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 C0B3530E83F; Thu, 13 Aug 2026 09:18:42 +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=1786612724; cv=none; b=ELxnOS0jgC6+lLqWE2cfZ0VNHNZK/tB/5GEU2rhx2kDiK9ze3u8d2XcCJFXXA3b/QDgdYdK9Yh1BQPovlx8rHN7wZSCG22RqCWU5Fv44Ol/ur8yKEJneuAuyxWvswa4gKnxoD17XTx7pjn+q6GUfOtHOoVk3XT5SDLbvPIJezyE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786612724; c=relaxed/simple; bh=zKBAATRQUP95DSf0gemyDrU5jRKGs8XyjRFkQQ+49uw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OFNEJ644twr178ji4MmUGn6M7NCBVQPX0ZGUjQnxWv51M6Fb3dOSSfrT4xRX5rVJprYF1dG7eZFWo6eR8UaYTUJMcycnUOFn46IFRTZw5pL5fO5Fh/EeQYd/Btjhp+Y/G84AXExs/EFN6CJg9rj1PVR5gny2k1fv178WgnkJhyo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bWIcRYVo; 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="bWIcRYVo" 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 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 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 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