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=-3.0 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_NEOMUTT autolearn=unavailable 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 6DDDDC072B1 for ; Mon, 27 May 2019 23:30:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 458F3208C3 for ; Mon, 27 May 2019 23:30:31 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=shutemov-name.20150623.gappssmtp.com header.i=@shutemov-name.20150623.gappssmtp.com header.b="Pht68gmg" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727509AbfE0Xaa (ORCPT ); Mon, 27 May 2019 19:30:30 -0400 Received: from mail-ed1-f67.google.com ([209.85.208.67]:41372 "EHLO mail-ed1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727465AbfE0Xaa (ORCPT ); Mon, 27 May 2019 19:30:30 -0400 Received: by mail-ed1-f67.google.com with SMTP id m4so28812348edd.8 for ; Mon, 27 May 2019 16:30:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=shutemov-name.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=Yx/t5JyNPq2CfWKyPo8S5jl7B36K3tOaXOqgll7VB3Y=; b=Pht68gmgXm2tUcD7wDY+aRLSo7xgBF3+W0lFrGP1dCtvsygSYQbwVnPbxlr3KTJiXO FYJNp7IT818BdApjvsXCeXP2vbj/wNGLbfkH6GDK51LvyDVgtaCNNyGaFohNyCyLFloc 381SYX6vJ7L7VQ0oUNClrVqNmNjnOlGaaU3f3Uw0WT66E22fSPM2u7bDmtoSOrhzo0lY lIXct0O9UkE+thCN4inaK0oYimDL/qUVHy4kn6NESWaBHUr3P+p78mEVTKffED8NaADK NkhQkZtW87MrsG9JfHCdGDx3eKzr7sA+L0JVa4o8hB+bL7rV6jn8a9KwvSSs5iagmCyK SgUg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=Yx/t5JyNPq2CfWKyPo8S5jl7B36K3tOaXOqgll7VB3Y=; b=QCX2bJ7TQtc8LSzuCStn/RTwVwroncBj/LXKM8Rvkt1qm8qmtcNnOIGKtnzBB9g2mk Ko4Om3NBp0VKrFG4kT6TmbHhChZpB+M6E7sULU/uZek53CZQ1JkEAv4IxY+L9TKo4V9G IQzqo+cii47X4xKv91OzdNK0GJ7DGw4VDRxZoZxSP/z556SpYSgTG5mDveeONeN1N4KW atTTkI1WacVinjvzaW+SzPfi3rNuKGbqCCvFPdlnOxB9IuAu5WwWrOwUbhXvV2dgjokV +XviNzqPcGqOxcTeFIJW3m8bfnMQlEmlNbkvQuM3/UDJ4uESkbGbqb8nqfNS/FKI2i2B 6CyQ== X-Gm-Message-State: APjAAAUOacmawRJQs7CAgO22aXU8FnJiLJT3g6nNnUQ0Yt2c26za87lh J0rBInphqJ/MpRzkQ4ABHzlDbA== X-Google-Smtp-Source: APXvYqwNsccQYDQ+T/DIEr4MpbjqeYPxkwVJIYO898WWuLI4de+wC1sH3D8WOusY2p8TGpyiYrsCyg== X-Received: by 2002:a50:add7:: with SMTP id b23mr125171875edd.215.1558999827807; Mon, 27 May 2019 16:30:27 -0700 (PDT) Received: from box.localdomain ([86.57.175.117]) by smtp.gmail.com with ESMTPSA id b45sm1365013edb.28.2019.05.27.16.30.26 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 27 May 2019 16:30:26 -0700 (PDT) Received: by box.localdomain (Postfix, from userid 1000) id E7ADE102832; Tue, 28 May 2019 02:30:30 +0300 (+03) Date: Tue, 28 May 2019 02:30:30 +0300 From: "Kirill A. Shutemov" To: Kirill Tkhai Cc: akpm@linux-foundation.org, dan.j.williams@intel.com, mhocko@suse.com, keith.busch@intel.com, kirill.shutemov@linux.intel.com, alexander.h.duyck@linux.intel.com, ira.weiny@intel.com, andreyknvl@google.com, arunks@codeaurora.org, vbabka@suse.cz, cl@linux.com, riel@surriel.com, keescook@chromium.org, hannes@cmpxchg.org, npiggin@gmail.com, mathieu.desnoyers@efficios.com, shakeelb@google.com, guro@fb.com, aarcange@redhat.com, hughd@google.com, jglisse@redhat.com, mgorman@techsingularity.net, daniel.m.jordan@oracle.com, jannh@google.com, kilobyte@angband.pl, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 0/7] mm: process_vm_mmap() -- syscall for duplication a process mapping Message-ID: <20190527233030.hpnnbi4aqnu34ova@box> References: <155836064844.2441.10911127801797083064.stgit@localhost.localdomain> <20190522152254.5cyxhjizuwuojlix@box> <358bb95e-0dca-6a82-db39-83c0cf09a06c@virtuozzo.com> <20190524115239.ugxv766doolc6nsc@box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 24, 2019 at 05:00:32PM +0300, Kirill Tkhai wrote: > On 24.05.2019 14:52, Kirill A. Shutemov wrote: > > On Fri, May 24, 2019 at 01:45:50PM +0300, Kirill Tkhai wrote: > >> On 22.05.2019 18:22, Kirill A. Shutemov wrote: > >>> On Mon, May 20, 2019 at 05:00:01PM +0300, Kirill Tkhai wrote: > >>>> This patchset adds a new syscall, which makes possible > >>>> to clone a VMA from a process to current process. > >>>> The syscall supplements the functionality provided > >>>> by process_vm_writev() and process_vm_readv() syscalls, > >>>> and it may be useful in many situation. > >>> > >>> Kirill, could you explain how the change affects rmap and how it is safe. > >>> > >>> My concern is that the patchset allows to map the same page multiple times > >>> within one process or even map page allocated by child to the parrent. > >>> > >>> It was not allowed before. > >>> > >>> In the best case it makes reasoning about rmap substantially more difficult. > >>> > >>> But I'm worry it will introduce hard-to-debug bugs, like described in > >>> https://lwn.net/Articles/383162/. > >> > >> Andy suggested to unmap PTEs from source page table, and this make the single > >> page never be mapped in the same process twice. This is OK for my use case, > >> and here we will just do a small step "allow to inherit VMA by a child process", > >> which we didn't have before this. If someone still needs to continue the work > >> to allow the same page be mapped twice in a single process in the future, this > >> person will have a supported basis we do in this small step. I believe, someone > >> like debugger may want to have this to make a fast snapshot of a process private > >> memory (when the task is stopped for a small time to get its memory). But for > >> me remapping is enough at the moment. > >> > >> What do you think about this? > > > > I don't think that unmapping alone will do. Consider the following > > scenario: > > > > 1. Task A creates and populates the mapping. > > 2. Task A forks. We have now Task B mapping the same pages, but > > write-protected. > > 3. Task B calls process_vm_mmap() and passes the mapping to the parent. > > > > After this Task A will have the same anon pages mapped twice. > > Ah, sure. > > > One possible way out would be to force CoW on all pages in the mapping, > > before passing the mapping to the new process. > > This will pop all swapped pages up, which is the thing the patchset aims > to prevent. > > Hm, what about allow remapping only VMA, which anon_vma::rb_root contain > only chain and which vma->anon_vma_chain contains single entry? This is > a vma, which were faulted, but its mm never were duplicated (or which > forks already died). The requirement for the VMA to be faulted (have any pages mapped) looks excessive to me, but the general idea may work. One issue I see is that userspace may not have full control to create such VMA. vma_merge() can merge the VMA to the next one without any consent from userspace and you'll get anon_vma inherited from the VMA you've justed merged with. I don't have any valid idea on how to get around this. -- Kirill A. Shutemov