMTP id c20-v6so2335882eds.21 for ; Thu, 28 Jun 2018 12:03:52 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id u12-v6si4246205edl.395.2018.06.28.12.03.48 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 28 Jun 2018 12:03:48 -0700 (PDT) Date: Thu, 28 Jun 2018 13:51:01 +0200 From: Michal Hocko 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: Sender: owner-linux-mm@kvack.org List-ID: 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 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