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[2003:cb:c70c:a600:1e3e:c75:d269:867a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4395a06e309sm28144855e9.24.2025.02.12.11.07.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 12 Feb 2025 11:07:47 -0800 (PST) Message-ID: Date: Wed, 12 Feb 2025 20:07:44 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/6] rust: page: Support borrowing `struct page` and physaddr conversion To: Asahi Lina , Zi Yan , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Jann Horn , Matthew Wilcox , Paolo Bonzini , Danilo Krummrich , Wedson Almeida Filho , Valentin Obst , Andrew Morton , linux-mm@kvack.org, airlied@redhat.com, Abdiel Janulgue , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, asahi@lists.linux.dev, Oscar Salvador , Muchun Song References: <20250202-rust-page-v1-0-e3170d7fe55e@asahilina.net> <41ca3445-80cd-43c1-8f9e-634c195c9187@asahilina.net> <37A0729B-A711-4D45-B9F0-328FDB9ADD28@nvidia.com> <0e19e1c3-293b-4740-93f3-2c410893288b@redhat.com> <82047858-480a-45e3-b826-3a46fbebe842@asahilina.net> <1e9ae833-4293-4e48-83b2-c0af36cb3fdc@asahilina.net> <026c1a0c-e53a-4a5e-92da-6e4f18ce0fee@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: B32EDC0012 X-Stat-Signature: 8q63mekzzccbkotkp6reg97dwx69echq X-Rspam-User: X-HE-Tag: 1739387271-238472 X-HE-Meta: U2FsdGVkX19LdoOIwaRfMmw+RDx01yhrYfIMVAXCKW7QIz5LuvMzznhTs2BpBAvh6Q5RSKKTOJ8K6l+clSH6tEKJ1kQv+Ke9HJXGQe89rt3TJdt7MhrgwH+3+2kl0eBd2yUORw8txt74JhydRBdQKvBEcU6Bs5/4++js+k0KmgKzbWP3JA9hK18FmkcF5uZhReLs8IzkMLW3DYQ+zWgsCPaG6WS1cpQk6/8t9JwwxM+tYQjxyJ8H7sJUxJJ8tODjbw9OcGxT4byP8uA4xhDm/yDLrFQPwoBjtTFN4HWnHmsdhMQUS6N/LCKm7k8RI6VNoQmUqQW2Wd59tCXVUjlB5L/TH69GVyEPF/LseJ2AmkW19oZAE0EEqdvlU0vZIcZaLl0w/KWOqCqzqai6ri51ZnPw6HNe1yXrsCHfSGLMc5bDdAKHW3aLKrUSUUSmzkyZ25idEBN7LqkZqvW9JFgE2bm52dhA37WsfnmEi3Hn/NkGD9U7zYJopwPcduFKgCTmyS114GOX0uAJcJKwYC5YatpZQKHnjp+CtGGwpC8B6KLwV8QMQz7uEqrHCT8y0gG2wbNGgzABmXwW81nunqCxe1l1JMrFagLVI6fccnfc/poTELZrhnUz6Spzk9UtE2DcpU+GLc31U/8m8uFgl3ZiLofvU9g+VivuP/s4XtNN7l+QVyeixZlHtfoEOojs3lyBjFW6NSa7/iQm0TKm+94Ey45EK47b3H9FzoiA/0kIt4HDahrFB2lkDwmmQC/x7wgTZSYPi5B8QG+IqKPOpnCfuQeVQJYOcJhDWBOJpolnTV9vaCqKPVytlej1+7bVWiQpopIOsamoNUnRCAyNGdG1I7Ty2ulO/0z4WWgGyEj+RJuelD5kJMbf2vW/Q+xRcEgYZp5+FZY1kFcatfrjGWN98nxK/FT3LJQOrT31cG7UOQfwVih4BQp2MZl2IGS2YCP+uNcm6aLnh/p4pRSfcwc fIIPLGQN yN056xl0PYwytBRA/qmuPRCzAEuTg0YppK2mBSiq4ImSS9taSKWAj5BJnsZUngXXitZTBJbpXOC5HqerAnDxbYJNMFAXl0CiX3ue0R8r6UDHumsNA062v3Q53XEW3QN0KritUODZF6rvaY7qrWY99JbNIvTw2JLxK8AOC2kZ7GsnWcuFoBLJGCyzU1OjhZfHbZvLmRTj5lqw6svOVUGpiJnyNnmCPApmbmMTRW6ReV/sl7/i7qB/w5DhGedB44wgGcu86SLkncSSABCz2yZoCtcbnzvKNR4mfweJNwV0a31ssZq2PShpXXf1QNq8ogS3to3EABgCy9RRtz9iYfJsEHFfuZUIcG72ip4XRaN4qTiTRyiDgi9YXFgnu7PcLYB8g8igsWPdK1CedPg9xg/veolzWDWxuNSbbG438bL62XzINLyHFTzJr9Ee5boX/f4bDC2MRfj6k7kCRNCBzqnIEdlmtxQ== X-Bogosity: Unsure, tests=bogofilter, spamicity=0.499354, 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 05.02.25 08:40, Simona Vetter wrote: > On Tue, Feb 04, 2025 at 03:38:17PM +0100, David Hildenbrand wrote: >>>> It can still race with memory offlining, and it