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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4924923392dsm530639635e9.2.2026.06.23.01.44.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 23 Jun 2026 01:44:48 -0700 (PDT) Date: Tue, 23 Jun 2026 09:44:46 +0100 From: David Laight To: David Hu Cc: Sumit Semwal , "Christian =?UTF-8?B?S8O2bmln?=" , Jason Gunthorpe , Nicolin Chen , Leon Romanovsky , Kevin Tian , Ankit Agrawal , Alex Williamson , linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org, iommu@lists.linux.dev, jmoroni@google.com, praan@google.com, kpberry@google.com, chriscli@google.com, sashiko-bot@kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v2] dma-buf: Split sgl into page-aligned 2G chunks Message-ID: <20260623094446.4a8fc2ed@pumpkin> In-Reply-To: <20260623015459.1153884-1-xuehaohu@google.com> References: <20260621222130.1667453-1-xuehaohu@google.com> <20260623015459.1153884-1-xuehaohu@google.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 23 Jun 2026 01:54:59 +0000 David Hu wrote: > Currently, `fill_sg_entry()` splits the scatterlist using `UINT_MAX`. > This creates a non-page-aligned DMA length (`0xFFFFFFFF`) for the > first entry, resulting in non-page-aligned DMA addresses for all > subsequent entries. There is a separate issue of whether this code is even needed at all. Where can transfers over 2G (never mind 4G) actually come from. The read, write and similar system calls limit transfers to INT_MAX (even on 64bit) and a lot of driver code will need fixing it longer lengths are allowed though. io_uring better enforce the same limits. So the transfers can come directly from userspace. Not only that but you also need a single physically contiguous buffer. Good luck allocating that! Now maybe there are some peer-to-peer places where the large buffer is device memory, but they will be unusual and probably need special treatment anyway. David