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mkyJSI1E3mNhcH5L2K4oob0nKA+eD2ltYPnul6zwWnH1H6Wf7jq4CoisUjl+WA7zDAoY8o72aUhOwoPLjhYEugVJFJaM7QUo3ko4guKuoRHyaumuGJ/pBcZ9mooFXjn4/6s3GH0QHiOwKb8DH3jP4BaRXxJNRrUa0Q4EFYM/3uN4CTYLGcnuzpEiwVloQ7X/ofHvTg/+pgMMOAYgFoFkzdrrnwAvUlpETe3uUQLA+honEoJphHbtRk21ETLk4md/zq0+cGUledG0DvR2NJkXE9PokVUaokAA+uuXD7xp25xFrvIlQXauEQ== 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 6/6/25 11:52, wangtao wrote: > > >> -----Original Message----- >> From: Christoph Hellwig >> Sent: Tuesday, June 3, 2025 9:20 PM >> To: Christian König >> Cc: Christoph Hellwig ; wangtao >> ; sumit.semwal@linaro.org; kraxel@redhat.com; >> vivek.kasireddy@intel.com; viro@zeniv.linux.org.uk; brauner@kernel.org; >> hughd@google.com; akpm@linux-foundation.org; amir73il@gmail.com; >> benjamin.gaignard@collabora.com; Brian.Starkey@arm.com; >> jstultz@google.com; tjmercier@google.com; jack@suse.cz; >> baolin.wang@linux.alibaba.com; linux-media@vger.kernel.org; dri- >> devel@lists.freedesktop.org; linaro-mm-sig@lists.linaro.org; linux- >> kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org; linux- >> mm@kvack.org; wangbintian(BintianWang) ; >> yipengxiang ; liulu 00013167 >> ; hanfeng 00012985 >> Subject: Re: [PATCH v4 0/4] Implement dmabuf direct I/O via >> copy_file_range >> >> On Tue, Jun 03, 2025 at 03:14:20PM +0200, Christian König wrote: >>> On 6/3/25 15:00, Christoph Hellwig wrote: >>>> This is a really weird interface. No one has yet to explain why >>>> dmabuf is so special that we can't support direct I/O to it when we >>>> can support it to otherwise exotic mappings like PCI P2P ones. >>> >>> With udmabuf you can do direct I/O, it's just inefficient to walk the >>> page tables for it when you already have an array of all the folios. >> >> Does it matter compared to the I/O in this case? >> >> Either way there has been talk (in case of networking implementations) that >> use a dmabuf as a first class container for lower level I/O. >> I'd much rather do that than adding odd side interfaces. I.e. have a version >> of splice that doesn't bother with the pipe, but instead just uses in-kernel >> direct I/O on one side and dmabuf-provided folios on the other. > If the VFS layer recognizes dmabuf type and acquires its sg_table > and folios, zero-copy could also be achieved. I initially thought > dmabuf acts as a driver and shouldn't be handled by VFS, so I made > dmabuf implement copy_file_range callbacks to support direct I/O > zero-copy. I'm open to both approaches. What's the preference of > VFS experts? That would probably be illegal. Using the sg_table in the DMA-buf implementation turned out to be a mistake. The question Christoph raised was rather why is your CPU so slow that walking the page tables has a significant overhead compared to the actual I/O? Regards, Christian. > > Regards, > Wangtao. >