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 219DE49EC4C; Fri, 2 Oct 2026 13:12:00 +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=1790946723; cv=none; b=IM/oW2cJ5QO7L30D/Cp5czqs4/w3NMqdNOYwva6iIdlgAKLD/WUHN3B19iLnnNMjylNsvSUSMPb+jeWACDkMjjSdT/dq3lYroEQzD8MJxmXSmq2ani5IqaGrYPUL2KB+IWzJb2iXc8vMl7fIQavXEHs6j8k6axiRgssbOlAES8Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790946723; c=relaxed/simple; bh=jMQr+27cYOj9tTCex41lz4ztYXg71sAZs6qJakSxu24=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ejv/7QNbCnk2uz2NkFkJhUF8k0BspoEu8u0uLm2uSz2rd/WcnEhawpEItqYEMEEm20ipUhAuL34pYpVez/pLhP0EexfLFnHLLlC4XmQLKAkOYOL7OnTPfJcOY49ToSD+xkYqwcZ14Hs3t3V61xavuhKBMPHdNEaWoEJUqxRqzeA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EHWU0mbT; 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="EHWU0mbT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DDC6B1F000FF; Fri, 2 Oct 2026 13:11:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790946720; bh=QbhUP9iXRT9busjJARvsVsEKsH2Ps/ZqLFS4bca4bFI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=EHWU0mbTANCXNOi3Zz8vfaluwXiHb6NrEeQE3579lOgrAlHyG4bhLJxtrP5SUxJ53 qyuyt7WwDms+J9SU6tj1doNb8ixs5S/dfzu1LKAyzLmCA11j1Lj0zCtK60fbB19ybx 3qiK3mWy3UStlhPrmaJU1AZq+hxZAKISulldff17r1WWYubrmXtDF0LmP4+X+Wm6bd BOzsfTZ78ABQCyKcm+yss2YPRtG6Dauzuh8gKGZxzEuKnmayEwg8S4NJjxmkbEjVzs TlQyReq4OVNz5qAdPorvbksNtN11kzuoPIYJM5l9/ocKXRgGPmsMbmcutbMweewTer BEcqQ9TLZwBbw== Message-ID: <3387642b-c39f-4f19-ba0c-267bbd2fe91d@kernel.org> Date: Fri, 2 Oct 2026 15:11:27 +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 v3 31/40] mm/vma: introduce vma[_flags]_is_persistent() To: "Lorenzo Stoakes (ARM)" Cc: Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Greg Kroah-Hartman , Dennis Dalessandro , Jason Gunthorpe , Leon Romanovsky , Paul Moore , Stephen Smalley , Jaroslav Kysela , Takashi Iwai , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Doug Gilbert , "James E.J. Bottomley" , "Martin K. 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Miller" , Andreas Larsson , Alexander Viro , Christian Brauner , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Youngjun Park , Johannes Weiner , Qi Zheng , Shakeel Butt , Axel Rasmussen , Yuanchu Xie , Wei Xu , Chengming Zhou , Michal Hocko , Miklos Szeredi , Xu Xin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-usb@vger.kernel.org, linux-rdma@vger.kernel.org, selinux@vger.kernel.org, linux-sound@vger.kernel.org, bpf@vger.kernel.org, linux-scsi@vger.kernel.org, linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-arch@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, fuse-devel@lists.linux.dev References: <20260917-b4-mmap-prepare-vma-flag-sanify-v3-0-4583d8a23bca@kernel.org> <20260917-b4-mmap-prepare-vma-flag-sanify-v3-31-4583d8a23bca@kernel.org> <9bbdf4e8-6984-4477-a183-f0b236047381@kernel.org> <3557015d-23dd-41ef-9832-78b58587f3f4@kernel.org> <81e1e5ae-0aab-4584-81e6-3fc9c2634d07@kernel.org> <2e5a4e6c-8f9c-4050-8998-454b83f4823b@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 10/2/26 14:48, Lorenzo Stoakes (ARM) wrote: > On Fri, Oct 02, 2026 at 02:35:06PM +0200, David Hildenbrand (Arm) wrote: >> On 10/2/26 14:08, Lorenzo Stoakes (ARM) wrote: >>> >>> No, see below. >>> >>> >>> Anything that requires stuff not to be dropped behind the user's back, which is >>> at least 4 cases! >>> >>> That being open-coded all over the place is a problem I think, and I think stuff >>> like the PMD device private are a reminder that open-coding all over can cause >>> problems. >>> >>> >>> There are 4 open-coded checks that test four ad-hoc flag combinations checking >>> for the same thing - 'can the kernel or a driver change things or discard stuff >>> behind my back?' >>> >>> So abstracting that to a helper, alongside the other 'let's ask based on >>> semantics' helpers, seems sensible. >>> >>> Maybe invert the meaning to make it clearer? >>> >>> vma_kernel_may_change_contents()? >>> >> >> This