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[2003:cb:c70a:300:3ae1:c3c0:cef:8413]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438e23de016sm191286705e9.9.2025.02.04.03.59.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 04 Feb 2025 03:59:37 -0800 (PST) Message-ID: Date: Tue, 4 Feb 2025 12:59:35 +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 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> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: 591irhjc77yy9oanpuccd7wor9kwxad9 X-Rspamd-Queue-Id: 930EDC0008 X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1738670383-380935 X-HE-Meta: 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 YKKbsmWO GqbFBAIlYV/XVKoD0fdIVSXbO2ygm6JtqS1xaMNYpG9rqjacfhVGw/Msa0ESx6MPkx1bxfpfgd8IClFXaLFFXinhP6IazZYOff2LDJ5mGNwMxRH4Erht6RNTbKjxZIr84GOsupVc1H7luv8gRGPFtX6Hmtkxu/1/VtP7C9HBNbxUykvmR8MK/WN8I1e6bquixBMe5yfTacyzSBTOtl4P2lvAAhh8LDR3elGcXIu5ACU4sFMXz7QnLMkIWe8i2Xo/i5ONS8EZNIeuM/05n4S3ls9r/mSiElAUv1ahU75gMHox2PbqtWjsbfSPHyXL5Nhz1bm2Wwx7mGiv1taPPmj2dfX/iZ7MqnqrC4C1ixWvYO/YyZUa3lE5Q0+JO8LiDW3me67+L4L+lle1SMaz63QjjPzkAj5W8U7gDix9Ov4B9YYeUswx37MzxvX1w8RvmP/5nkzFWTw1mMj731nIqKR2P1gbZQg== X-Bogosity: Unsure, tests=bogofilter, spamicity=0.463919, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >>> 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. > > The physaddr to page function does look up pages by pfn, but it's > intended to be used by drivers that know what they're doing. There are > two variants of the API, one that is completely unchecked (a fast path > for cases where the driver definitely allocated these pages itself, for > example just grabbing the `struct page` back from a decoded PTE it > wrote), and one that has this check: > > pfn_valid(pfn) && page_is_ram(pfn) > > Which is intended as a safety net to allow drivers to look up > firmware-reserved pages too, and fail gracefully if the kernel doesn't > know about them (because they weren't declared in the > bootloader/firmware memory map correctly) or doesn't have them mapped in > the direct map (because they were declared no-map). > > Is there anything else that can reasonably be done here to make the API > safer to call on an arbitrary pfn? In PFN walkers we use pfn_to_online_page() to make sure that (1) the memmap really exists; and (2) that it has meaning (e.g., was actually initialized). 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(). > > 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? > - Raw pages allocated from the page allocator for use as GPU page tables, That makes sense. > - System memory that is marked reserved by the firmware/bootloader, E.g., in arch/x86/mm/ioremap.c:__ioremap_check_ram() we refuse anything that has a valid memmap and is *not* marked as PageReserved, to prevent remapping arbitrary *real* RAM. Is that case here similar? > - (Potentially) invalid PFNs that aren't part of the System RAM region > at all and don't have a struct page to begin with, which we check for, > so the API returns an error. This would only happen if the bootloader > didn't declare some used firmware ranges at all, so Linux doesn't know > about them. > >> >>> >>> 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. > > This is what the safe API does. (Also the unsafe physaddr APIs if all > you ever do is convert an allocated page to a physaddr and back, which > is the only thing the GPU page table code does during normal use. The > walking leaf PFNs story is only for GPU device coredumps when the > firmware crashes.) I would hope that we can lock down this interface as much as possible. Ideally, we would never go from pfn->page, unless (a) we remember somehow that we came from page->pfn. E.g., we allocated these pages or someone else provided us with these pages. The memmap cannot go away. I know it's hard. (b) the pages are flagged as being special, similar to __ioremap_check_ram(). -- Cheers, David / dhildenb