From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f200.google.com (mail-pf0-f200.google.com [209.85.192.200]) by kanga.kvack.org (Postfix) with ESMTP id BA20F6B0005 for ; Thu, 28 Jun 2018 20:59:55 -0400 (EDT) Received: by mail-pf0-f200.google.com with SMTP id d4-v6so3568510pfn.9 for ; Thu, 28 Jun 2018 17:59:55 -0700 (PDT) Received: from out4438.biz.mail.alibaba.com (out4438.biz.mail.alibaba.com. [47.88.44.38]) by mx.google.com with ESMTPS id y124-v6si7061751pgy.228.2018.06.28.17.59.52 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 28 Jun 2018 17:59:54 -0700 (PDT) Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping From: Yang Shi 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 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: owner-linux-mm@kvack.org List-ID: 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 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) >>> A A A A A A A A A A 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 >> A A A A down_write >> A A A A split & set up VM_DEAD >> A A A A downgrade_write >> A A A A unmap >> A A A A upgrade_read >> A A A A zap ptes >> A A A A 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. >