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Thu, 30 Jan 2025 05:31:52 -0800 (PST) Received: from phenom.ffwll.local ([2a02:168:57f4:0:5485:d4b2:c087:b497]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c5c1cf831sm2033238f8f.90.2025.01.30.05.31.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jan 2025 05:31:51 -0800 (PST) Date: Thu, 30 Jan 2025 14:31:49 +0100 From: Simona Vetter To: David Hildenbrand Cc: 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 , =?iso-8859-1?B?Suly9G1l?= Glisse , Jonathan Corbet , Alex Shi , Yanteng Si , Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pasha Tatashin , Peter Xu , Jason Gunthorpe Subject: Re: [PATCH v1 4/4] mm/memory: document restore_exclusive_pte() Message-ID: Mail-Followup-To: David Hildenbrand , 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 , =?iso-8859-1?B?Suly9G1l?= Glisse , Jonathan Corbet , Alex Shi , Yanteng Si , Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pasha Tatashin , Peter Xu , Jason Gunthorpe References: <20250129115803.2084769-1-david@redhat.com> <20250129115803.2084769-5-david@redhat.com> <7vejbjs7btkof4iguvn3nqvozxqpnzbymxbumd7pant4zi4ac4@3ozuzfzsm5tp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Operating-System: Linux phenom 6.12.11-amd64 X-Rspamd-Queue-Id: AA854100015 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: y47jet5q5dd5cqmwbjwwjwzyfiqq5bd3 X-HE-Tag: 1738243913-403178 X-HE-Meta: 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 Y5miNKwG ZolRWu6a2XEDIecM6nv+/FFESluy4pUl51qbM3Z5Mr3lyD9COKAg3zUqSyQzqlWa1DPbDQYk4yjFEVT/Iiu3fFz0UvjUUbaNG6RS2dj7JaMCD3M/BUYA2IjTsqH+aRytjyY4I1hB3ob1JOxba2jKGhcn8Bj770YEQ2mbZEKN9DaroE2V4hBQgWXHBywQdTso/JFncnlxrse1/MvV1a7zxStieh2yzgPrHcV9riyhmcId0nq+0dU74luDEcN+EF60jy6WDnKM2Va57qDsR4NRcXOa+lQNO0SoaJ3KyDajG+9hR9oZGNqoZmZ6RJdIKvgyLRUn9JTrHM9JaTbb1FGwUjdulkT1pECxFupj1 X-Bogosity: Ham, tests=bogofilter, spamicity=0.008252, 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 Thu, Jan 30, 2025 at 10:37:06AM +0100, David Hildenbrand wrote: > On 30.01.25 01:27, Alistair Popple wrote: > > On Wed, Jan 29, 2025 at 12:58:02PM +0100, David Hildenbrand wrote: > > > Let's document how this function is to be used, and why the requirement > > > for the folio lock might maybe be dropped in the future. > > > > Sorry, only just catching up on your other thread. The folio lock was to ensure > > the GPU got a chance to make forward progress by mapping the page. Without it > > the CPU could immediately invalidate the entry before the GPU had a chance to > > retry the fault. > > > Obviously performance wise having such thrashing is terrible, so should > > really be avoided by userspace, but the lock at least allowed such programs > > to complete. > > Thanks for the clarification. So it's relevant that the MMU notifier in > remove_device_exclusive_entry() is sent after taking the folio lock. > > However, as soon as we drop the folio lock, remove_device_exclusive_entry() > will become active, lock the folio and trigger the MMU notifier. > > So the time it is actually mapped into the device is rather Looks like you cut off a bit here (or mail transport did that somewhere), but see my other reply I don't think this is a legit use-case. So we don't have to worry. Well beyond documenting that if userspace concurrently thrashes the same page with both device atomics and cpu access it will stall real bad. -Sima -- Simona Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch