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[2003:cb:c707:3000:a06b:56f1:d152:db83]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4391dfd84b9sm25919955e9.39.2025.02.06.09.58.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 06 Feb 2025 09:58:03 -0800 (PST) Message-ID: <0dffaa7d-340f-4ce1-9a2e-54cfd9079266@redhat.com> Date: Thu, 6 Feb 2025 18:58:01 +0100 Precedence: bulk X-Mailing-List: asahi@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: 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 Cc: 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> <6bcd3315-a0f9-463c-ab97-a43736f9b4f4@redhat.com> <2a513c3e-818c-4040-b3d3-7835861bab4f@asahilina.net> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 04.02.25 22:06, Asahi Lina wrote: > > > On 2/5/25 5:10 AM, David Hildenbrand wrote: >> On 04.02.25 18:59, Asahi Lina wrote: >>> On 2/4/25 11:38 PM, David Hildenbrand wrote: >>>>>>> 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 :) >>> >>> I think this is a little over my head. Though I only just realized that >>> we seem to be keeping the GEM objects pinned forever, even after unmap, >>> in the drm-shmem core API (I see no drm-shmem entry point that would >>> allow the sgt to be freed and its corresponding pages ref to be dropped, >>> other than a purge of purgeable objects or final destruction of the >>> object). I'll poke around since this feels wrong, I thought we were >>> supposed to be able to have shrinker support for swapping out whole GPU >>> VMs in the modern GPU MM model, but I guess there's no implementation of >>> that for gem-shmem drivers yet...? >> >> I recall that shrinker as well, ... or at least a discussion around it. >> >> [...] >> >>>> >>>> If it's only for crash dumps etc. that might even be opt-in, it makes >>>> the whole thing a lot less scary. Maybe this could be opt-in somewhere, >>>> to "unlock" this interface? Just an idea. >>> >>> Just to make sure we're on the same page, I don't think there's anything >>> to unlock in the Rust abstraction side (this series). At the end of the >>> day, if nothing else, the unchecked interface (which the regular >>> non-crash page table management code uses for performance) will let you >>> use any pfn you want, it's up to documentation and human review to >>> specify how it should be used by drivers. What Rust gives us here is the >>> mandatory `unsafe {}`, so any attempts to use this API will necessarily >>> stick out during review as potentially dangerous code that needs extra >>> scrutiny. >>> >>> For the client driver itself, I could gate the devcoredump stuff behind >>> a module parameter or something... but I don't think it's really worth >>> it. We don't have a way to reboot the firmware or recover from this >>> condition (platform limitations), so end users are stuck rebooting to >>> get back a usable machine anyway. If something goes wrong in the >>> crashdump code and the machine oopses or locks up worse... it doesn't >>> really make much of a difference for normal end users. I don't think >>> this will ever really happen given the constraints I described, but if >>> somehow it does (some other bug somewhere?), well... the machine was >>> already in an unrecoverable state anyway. >>> >>> It would be nice to have userspace tooling deployed by default that >>> saves off the devcoredump somewhere, so we can have a chance at >>> debugging hard-to-hit firmware crashes... if it's opt-in, it would only >>> really be useful for developers and CI machines. >> >> Is this something that possibly kdump can save or analyze? Because that >> is our default "oops, kernel crashed, let's dump the old content so we >> can dump it" mechanism on production systems. > > kdump does not work on Apple ARM systems because kexec is broken and > cannot be fully fixed, due to multiple platform/firmware limitations. A > very limited version of kexec might work well enough for kdump, but I > don't think anyone has looked into making that work yet... > >> but ... I am not familiar with devcoredump. So I don't know when/how it >> runs, and if the source system is still alive (and remains alive --  in >> contrast to a kernel crash). > > Devcoredump just makes the dump available via /sys so it can be > collected by the user. The system is still alive, the GPU is just dead > and all future GPU job submissions fail. You can still SSH in or (at > least in theory, if enough moving parts are graceful about it) VT-switch > to a TTY. The display controller is not part of the GPU, it is separate > hardware. Thanks for all the details (and sorry for the delay, I'm on PTO until Monday ... :) (regarding the other mail) Adding that stuff to rust just so we have a devcoredump that ideally wouldn't exist is a bit unfortunate. So I'm curious: we do have /proc/kcore, where we do all of the required filtering, only allowing for reading memory that is online, not hwpoisoned etc. makedumpfile already supports /proc/kcore. Would it be possible to avoid Devcoredump completely either by dumping /proc/kcore directly or by having a user-space script that walks the page tables to dump the content purely based on /proc/kcore? If relevant memory ranges are inaccessible from /proc/kcore, we could look into exposing them. -- Cheers, David / dhildenb