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 5013A396579 for ; Fri, 19 Jun 2026 14:27:16 +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=1781879237; cv=none; b=toN7YQ04AwHZGFUS+POubZjAPGPdvjxi+zdGUieEENBWa+XaMT0MGB13+XWboi4Yju5lH2NqF78tEs3bdTqIF/MtUU/TYf8tSIcqMhUYIJDZBrq9UCbrkyuOcKSrYcfPqcABLZxCOLNLPWnwZ05Wp5HWAILO/jKBkNNGr54agAM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781879237; c=relaxed/simple; bh=f6Px926nt5XiAOTF2Z8Jpvr7UXfYyElQLc/dyxoRFHY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kE/uLIBepnLc71NXBJk6ZIa7PpH/rS+Rds/GOVXmUJllQrS7g9UbKTGVRGvsGFLJP7L8TADE0GTPVm7dh+NmehGsPaNqajJ5Y0rfr4g4KkihWJhOopPbe/+Z2QMcFuuynr1YLf5fqE4nqH2MgNlAGnaDamh43BwpVHjDWOyLXzQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=giXjF4dP; 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="giXjF4dP" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 045101F00AC4; Fri, 19 Jun 2026 14:27:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781879236; bh=yvmmWMqqk4mhEEd2iTTFb0La55bI1ogNn7PI48Pfp9w=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=giXjF4dP8PaD3YUUoznlSm3gM8M8eSAXQDkGgo2+RIIBFUXRpWiJRU37xV7Vdreiv CuDFLJ+5jW9wp7M6ZS9NCfaZ8ftOswsLYb2mlg55GNr80Tsr+fy3w9+WuU+4Hyp7wK ZyJgrhNBfdFmiCRBaZ8SgfOp+Rzsn9UxFDH/WyIEvRAhWkavr+AA6IvlZfpMFgqS1J pqVeA2DIWeAG27drb3rW9c8m6q+ioNu33E1/qdYgsB2CMTTl/AgZrZW5f4hETN2V65 7kymJK5GawcPL3WdH+X5m0el/EcnfUJqPIxvfnhb4sFayelf48zHHOaaX4YnvcC5hi HUZsr6d9DvW4g== Message-ID: Date: Fri, 19 Jun 2026 16:27:11 +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 0/3] mm: __access_remote_vm with per-VMA lock To: Suren Baghdasaryan Cc: Lorenzo Stoakes , Rik van Riel , linux-kernel@vger.kernel.org, x86@kernel.org, linux-mm@kvack.org, Thomas Gleixner , Ingo Molnar , Dmitry Ilvokhin , Borislav Petkov , Dave Hansen , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka References: <20260616190300.1509639-1-riel@surriel.com> <603e002e-77f4-492e-9ad4-75572e18fc0b@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 6/19/26 16:19, Suren Baghdasaryan wrote: > On Fri, Jun 19, 2026 at 6:05 AM David Hildenbrand (Arm) > wrote: >> >>> >>> Well I think a critical problem here, as pointed out by Suren, is that holding a >>> VMA lock means that the VMAs around you can change and in ways that are quite >>> problematic. >>> >>> E.g. The moment you drop the VMA lock that VMA might get freed and then merged >>> with something else, and the next VMA you consume is the same one you just >>> partially walked, for instance. >>> >>> Now perhaps you could reason your way around this, but I'm pretty sure there are >>> cases where you might actually miss VMAs due to races (Suren knows best). >>> >>> And also without an mmap lock people can unmap and map new VMAs in the range as >>> you go through which might cause weirdness as well. >>> >>> Really, unless you are dealing with a single VMA in the range, I suspect GUP >>> needs to stabilise that whole range. >> >> Well, depends, really. It's not like a all GUP operation that target many pages >> runs exclusively under the mmap lock that would prevent any VMA changes. >> >> With userfaultfd, for example, we drop the lock in between, to lookup the VMA >> again later. There are various paths where __get_user_pages_locked() is >> instructed to grab the mmap lock itself, to even temporarily drop it if the mmap >> lock was dropped. >> >> gup_fast_fallback() grabs some pages to then take the mmap lock. And continue >> from the next address. >> >> >> So it really depends on the use case. I would actually be surprised if there a >> lot of use cases that strictly must block concurrent mremap operations etc. >> >> The important part is that you process each virtual page address requested >> exactly once. If the VMA was merged in the meantime, you continue from that >> address in the previously-processed VMA. >> >> >> Some use cases might indeed want to stabilize the whole range. But I wouldn't >> expect them to opt-in to using per-VMA locks. >> >> Just like with any other page table walker, we cannot just convert all in one >> shot to use per-VMA locks. >> >>> >>> If we could find a way to have GUP fast-path the single VMA case sensibly, then >>> that's probably workable? >> >> Right, that's what I said: start with a single-VMA interface that supports >> getting called with the per-vma lock or the mmap lock. >> >> If we have to fallback to the mmap lock (userfaultd? indicated back by the >> caller), handle it in the caller of that interface for now. >> >>> >>> And I agree special-casing only one place but not others sucks. >> >> Yeah, we're not doing that unless inevitable. >> >>> >>> Perhaps we could find a way to get this improvement without it being quite so >>> 'tacked on' but without needing significant rework of GUP, but in either case I >>> broadly agree we need to improve the codebase as part of the changes. >> >> We shouldn't fear extending GUP in a reasonable way that makes everyone out >> there profit ion the long run :) > > I do not disagree with the general premise of making existing > mechanisms work better rather than implementing parallel ones. I'm > just pointing out my findings so far when I moved in that direction > and I'm happy Rik posted an alternative simple way around large > refactoring and started this discussion. We should definitely try > reworking GUP to cause less contention. I just don't have enough time > ATM to drive that, but would be happy to help with the VMA-locking > parts. If we have a VMA-lock GUP variant, I guess supporting the write part would also be fairly easy? Not sure how performance-relevant that is, though. -- Cheers, David