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[47.55.120.4]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4aeeed669c0sm65687951cf.33.2025.08.05.07.09.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 05 Aug 2025 07:09:26 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1ujIM1-00000001XVj-3WEg; Tue, 05 Aug 2025 11:09:25 -0300 Date: Tue, 5 Aug 2025 11:09:25 -0300 From: Jason Gunthorpe To: Alistair Popple Cc: David Hildenbrand , Matthew Wilcox , Yonatan Maman , =?utf-8?B?SsOpcsO0bWU=?= Glisse , 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 Subject: Re: [PATCH v2 1/5] mm/hmm: HMM API to enable P2P DMA for device private pages Message-ID: <20250805140925.GO26511@ziepe.ca> References: <7lvduvov3rvfsgixbkyyinnzz3plpp3szxam46ccgjmh6v5d7q@zoz4k723vs3d> <20250801164058.GD26511@ziepe.ca> <20250801165749.GF26511@ziepe.ca> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Aug 04, 2025 at 11:51:38AM +1000, Alistair Popple wrote: > On Fri, Aug 01, 2025 at 01:57:49PM -0300, Jason Gunthorpe wrote: > > On Fri, Aug 01, 2025 at 06:50:18PM +0200, David Hildenbrand wrote: > > > 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. > > What callers need to do pfn to page when finding a device private pfn via > hmm_range_fault()? GPU drivers don't, they tend just to use the pfn as an offset > from the start of the pgmap to find whatever data structure they are using to > track device memory allocations. All drivers today must. You have no idea if the PFN returned is a private or CPU page. The only way to know is to check the struct page type, by looking inside the struct page. > So other than adding a HMM_PFN flag to say this is really a device index I don't > see too many issues here. Christoph suggested exactly this, and it would solve the issue. Seems quite easy too. Let's do it. Jason