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 579DC6B026B for ; Wed, 11 Jul 2018 13:04:58 -0400 (EDT) Received: by mail-pf0-f200.google.com with SMTP id t78-v6so16527756pfa.8 for ; Wed, 11 Jul 2018 10:04:58 -0700 (PDT) Received: from out30-132.freemail.mail.aliyun.com (out30-132.freemail.mail.aliyun.com. [115.124.30.132]) by mx.google.com with ESMTPS id d4-v6si19818274pla.81.2018.07.11.10.04.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 11 Jul 2018 10:04:57 -0700 (PDT) Subject: Re: [RFC v4 0/3] mm: zap pages with read mmap_sem in munmap for large mapping References: <1531265649-93433-1-git-send-email-yang.shi@linux.alibaba.com> <20180711111052.hbyukcwetmjjpij2@kshutemo-mobl1> From: Yang Shi Message-ID: <3d4c69c9-dd2b-30d2-5bf2-d5b108a76758@linux.alibaba.com> Date: Wed, 11 Jul 2018 10:04:48 -0700 MIME-Version: 1.0 In-Reply-To: <20180711111052.hbyukcwetmjjpij2@kshutemo-mobl1> 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: "Kirill A. Shutemov" Cc: mhocko@kernel.org, willy@infradead.org, ldufour@linux.vnet.ibm.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 7/11/18 4:10 AM, Kirill A. Shutemov wrote: > On Wed, Jul 11, 2018 at 07:34:06AM +0800, Yang Shi wrote: >> Background: >> Recently, when we ran some vm scalability tests on machines with large memory, >> we ran into a couple of mmap_sem scalability issues when unmapping large memory >> space, please refer to https://lkml.org/lkml/2017/12/14/733 and >> https://lkml.org/lkml/2018/2/20/576. >> >> >> History: >> Then akpm suggested to unmap large mapping section by section and drop mmap_sem >> at a time to mitigate it (see https://lkml.org/lkml/2018/3/6/784). >> >> V1 patch series was submitted to the mailing list per Andrew's suggestion >> (see https://lkml.org/lkml/2018/3/20/786). Then I received a lot great feedback >> and suggestions. >> >> Then this topic was discussed on LSFMM summit 2018. In the summit, Michal Hocko >> suggested (also in the v1 patches review) to try "two phases" approach. Zapping >> pages with read mmap_sem, then doing via cleanup with write mmap_sem (for >> discussion detail, see https://lwn.net/Articles/753269/) >> >> >> Approach: >> Zapping pages is the most time consuming part, according to the suggestion from >> Michal Hocko [1], zapping pages can be done with holding read mmap_sem, like >> what MADV_DONTNEED does. Then re-acquire write mmap_sem to cleanup vmas. >> >> But, we can't call MADV_DONTNEED directly, since there are two major drawbacks: >> * The unexpected state from PF if it wins the race in the middle of munmap. >> It may return zero page, instead of the content or SIGSEGV. >> * Cana??t handle VM_LOCKED | VM_HUGETLB | VM_PFNMAP and uprobe mappings, which >> is a showstopper from akpm >> >> And, some part may need write mmap_sem, for example, vma splitting. So, the >> design is as follows: >> acquire write mmap_sem >> lookup vmas (find and split vmas) >> set VM_DEAD flags >> deal with special mappings >> downgrade_write >> >> zap pages >> release mmap_sem >> >> retake mmap_sem exclusively >> cleanup vmas >> release mmap_sem >> >> Define large mapping size thresh as PUD size, just zap pages with read mmap_sem >> for mappings which are >= PUD_SIZE. So, unmapping less than PUD_SIZE area still >> goes with the regular path. >> >> All vmas which will be zapped soon will have VM_DEAD flag set. Since PF may race >> with munmap, may just return the right content or SIGSEGV before the optimization, >> but with the optimization, it may return a zero page. Here use this flag to mark >> PF to this area is unstable, will trigger SIGSEGV, in order to prevent from the >> unexpected 3rd state. >> >> If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, they are considered >> as special mappings. They will be dealt with before zapping pages with write >> mmap_sem held. Basically, just update vm_flags. The actual unmapping is still >> done with read mmap_sem. >> >> And, since they are also manipulated by unmap_single_vma() which is called by >> zap_page_range() with read mmap_sem held in this case, to prevent from updating >> vm_flags in read critical section and considering the complexity of coding, just >> check if VM_DEAD is set, then skip any VM_DEAD area since they should be handled >> before. >> >> When cleaning up vmas, just call do_munmap() without carrying vmas from the above >> to avoid race condition, since the address space might be already changed under >> our feet after retaking exclusive lock. >> >> For the time being, just do this in munmap syscall path. Other vm_munmap() or >> do_munmap() call sites (i.e mmap, mremap, etc) remain intact for stability reason. >> And, make this 64 bit only explicitly per akpm's suggestion. > I still see VM_DEAD as unnecessary complication. We should be fine without it. > But looks like I'm in the minority :/ > > It's okay. I have another suggestion that also doesn't require VM_DEAD > trick too :) > > 1. Take mmap_sem for write; > 2. Adjust VMA layout (split/remove). After the step all memory we try to > unmap is outside any VMA. > 3. Downgrade mmap_sem to read. > 4. Zap the page range. > 5. Drop mmap_sem. > > I believe it should be safe. Yes, it looks so. But, a further question is all the vmas have been removed, how zap_page_range could do its job? It depends on the vmas. One approach is to save all the vmas on a separate list, then zap_page_range does unmap with this list. Yang > > The pages in the range cannot be re-faulted after step 3 as find_vma() > will not see the corresponding VMA and deliver SIGSEGV. > > New VMAs cannot be created in the range before step 5 since we hold the > semaphore at least for read the whole time. > > Do you see problem in this approach? >