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([2a01:599:904:96e0:a245:aa9f:6c57:eb41]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-46fdf0e0d18sm5360991cf.40.2025.01.30.01.47.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 30 Jan 2025 01:47:34 -0800 (PST) Message-ID: <54a55ff7-38c8-42c2-886f-d6d1985072a9@redhat.com> Date: Thu, 30 Jan 2025 10:47:29 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 04/12] mm/rmap: implement make_device_exclusive() using folio_walk instead of rmap walk To: Alistair Popple , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-mm@kvack.org, nouveau@lists.freedesktop.org, Andrew Morton , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Jonathan Corbet , Alex Shi , Yanteng Si , Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , "Liam R. 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 5BD014000B X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: tdmgj4osc33q1arz6cqhihf7919ess44 X-HE-Tag: 1738230460-623389 X-HE-Meta: U2FsdGVkX1991R/Nm286aH45UM40s/KdByJMc6MbxM+20FnqeZ2PrSJ1GaYIv4AkIjTSxSIsYDn7G0kWLx3Omp8Kq/5tKgdKeFZuEvtxLN4TQ80ly5yPr7F6trAOsplynEcDYwfndOV+VrfonOvcBdmbmHW4r/ciLLjyQ6oa3MMvyYtz8EBkqYY9Z/4jajaKLyssSuf4BL06fBvwjFJ+zBFXkFJxiqw9txDvj1ynbIzZ9Ch3dSTKbv4vitkMR30m9kHWIkVrojFJaUGxo3DoPw2VaWGZu0zsFUBsNV1B91hzKlp9Du3W8zsL4l21p/HKJwQsAWiTKoOYE/1MOAeFn109wr8nf4uwMt6TB7kLxDbtXy89JVj/IvbgnkBp9y4/nOf6+r5kwFih8t9QQImkAww6QKIgZisQxeYUekN0LVTj4/bkr276sDj2SeCi22zhWM/1XBXbDXa9DXothiRpx7MAA5OMpGIKRY8ZdYV+HldOftGCsoGKQl9t60TQc7xA8PCFQq3lsyxb3zLUmWa5XMVLGQkX+1idpryPVRVb2eaoHdgvJNEoBO9bOBSjHjEnjokhDNv1HN5w8LZXY2Fj232gvL+Is+XNiDbzuSbnNmcKtaojNiNUX4FV/rAwZUA/SEvykt95gU7MEFxKTvfRiMax8py06+3uHsHW+6+/r2nnGR7UgWeeyhzWxEP2kwxKyVKgIitemmvTHhL/P8dWjYpp6E9WTPa1wskUn2imDXLX+mjKKLTmAoMFo4yfMFsJCAfUpbBu2A3jgnVQ6WlcPscvx132rts1t2FuVrju/7UtadTuaKSyGLiZRxT0muQN+uYZ6RStYJGopk35dzBa4SXyPofRtUOWGkyLQm7GNMH/mN2H6cVWJo5qII4XrRxgZfAw2rcWjDe9Z/qQ9oVzRScoWNen4AVHfff1i2LbGCtNDaViyh1qfdrWjHr+DAmmXV5BLUh6JiXKWrPFPQk t3htjWRx 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000307, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 30.01.25 10:40, Simona Vetter wrote: > On Thu, Jan 30, 2025 at 05:11:49PM +1100, Alistair Popple wrote: >> On Wed, Jan 29, 2025 at 12:54:02PM +0100, David Hildenbrand wrote: >>> We require a writable PTE and only support anonymous folio: we can only >>> have exactly one PTE pointing at that page, which we can just lookup >>> using a folio walk, avoiding the rmap walk and the anon VMA lock. >>> >>> So let's stop doing an rmap walk and perform a folio walk instead, so we >>> can easily just modify a single PTE and avoid relying on rmap/mapcounts. >>> >>> We now effectively work on a single PTE instead of multiple PTEs of >>> a large folio, allowing for conversion of individual PTEs from >>> non-exclusive to device-exclusive -- note that the other way always >>> worked on single PTEs. >>> >>> We can drop the MMU_NOTIFY_EXCLUSIVE MMU notifier call and document why >>> that is not required: GUP will already take care of the >>> MMU_NOTIFY_EXCLUSIVE call if required (there is already a device-exclusive >>> entry) when not finding a present PTE and having to trigger a fault and >>> ending up in remove_device_exclusive_entry(). >> >> I will have to look at this a bit more closely tomorrow but this doesn't seem >> right to me. We may be transitioning from a present PTE (ie. a writable >> anonymous mapping) to a non-present PTE (ie. a device-exclusive entry) and >> therefore any secondary processors (eg. other GPUs, iommus, etc.) will need to >> update their copies of the PTE. So I think the notifier call is needed. > > I guess this is a question of semantics we want, for multiple gpus do we > require that device-exclusive also excludes other gpus or not. I'm leaning > towards agreeing with you here. See my reply, it's also relevant for non-device, such as KVM. So it's the right thing to do. > >>> Note that the PTE is >>> always writable, and we can always create a writable-device-exclusive >>> entry. >>> >>> With this change, device-exclusive is fully compatible with THPs / >>> large folios. We still require PMD-sized THPs to get PTE-mapped, and >>> supporting PMD-mapped THP (without the PTE-remapping) is a different >>> endeavour that might not be worth it at this point. > > I'm not sure we actually want hugepages for device exclusive, since it has > an impact on what's allowed and what not. If we only ever do 4k entries > then userspace can assume that as long atomics are separated by a 4k page > there's no issue when both the gpu and cpu hammer on them. If we try to > keep thp entries then suddenly a workload that worked before will result > in endless ping-pong between gpu and cpu because the separate atomic > counters (or whatever) now all sit in the same 2m page. Agreed. And the conversion + mapping into the device gets trickier. > > So going with thp might result in userspace having to spread out atomics > even more, which is just wasting memory and not saving any tlb entries > since often you don't need that many. > > tldr; I think not supporting thp entries for device exclusive is a > feature, not a bug. So, you agree with my "different endeavour that might not be worth it" statement? -- Cheers, David / dhildenb