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[2003:d8:2f1f:5e00:c941:d6fb:3e30:b42]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b76fcc43c2sm1436739f8f.83.2025.07.24.01.52.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 24 Jul 2025 01:52:56 -0700 (PDT) Message-ID: Date: Thu, 24 Jul 2025 10:52:54 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/5] mm/hmm: HMM API to enable P2P DMA for device private pages To: Alistair Popple , Jason Gunthorpe Cc: Matthew Wilcox , Yonatan Maman , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Andrew Morton , Leon Romanovsky , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , Ben Skeggs , Michael Guralnik , Or Har-Toov , Daisuke Matsuda , Shay Drory , linux-mm@kvack.org, linux-rdma@vger.kernel.org, dri-devel@lists.freedesktop.org, nouveau@lists.freedesktop.org, linux-kernel@vger.kernel.org, Gal Shalom References: <20250718115112.3881129-1-ymaman@nvidia.com> <20250718115112.3881129-2-ymaman@nvidia.com> <20250718144442.GG2206214@ziepe.ca> <7lvduvov3rvfsgixbkyyinnzz3plpp3szxam46ccgjmh6v5d7q@zoz4k723vs3d> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: B69A120002 X-Stat-Signature: umzhx6y3btijq1bbg5xcodggdtbpkoic X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1753347183-621890 X-HE-Meta: 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 MEDfaPFE +XKh7XJcYYQVEJqqxJHH8O8QLSylVRjKSRjE/G6ImDocV2+PzLrFhLTkr358iK7tUPo7iwlCPFyFcj1uqcjQC1UwB5QSgwFxJh5QiG6KpLD+lUTMuN0TyDyw4cImWo2rtCj1AIiAwUVIW5Y2Uqw8itMhygpAwlciiM+Wc9jyq+9mSh4MOkTazEOdSL7xK9/xylhPal2gYlQIi3WGgy6dVlxDhAEGNFlgbe+49eHP4O1QzQ/OYhwmokmYMU22CDRPnmuF8/WaOvMlQ3nd8QdAeTW9oJssYfJZmnQKHaDdBO7bpz6VVffejJjHIzqDfeSm3PXdMAhDZ/cQqckryFvsMNliHI0gBVSN4c+3GF8UH4BpSXN5g0ZGdm42ftLhYYx7JJNZDJZX2k5/6cbjCOEUyO4MOBBIPYod4XDdS8tNkAozaTxgZJ9+RUsuVaMcp7COPQPz9UAaaCIFNxTNgUnw6G+Sh3g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, 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 23.07.25 06:10, Alistair Popple wrote: > On Wed, Jul 23, 2025 at 12:51:42AM -0300, Jason Gunthorpe wrote: >> On Tue, Jul 22, 2025 at 10:49:10AM +1000, Alistair Popple wrote: >>>> So what is it? >>> >>> IMHO a hack, because obviously we shouldn't require real physical addresses for >>> something the CPU can't actually address anyway and this causes real >>> problems >> >> IMHO what DEVICE PRIVATE really boils down to is a way to have swap >> entries that point to some kind of opaque driver managed memory. >> >> We have alot of assumptions all over about pfn/phys to page >> relationships so anything that has a struct page also has to come with >> a fake PFN today.. > > Hmm ... maybe. To get that PFN though we have to come from either a special > swap entry which we already have special cases for, or a struct page (which is > a device private page) which we mostly have to handle specially anyway. I'm not > sure there's too many places that can sensibly handle a fake PFN without somehow > already knowing it is device-private PFN. > >>> (eg. it doesn't actually work on anything other than x86_64). There's no reason >>> the "PFN" we store in device-private entries couldn't instead just be an index >>> into some data structure holding pointers to the struct pages. So instead of >>> using pfn_to_page()/page_to_pfn() we would use device_private_index_to_page() >>> and page_to_device_private_index(). >> >> It could work, but any of the pfn conversions would have to be tracked >> down.. Could be troublesome. > > I looked at this a while back and I'm reasonably optimistic that this is doable > because we already have to treat these specially everywhere anyway. How would that look like? E.g., we have code like if (is_device_private_entry(entry)) { page = pfn_swap_entry_to_page(entry); folio = page_folio(page); ... folio_get(folio); ... } We could easily stop allowing pfn_swap_entry_to_page(), turning these into non-pfn swap entries. Would it then be something like if (is_device_private_entry(entry)) { page = device_private_entry_to_page(entry); ... } Whereby device_private_entry_to_page() obtains the "struct page" not via the PFN but some other magical (index) value? -- Cheers, David / dhildenb