From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7BEEA36215D; Thu, 13 Aug 2026 09:30:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786613408; cv=none; b=Jsn8XfKevKkalOBxUBrbM71g2ud9REmXHPcrZTmI6t+5ASXWcLAtb0leJE33q7mLjNi761MPQ/8ely9M0LAk5/TRSuLyjsCJC9iDGSEyKpCtrEznW46yq5s2OjiHUM5Jlw3fe/pGHHSnd7md3Bu93s1qgxQ8XxfMfhfNFJ3ltgM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786613408; c=relaxed/simple; bh=u23II3fMSzIgbfJuem0s6lq3oAzEoluYKiOoxHn0MCQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=bR1t2jSnUboxtuplUet7s05gHaz6U1Lt7kJJUVlwkpU8hY3eHB4aqrvchOrhivQm+7GyPThRN0Wft2FDP7lhl41VJtkXELaoWt4088bMt6AWY+ocs68e/Pk6VZRDy+bSIirA/EvvIMQO86Qxyeweu7U/qV5PHXqey8vnneCRNEA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ClERN4IN; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ClERN4IN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 23CB81F000E9; Thu, 13 Aug 2026 09:29:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786613407; bh=AbNB7YmIIEAsBupr3N2a7aEdIcPITLg2I2/18fsoCbY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=ClERN4INhPkBslxL5w4ZJcA1HiSs+6EJS5j2wIA/YNYxGzQCzzOltTl8mtVstS2Zv v6pbP1+6xy3WTefzm7r5D1n78dDzwYDPCLqM3C2uMBinPgnhev/yTaEWJUu2+3LCpE /oC3GCv+k+6f+600guIAvTW53T5sKQHXS5FRkrKjlATP4H8E92JsuOrLfoYUk6rT5M Es7CfwkPX97KRxwE72qlmu+QZ3FS51F5N5fgbF6KJ044HpYpGF6IF7YHBQx51Gf+Sm eGfAOyFrSE+Va+lns8GxKRnZ5JhnebCnBwPc1nogAQzkdqrN4uwRjCOnoRNzcPaOIp gHZjz5IjIBrlw== Message-ID: <517ea018-d6a9-4c78-b0a6-4f002b3b72ca@kernel.org> Date: Thu, 13 Aug 2026 11:29:43 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 18/20] mm/vma: make MAP_PRIVATE-mapped /dev/zero mappings truly anonymous To: "Lorenzo Stoakes (ARM)" , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , "Matthew Wilcox (Oracle)" , Jan Kara , Miaohe Lin , Naoya Horiguchi , Rik van Riel , Harry Yoo , Lance Yang , Kees Cook , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Usama Arif , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Peter Xu , Xu Xin , Chengming Zhou , Arnd Bergmann , Greg Kroah-Hartman , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Sven Schnelle , Alex Deucher , =?UTF-8?Q?Christian_K=C3=B6nig?= , David Airlie , Simona Vetter , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Boris Brezillon , Steven Price , Liviu Dudau , Huang Rui , Matthew Auld , =?UTF-8?Q?Thomas_Hellstr=C3=B6m?= , Rodrigo Vivi , Masami Hiramatsu , Oleg Nesterov , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , James Clark , Jason Gunthorpe , John Hubbard , Muchun Song , Oscar Salvador , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Youngjun Park Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, kvm@vger.kernel.org, linux-s390@vger.kernel.org, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, intel-xe@lists.freedesktop.org, linux-perf-users@vger.kernel.org, linux-trace-kernel@vger.kernel.org References: <20260806-b4-scalable-cow-virt-pgoff-v4-0-ab318a350404@kernel.org> <20260806-b4-scalable-cow-virt-pgoff-v4-18-ab318a350404@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/6/26 22:21, Lorenzo Stoakes (ARM) wrote: > When mapping /dev/zero with MAP_PRIVATE, one ends up with strange VMAs > originating from Linux's distant past. > > These have vma->vm_file set but NULL vma->vm_ops, meaning they satisfy > vma_is_anonymous() but otherwise resemble a file-backed VMA. > > The introduction of anonymous page offsets and their subsequent use as > indexes for MAP_PRIVATE-file-backed mappings mean the rmap does the right > thing with these but we are left with inconsistencies. > > The vma_start_pgoff(vma) == vma_start_anon_pgoff(vma) invariant is true for > all other anonymous VMAs, but not these. > > These VMAs are also observable as files in /proc//[maps, smaps, > map_files] but otherwise behave like anonymous mappings. > > Therefore let's make these VMAs actually anonymous at mapping time which > will activate the anonymous code path for mappings. > > This means we no longer have to account for this discrepancy anywhere and > no longer have to think about these at all. > > This is user-observable, as MAP_PRIVATE-/dev/zero will no longer appear in > procfs as a file-backed mapping, but the impact of this change should be > low as likely nobody is relying upon this. As discussed off-list, it's best to discuss the impact with the wider community. I'm afraid not many people made it to patch #18 in this series. :) I don't expect this to actually break something, but we should be clear that doing a mmap("/dev/zero") will no longer indicate that as a file mapping in /proc/. You could mention that e.g., criu checks for "/dev/zero" mappings explicitly, but will just keep doing the right thing once this is a proper anon mapping; we expect similar use cases to do exactly that, and rather check for "/dev/zero" only to conclude themselves "this is just an anon mapping". Doing some digging, AI raised that there is the potential of some workload explicitly using "/dev/zero" to get an entry in map_files. I think that's essentially what our selftests do that you have to modify. I cannot thing of a good reason why someone would do that in their workload, and at least AI wasn't able to easily identify any such users. -- Cheers, David