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[34.142.210.164]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-85dc003aee2sm1404704b3a.30.2026.09.02.06.42.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 02 Sep 2026 06:42:39 -0700 (PDT) Date: Wed, 2 Sep 2026 13:42:31 +0000 From: Pranjal Shrivastava To: Christian =?iso-8859-1?Q?K=F6nig?= Cc: Leon Romanovsky , David Hu , sumit.semwal@linaro.org, alex@shazbot.org, ankita@nvidia.com, chriscli@google.com, david.laight.linux@gmail.com, dri-devel@lists.freedesktop.org, iommu@lists.linux.dev, jgg@ziepe.ca, jmoroni@google.com, kevin.tian@intel.com, kpberry@google.com, linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, nicolinc@nvidia.com, sashiko-bot@kernel.org, stable@vger.kernel.org, viursachi@google.com, xuehaohu@google.com Subject: Re: [PATCH v8 0/2] dma-buf: Fix silent overflow and alignment Message-ID: References: <20260901170849.4052816-1-dhu@x6u.co> <2bf581db-7cc5-4f38-a075-1b988c332f3c@amd.com> <20260902073936.GR24140@unreal> <20260902083232.GT24140@unreal> <20260902095338.GU24140@unreal> <65dc4ca1-6882-490d-baad-3e0abd708af3@amd.com> Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <65dc4ca1-6882-490d-baad-3e0abd708af3@amd.com> On Wed, Sep 02, 2026 at 12:00:25PM +0200, Christian König wrote: > On 9/2/26 11:53, Leon Romanovsky wrote: > > On Wed, Sep 02, 2026 at 10:44:59AM +0200, Christian König wrote: > >> On 9/2/26 10:32, Leon Romanovsky wrote: > >>> On Wed, Sep 02, 2026 at 09:56:06AM +0200, Christian König wrote: > >>>> On 9/2/26 09:39, Leon Romanovsky wrote: > >>>>> On Wed, Sep 02, 2026 at 09:00:46AM +0200, Christian König wrote: > >>>>>> On 9/1/26 19:08, David Hu wrote: > >>>>>>> From: David Hu > >>>>>>> > >>>>>>> This series address two related issues in scatter-gather mapping, > >>>>>>> specifically for the MMIO based dma-buf mapping. The fixes ensure > >>>>>>> sgt mapping is correct, and proper for large MMIO regions. > >>>>>>> > >>>>>>> Patch 1 fixes a silent integer overflow for mapping length exceeding 4G > >>>>>>> (Previously submitted as [PATCH v7] dma-buf: Fix silent overflow for > >>>>>>> phys vec to sgt) > >>>>>>> https://lore.kernel.org/all/20260609164047.486227-1-xuehaohu@google.com/ > >>>>>>> > >>>>>>> Patch 2 Splits sgl by largest page aligned chunk > >>>>>>> (Previously submitted as [PATCH v3] dma-buf: Split sgl by largest page-aligned chunk) > >>>>>>> https://lore.kernel.org/all/20260722233806.3922093-1-dhu@x6u.co/ > >>>>>> > >>>>>> *sigh* such issues are exactly the reason why I didn't wanted the dma-mapping stuff inside DMA-buf. That clearly doesn't belong here. > >>>>> > >>>>> And this is why so many in the kernel community want to get rid of SG > >>>>> lists. It would be great if DMA-BUF could also eliminate the need to > >>>>> convert to an SGL, like Jason proposed. > >>>>> > >>>>> The DMA layer no longer needs SGL. These bugs belong to the DMA-BUF layer, > >>>>> which is the one that depends on it. > >>>> > >>>> I'm all fine using an array/xarray of dma_addr_t in DMA-buf, just phys_vec is a clear no-go. > >>> > >>> You are proposing the same thing as an SGL, just in a different format. > >> > >> Yes, because that is the right thing todo as far as I can see. > >> > >>> It does not address the issue that dma_addr_t is expected to hold a DMA > >>> address, while that is not always the case. For example, in the P2P case, > >>> the addresses are not DMA addresses. > >> > >> Yes they are. They must be DMA addresses because that is the only thing the importer needs to do it's DMA. > > > > They can perform DMA, but that still does not make them suitable for the > > dma_addr_t type. For the PCI_P2PDMA_MAP_BUS_ADDR flow, these addresses > > follow completely different rules: they are not unmapped, require no cache > > synchronization, are valid only for peer access, and require separate error > > handling. > > The PCI_P2PDMA_MAP_BUS_ADDR is not supported by DMA-buf and as far as I can see is a complete dead end. > > > All of this information is lost if only the dma_addr_t is stored. > > Yes and that is fully intentional. > > DMA-buf handles that cleanly on the buffer object level and not like PCI_P2PDMA_MAP_BUS_ADDR as a completely broken design on a per address/page basis. > > Technical background is that the PCI_P2PDMA_MAP_BUS_ADDR approach can only be handled by a very very small subset of HW. > With the rise of accelerators, the P2PDMA_MAP_BUS_ADDR is going to be increasingly more common where accelerators directly transfer data to NICs, storage devices, other accelerators etc. With VFIO gaining a DMABUF exporter the use has already spread to RDMA & NVMe devices (w/ SPDK). In fact, we found these bugs while trying to map large BAR regions for RDMA. Thus, it would be great if we could find alignment here. AFAICT, I foresee the use of dmabufs to only increase for PCI_P2PDMA_MAP_BUS_ADDR. IIRC when the network stack moved to net_iovs to support dmabufs, the SGL became a primary concern and partly the reason why we have "unreadable" skbs for memory we can indeed access if mapped correctly. Thanks, Praan