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[2003:cb:c70a:300:3ae1:c3c0:cef:8413]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438dcc80e61sm239135565e9.34.2025.02.04.12.05.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 04 Feb 2025 12:05:49 -0800 (PST) Message-ID: <1d379007-97c3-4bb9-93da-1a828f955fc4@redhat.com> Date: Tue, 4 Feb 2025 21:05:47 +0100 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 0/6] rust: page: Support borrowing `struct page` and physaddr conversion To: Jason Gunthorpe Cc: Asahi Lina , 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 References: <20250202-rust-page-v1-0-e3170d7fe55e@asahilina.net> <1eaf66ba-b2d3-448f-938b-913f17ca98a4@redhat.com> <20250204183933.GJ2296753@ziepe.ca> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 04.02.25 19:39, Jason Gunthorpe wrote: > On Tue, Feb 04, 2025 at 11:33:24AM +0100, David Hildenbrand wrote: >> On 03.02.25 10:58, Simona Vetter wrote: >>> On Sun, Feb 02, 2025 at 10:05:42PM +0900, Asahi Lina wrote: >>>> This series refactors the existing Page wrapper to support borrowing >>>> `struct page` objects without ownership on the Rust side, and converting >>>> page references to/from physical memory addresses. >>>> >>>> The series overlaps with the earlier submission in [1] and follows a >>>> different approach, based on the discussion that happened there. >>>> >>>> The primary use case for this is implementing IOMMU-style page table >>>> management in Rust. This allows drivers for IOMMUs and MMU-containing >>>> SoC devices to be written in Rust (such as embedded GPUs). The intended >>>> logic is similar to how ARM SMMU page tables are managed in the >>>> drivers/iommu tree. >>>> >>>> First, introduce a concept of Owned and an Ownable trait. These are >>>> similar to ARef and AlwaysRefCounted, but are used for types which >>>> are not ref counted but rather have a single intended owner. >>>> >>>> Then, refactor the existing Page support to use the new mechanism. Pages >>>> returned from the page allocator are not intended to be ref counted by >>>> consumers (see previous discussion in [1]), so this keeps Rust's view of >>>> page ownership as a simple "owned or not". Of course, this is still >>>> composable as Arc> if Rust code needs to reference count its >>>> own Page allocations for whatever reason. >>> >>> I think there's a bit a potential mess here because the conversion to >>> folios isn't far enough yet that we can entirely ignore page refcounts and >>> just use folio refcounts. But I guess we can deal with that oddity if we >>> hit it (maybe folio conversion moves fast enough), since this only really >>> starts to become relevant for hmm/svm gpu stuff. >> >> I'll note that in the future only selected things will be folios (e.g., >> pagecache, anonymous memory). Everything else will either get a separate >> memdesc (e.g., ptdesc), or work on bare pages. >> >> Likely, when talking about page tables, "ptdesc" might be what you want to >> allocate here, and not "folios". > > I just posted a series to clean up the iommu code that this is > cribbing the interface from: add an ioptdesc, remove all the struct > page from the API and so forth: > > https://lore.kernel.org/all/0-v1-416f64558c7c+2a5-iommu_pages_jgg@nvidia.com/ > > I strongly suggest that rust not expose these old schemes that will > need another cleanup like the above. Page tables just need an > allocator using simple void *, kmalloc is good enough for simple > cases. > > Drivers should not be exposing or touching struct page just to > implement a page table. Fully agreed, this is going into the right direction. Dumping what's mapped is a different story. Maybe that dumping logic could simply be written in C for the time being? -- Cheers, David / dhildenb