refuses ZONE_DEVICE >>>> pages. For the latter, we have a different way to check validity. See >>>> memory_failure() that first calls pfn_to_online_page() to then check >>>> get_dev_pagemap(). >>> >>> I'll give it a shot with these functions. If they work for my use case, >>> then it's good to have extra checks and I'll add them for v2. Thanks! >> >> Let me know if you run into any issues. >> >>> >>>> >>>>> >>>>> If the answer is "no" then that's fine. It's still an unsafe function >>>>> and we need to document in the safety section that it should only be >>>>> used for memory that is either known to be allocated and pinned and will >>>>> not be freed while the `struct page` is borrowed, or memory that is >>>>> reserved and not owned by the buddy allocator, so in practice correct >>>>> use would not be racy with memory hot-remove anyway. >>>>> >>>>> This is already the case for the drm/asahi use case, where the pfns >>>>> looked up will only ever be one of: >>>>> >>>>> - GEM objects that are mapped to the GPU and whose physical pages are >>>>> therefore pinned (and the VM is locked while this happens so the objects >>>>> cannot become unpinned out from under the running code), >>>> >>>> How exactly are these pages pinned/obtained? >>> >>> Under the hood it's shmem. For pinning, it winds up at >>> `drm_gem_get_pages()`, which I think does a `shmem_read_folio_gfp()` on >>> a mapping set as unevictable. >> >> Thanks. So we grab another folio reference via >> shmem_read_folio_gfp()->shmem_get_folio_gfp(). >> >> Hm, I wonder if we might end up holding folios residing in >> ZONE_MOVABLE/MIGRATE_CMA longer than we should. >> >> Compared to memfd_pin_folios(), which simulates FOLL_LONGTERM and makes sure >> to migrate pages out of ZONE_MOVABLE/MIGRATE_CMA. >> >> But that's a different discussion, just pointing it out, maybe I'm missing >> something :) > > Good GPU Drivers (tm) are supposed to have a shrinker so we can at least > nuke some of them again. Some folks even looked into hooking up a migrate > callback through the address_space (or wherever that hook was, this is > from memory) so we can make this somewhat reliable. So yeah we're hogging > ZONE_MOVEABLE unduly still. Hmmm, so we should really be migrating pages out of ZONE_MOVABLE/MIGRATE_CMA here, just like we now properly do in memfd_pin_folios(). > > The other side is that there's about 2-3 good drivers (msm, i915, xe > should have a shrinker now too but I didn't check). The others all fall > various levels of short, or still have 3 times cargo-culted versions of > i915's pin-as-a-lock design and get it completely wrong. :( > > So yeah I'm aware this isn't great, and we're at least glacially slowly > moving towards a common shrinker infrastructure that maybe in a glorious > future gets all this right. I mean it took us 15+ years to get to a > cgroups controller after all too, and that was also a well known issue of > just being able to hog memory with no controls and potentially cause > havoc. I guess that means job security for us, haha :) -- Cheers, David / dhildenb