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[2003:d8:2f20:7500:5f99:9633:990e:138]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b79c467994sm6378420f8f.50.2025.08.01.09.50.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 01 Aug 2025 09:50:20 -0700 (PDT) Message-ID: Date: Fri, 1 Aug 2025 18:50:18 +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: Jason Gunthorpe , Alistair Popple 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> <20250801164058.GD26511@ziepe.ca> 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: rspam05 X-Rspamd-Queue-Id: CDDC740009 X-Stat-Signature: n6zhu75gmxucrcn66eukhbfdmqj83j1d X-HE-Tag: 1754067024-164641 X-HE-Meta: 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 nGknVh8T kYcv53rU1va8fJG8T59Ike6yDwZ2CKWo2wh4bMiFUm8rN29/eTIo3hd8rJQSfNtwpUIJD9Ys+fM86jfSb4k4XkP0MI/RAzinbNV2Uk364rWJ2agKbkBbiZHCgYlXEgCmsSnSz3V/N4iH4N170TYT4ZO6y15s2irJCy3X0u9A7BLkyxfGunEZ0g4hVGStbX4pCpS/7DOigwdvOB1Sa+Fg4bizdvOkRvYYy6vpwBUyzgQvCoc4ZuBfifmJiok4ioVfdotW+cvGUA2n113fRQ3GOJ55aJEMh5u4Xpa5JfDXh91CubR+1MfrBbDTQDk4WPKLZJPCXWeljSjwHdQoWo9l8p3dnzJR2JsaCNqPDUsf52yGHAPDzOOSMxKcqcevHLO6BrB1RNlpPXN6/SbMkvRCcOeuBIlJPO9CPtA3QX125GeNqoZa6phweSgfPGHl6Sug3Al+S9R5vCliCNYeSuqSpGJWx0Q== 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 01.08.25 18:40, Jason Gunthorpe wrote: > On Fri, Jul 25, 2025 at 10:31:25AM +1000, Alistair Popple wrote: > >> 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. > > hmm does it, it encodes the device private into a pfn and expects the > caller to do pfn to page. > > This isn't set in stone and could be changed.. > > But broadly, you'd want to entirely eliminate the ability to go from > pfn to device private or from device private to pfn. > > Instead you'd want to work on some (space #, space index) tuple, maybe > encoded in a pfn_t, but absolutely and typesafely distinct. Each > driver gets its own 0 based space for device private information, the > space is effectively the pgmap. > > And if you do this, maybe we don't need struct page (I mean the type!) > backing device memory at all.... Which would be a very worthwhile > project. > > Do we ever even use anything in the device private struct page? Do we > refcount it? ref-counted and map-counted ... -- Cheers, David / dhildenb