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[2003:cb:c70a:300:3ae1:c3c0:cef:8413]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438e244eb0dsm187009005e9.27.2025.02.04.02.26.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 04 Feb 2025 02:26:22 -0800 (PST) Message-ID: <0e19e1c3-293b-4740-93f3-2c410893288b@redhat.com> Date: Tue, 4 Feb 2025 11:26:19 +0100 Precedence: bulk X-Mailing-List: rust-for-linux@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: Zi Yan , Asahi Lina 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> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 03.02.25 22:05, Zi Yan wrote: > On 3 Feb 2025, at 9:32, Asahi Lina wrote: > >> On 2/3/25 6:58 PM, 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. >>> >>> iow I think anticipating the future where struct page really doesn't have >>> a refcount is the right move. Aside from that it's really not a refcount >>> that works in the rust ARef sense, since struct page cannot disappear for >>> system memory, and for dev_pagemap memory it's an entirely different >>> reference you need (and then there's a few more special cases). >> >> Right, as far as this abstraction is concerned, all that needs to hold >> is that: >> >> - alloc_pages() and __free_pages() work as intended, however that may >> be, to reserve and return one page (for now, though I think extending >> the Rust abstraction to handle higher-order folios is pretty easy, but >> that can happen later). >> - Whatever borrows pages knows what it's doing. In this case there's >> only support for borrowing pages by physaddr, and it's only going to be >> used in a driver for a platform without memory hot remove (so far) and >> only for pages which have known usage (in principle) and are either >> explicitly allocated or known pinned or reserved, so it's not a problem >> right now. Future abstractions that return borrowed pages can do their >> own locking/bookkeeping/whatever is necessary to keep it safe. >> >> I would like to hear how memory hot-remove is supposed to work though, >> to see if we should be doing something to make the abstraction safer >> (though it's still unsafe and always will be). Is there a chance a >> `struct page` could vanish out from under us under some conditions? > > Add DavidH and OscarS for memory hot-remove questions. > > IIUC, struct page could be freed if a chunk of memory is hot-removed. Right, but only after there are no users anymore (IOW, memory was freed back to the buddy). PFN walkers might still stumble over them, but I would not expect (or recommend) rust to do that. > > Another case struct page can be freed is when hugetlb vmemmap optimization > is used. Muchun (cc'd) is the maintainer of hugetlbfs. Here, the "struct page" remains valid though; it can still be accessed, although we disallow writes (which would be wrong). If you only allocate a page and free it later, there is no need to worry about either on the rust side. -- Cheers, David / dhildenb