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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id F2C1CC5CFDB for ; Thu, 13 Aug 2026 09:18:46 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DAA4E6B0252; Thu, 13 Aug 2026 05:18:45 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D5ADE6B043A; Thu, 13 Aug 2026 05:18:45 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C229C6B043B; Thu, 13 Aug 2026 05:18:45 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 8C0D06B0252 for ; Thu, 13 Aug 2026 05:18:45 -0400 (EDT) Received: from smtpin12.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 0333980537 for ; Thu, 13 Aug 2026 09:18:44 +0000 (UTC) X-FDA: 85095696210.12.93851DE Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf28.hostedemail.com (Postfix) with ESMTP id 49D62C0006 for ; Thu, 13 Aug 2026 09:18:43 +0000 (UTC) Authentication-Results: imf28.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=bWIcRYVo; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf28.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1786612723; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=GetBiy0Zn1Zt+WVhmIztIBXSgbwgf2K+rHp84uzlz2E=; b=PbsPC1TAdJozN8l0dhGN7Q2Dtazu0uCoCOUwJ3lIeRTVmWdHq8iSFjjK81TIEImvcTfrkS 6l25LggJW6W1Y+hcyV6494nGifnyAtlmmxh5zKHKtzalzBX+npU643N88ekPm45c2Vxc4m 3pnUYCkDN7uQlisCgbcfp/gSW6072Kw= ARC-Authentication-Results: i=1; imf28.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=bWIcRYVo; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf28.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1786612723; b=KEvVg5xRXxNjoLZ30REjr8AGQp67upz/hl+Rd6AnhJiHOhEIGm8S5DQuPior4ZVkF8dPgC FYruSKjCiz9D0YNY5RFiaAEZt9RzbomubVwsYwoPRTHvminzKxWMnn4166Aazao9hs6p66 ze1WhMSr9GJxLd7qZ1fQZ22z9pxAjR0= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id B1C64600AD; Thu, 13 Aug 2026 09:18:42 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 07F781F000E9; Thu, 13 Aug 2026 09:18:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786612722; bh=GetBiy0Zn1Zt+WVhmIztIBXSgbwgf2K+rHp84uzlz2E=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=bWIcRYVoSsY+NP8RDYGqeCKfdNSxxjBsO0HFtspwgEIcp4mxv6MIb+2TtjGjCRLw9 hIFNHL3xy64xAuU96ONyJA6lJgCdNyALdgyI+DydU5l+v+zFz/J5b/Y1Kd30ZurMF5 LX4SUYoWQFIjEvpb0hBaybB0GmnOwdsg4e3tYQF94wvU9f1NxWbA9ptSNnR6vuNgF9 /pBx8beROUiTVDlXLjY9uCUrg/A3FYGguRxxKdoiDYMZa0ZmOsA30p8M+qmovrM09i ++Q2dp3raik6cTHMydkCPT3cK/35vli7LyQG8Ow9StXI6zByuJcR/6Mea1EDO0Ksop xQtWNqBuv2IBQ== Message-ID: Date: Thu, 13 Aug 2026 11:18:13 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 17/20] mm/vma: only permit MAP_PRIVATE /dev/zero to be mapped anonymous To: "Lorenzo Stoakes (ARM)" Cc: 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 , 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-v4-17-ab318a350404@kernel.org> <14a06774-4157-4629-8327-b1a6bae24ab7@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 X-Stat-Signature: 19m43nr79r3q7rbw1txkjc8uwr918e6c X-Rspamd-Queue-Id: 49D62C0006 X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1786612723-977623 X-HE-Meta: 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 PwK0W8BN SbNbkX4xs/k+hg550rWpFsoFIzVBlxylh9YJg9aSXp3Z9Beb3++KCygmqsLpb92Kf7A3XclsdPe9Q+PKfHsFnvZE01yHw8sENJfux8OiU/qYjNJjV8pwEgQmCAyrq7fiA06jOIhgFUrrqo/JNMT1XgnYWX/FEZUMLf96jOVrH7H9Av/ttFHBJfhJ1i0vadLtteftDtr4HO1OV56zX2cVCm8hGA6AIik7T5T+DriZVw6U0QMsPkUzNT594Svu5GRuVwIVh6htQmtPbMbxlFlaz59HXjsEBct47OxX5L0E0ptA2QyNdGjUqV72LqdB3abvSvi+dwKFtNuhypaD87aWbjzzyb4wmUzrA3Rl4hHIQERsvjsWdY2hilY32AolTz5qV7w0D Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/12/26 20:05, Lorenzo Stoakes (ARM) wrote: > TL;DR - either we need to find a way to uniquely identify it (keep my approach > or move it to core mm) or we should drop this patch. > > Given we're late in the cycle + you stringly objective to my approach I think > maybe best to drop it and respin? Yes, let's handle that separately. > > Can do a follow up to add unique identification later, somehow. > > The key change of 'make anon-assigned file-backed MAP_PRIVATE mappings not > special snowflakes' will still be achieved in the other patches (but have to > rework them a little obviously). > > On Wed, Aug 12, 2026 at 07:15:05PM +0200, David Hildenbrand (Arm) wrote: >>> >>> We don't assign vm_ops for a MAP_PRIVATE-/dev/zero mapping. So that won't work. >>> >>> We could expose zero->f_ops but then it's literally in drivers/char/ and that's >>> just weird to expose in mm.h or whatever. >> >> Thinking out loud: could we use a dummy (empty) vm_ops? >> >> We'd place it in mm.h (zero_vm_ops) and just use it in drivers/char/. > > The entire purpose of this patch is to _uniquely identify_ MAP_PRIVATE-mapped > /dev/zero and only permit this to make a mapping anonymous. > > As per the commit message: 'mm/vma: only permit MAP_PRIVATE /dev/zero to be > mapped anonymous' > > MAP_PRIVATE-/dev/zero to anon is a historical abberation. This is why it is a > unique case where semantics don't make sense. > > Doing things by semantics makes sense if the semantics are a _class_ of > thing. This is not a class of thing it is a one of one. > > Yes the method used is ugly, but the alternatives (I can think of) are: > > 1. (your proposal) allow any in-tree non-module mmap_prepare() driver to map > anonymous pages, creating an entirely new semantic for memory mapping. > > (this is currently something that is possible, unfortunately). > > To be safe we would have to implement then a series of checks to prevent true > insanities, because doing that would permit broken madness like a PFN map > being treated as anon. > > Once this stuff is out there as an API, even in-tree, even non-module, people > will abuse it because it'll go through some random tree and we'll have to > support it. > > I mean we may as well just not make a change in this case other than maybe > preventing .mmap from being able to do this. As discussed off-list, we could have in in mm/ and special-case drivers/char/mem.c to include a mm header like fs/exec.c similarly does. drivers/char/mem.c really belongs to core-mm. > > 2. My solution - ugly but uniquely identifies the single permitted situation > where this behaviour is desired. > > 3. Export something from drivers/char/mem.c to mm - horrific for multiple > reasons, violates separation of concerns etc. etc. Jup. > > 4. Move the driver code just for /dev/zero to mm - I thought a pretty horrible > idea, but maybe we could have some mm/ bit and some driver/char bit like > hugetlbfs or uffd? But I'm not sure there is a not-ugly way of doing this. > MAP_SHARED /dev/zero does shmem stuff so maybe could live there. Given that I think drivers/char/mem.c really is core-mm material, something along these lines also make sense. -- Cheers, David