is all super confusing and I don't think we should try to describe the >> semantics that way. >> >> Just imagine having udmabuf use a PFNMAP of folios obtained from shmem. For sure >> the kernel could now change the shmem pages that are mapped in some ordinary VMA. > > There's always edge cases like this. > > In each of these conditionals that this helper replaces, that's exactly what's > being checked for. > > A driver could GUP anything and access the page raw and do things like that > too... > > Without this we lose all that and are back to arbitrary flag checks again. It > seems unwise. > >> >> >> Likely we don't have to squeeze everything into a single helper that is hard to >> describe. > > I'm not trying to do that :) > > There are plenty of edge cases I left alone. > > This one explicitly is one that can end up very easily broken in the future. > > Already there's been issues as I recall with people forgetting about droppable. > >> >> Maybe we can pull parts of it into a separate helper with semantics that are >> easier to describe? > > But then you lose the whole purpose of this, which is to not miss things. I think it's perfectly fine to instead have helper like vma_is_dumpable() that maybe have a single purpose but are centralized and well documented. If we can construct them out of other helpers, great. > >> >> >> vma_is_user_memory() && !vma_is_droppable_memory() > > I find vma_is_user_memory() _very_ confusing :) I have no idea what that means. If we can find some way to describe "anon+pagecache" ... maybe something around folios (future oriented). > > And vma_is_droppable_memory() is a semi-pointless wrapper for checking the > droppable flag no? Sure, no strong opinion on that. It's a bit easier on the eye than bit checks. (and can have a nicer description maybe). > >> >> Although I am not sure user_memory is exactly precise (pagecache+anon) and what >> we want? It's all super confusing (thanks for deciphering it). > > That name isn't great, as userland memory is defined as that which can be mapped > in the userland portion of the virtual memory address map, and all VMAs describe > that :) > > And then you can go on from there as to pedantic issues, the very same shmem > stuff you mention above can be put forward as an argument, etc. etc. >> >> >>> vma_contents_may_change() is shorter but easily confused with something being >>> writable by userland etc. >>> >>> Or maybe: >>> >>> vma_is_volatile() >>> >>> ? >>> >>> Which is analogous to the meaning of the volatile keyword. >> This is all confusing because persistent and volatile are established concept >> when talking about memory. And see my example above, it's not even clear what it >> means that "the kernel can modify something". > > Yep I guess volatile vs. non-volatile. > > But a VMA describes a mapping not backing and 'volatile' in C is pretty clear - > something can be touched by something else so don't make assumptions. > > I mean if people are confused they can look at the comment. We argue about > details meanwhile the code is full of terrible naming that's duplicated 100 > times :) > > And you can come up with edge cases to a lot of things. That doesn't invalidate > the intent. > > Right now the code is duplicative and I found multiple instances of e.g. drivers > doing completely the wrong thing. > > In any case, you seem to feel very strongly about this - so maybe best I drop > this patch from the series to be revisited later? I'd be very happy if we could find a clear way to describe "this is what we call user memory: anon+pagecache", if that could come handy in such a context. In the end, it's mostly all about: this is nothing special (lol), it's just ordinary user memory (anon+pagecache), and in some cases we want to also ignore weird droppable mappings, because why e.g., dump them if the context is just irrelevant. -- Cheers, David