On 6/6/25 11:52, wangtao wrote: > > >> -----Original Message----- >> From: Christoph Hellwig <h...@infradead.org> >> Sent: Tuesday, June 3, 2025 9:20 PM >> To: Christian König <christian.koe...@amd.com> >> Cc: Christoph Hellwig <h...@infradead.org>; wangtao >> <tao.wang...@honor.com>; sumit.sem...@linaro.org; kra...@redhat.com; >> vivek.kasire...@intel.com; v...@zeniv.linux.org.uk; brau...@kernel.org; >> hu...@google.com; a...@linux-foundation.org; amir7...@gmail.com; >> benjamin.gaign...@collabora.com; brian.star...@arm.com; >> jstu...@google.com; tjmerc...@google.com; j...@suse.cz; >> baolin.w...@linux.alibaba.com; linux-me...@vger.kernel.org; dri- >> de...@lists.freedesktop.org; linaro-mm-...@lists.linaro.org; linux- >> ker...@vger.kernel.org; linux-fsde...@vger.kernel.org; linux- >> m...@kvack.org; wangbintian(BintianWang) <bintian.w...@honor.com>; >> yipengxiang <yipengxi...@honor.com>; liulu 00013167 >> <liulu....@honor.com>; hanfeng 00012985 <feng....@honor.com> >> 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. >
