From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) by kanga.kvack.org (Postfix) with ESMTP id 554256B0010 for ; Fri, 29 Jun 2018 07:34:53 -0400 (EDT) Received: by mail-ed1-f70.google.com with SMTP id c2-v6so972717edi.20 for ; Fri, 29 Jun 2018 04:34:53 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id p63-v6si3592051edp.313.2018.06.29.04.34.49 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 29 Jun 2018 04:34:49 -0700 (PDT) Date: Fri, 29 Jun 2018 13:34:47 +0200 From: Michal Hocko Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping Message-ID: <20180629113447.GA5963@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2ecdb667-f4de-673d-6a5f-ee50df505d0c@linux.alibaba.com> 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 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. -- Michal Hocko SUSE Labs