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[2003:d8:2f1a:f400:5a9f:b1bf:4bb3:99b1]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b7785d5e31sm207596f8f.6.2025.07.25.02.51.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 25 Jul 2025 02:51:56 -0700 (PDT) Message-ID: Date: Fri, 25 Jul 2025 11:51:54 +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 Cc: Jason Gunthorpe , 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-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: B718DA0007 X-Stat-Signature: wkrf6y5upd47wqccd6cufiw178xkadsa X-HE-Tag: 1753437120-312149 X-HE-Meta: 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 PdzW2jaz 0zvaXXPGj1034rdQWquIfZJox8ARKLELjExvogYLq/EEebT9N1tXKAe+WuYkCQfAFX5QT58mMr3HFaWGNy5yzPVThPmZS3+lHr8AhXUea+V9olnqACIfkslsIMPUeyraOCimu4rg1g3aen+aUqo6RGZaOcIryYcpjgumunpF2hm87NnJifjpvvETSQaw97LdutOdyIBSr8PGp56Xkv50BIFNSh3o/IqVu7+cQuuirlUTMENEurUooVYd2coFHbPCQ0gs0T2QKD7TwQf/RMVqCJRtWlJI839SqaJNse5Ra6YM1U7z5+pjJ0Z24STSMSgCyBylmXvIfWh/bDOyDhFvek0YLyyWzqnY5BAD5RLoF0BuMhiecD7+B7jG1CCu5JNJz36vU8+KVW04ctO0GD13V1HAk21Z1+/E2lPH4Ir2yoHi3vFZqeYvh1+BLrhKU7u04WdZuXXw+AuYPfmukUKRs9Rp/Hw== 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 25.07.25 02:31, Alistair Popple wrote: > On Thu, Jul 24, 2025 at 10:52:54AM +0200, David Hildenbrand wrote: >> 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? > > Exactly. The observation being that when you convert a PTE from a swap entry > to a page we already know it is a device private entry, so can go look up the > struct page with special magic (eg. an index into some other array or data > structure). > > And if you have a struct page you already know it's a device private page so if > you need to create the swap entry you can look up the magic index using some > alternate function. > > The only issue would be if there were generic code paths that somehow have a > raw pfn obtained from neither a page-table walk or struct page. My assumption > (yet to be proven/tested) is that these paths don't exist. I guess memory compaction and friends don't apply to ZONE_DEVICE, and even memory_failure() handling goes a separate path. -- Cheers, David / dhildenb