> -----Original Message----- > From: Christoph Hellwig <hch@xxxxxxxxxxxxx> > Sent: Tuesday, June 3, 2025 9:20 PM > To: Christian König <christian.koenig@xxxxxxx> > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>; wangtao > <tao.wangtao@xxxxxxxxx>; sumit.semwal@xxxxxxxxxx; kraxel@xxxxxxxxxx; > vivek.kasireddy@xxxxxxxxx; viro@xxxxxxxxxxxxxxxxxx; brauner@xxxxxxxxxx; > hughd@xxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; amir73il@xxxxxxxxx; > benjamin.gaignard@xxxxxxxxxxxxx; Brian.Starkey@xxxxxxx; > jstultz@xxxxxxxxxx; tjmercier@xxxxxxxxxx; jack@xxxxxxx; > baolin.wang@xxxxxxxxxxxxxxxxx; linux-media@xxxxxxxxxxxxxxx; dri- > devel@xxxxxxxxxxxxxxxxxxxxx; linaro-mm-sig@xxxxxxxxxxxxxxxx; linux- > kernel@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx; linux- > mm@xxxxxxxxx; wangbintian(BintianWang) <bintian.wang@xxxxxxxxx>; > yipengxiang <yipengxiang@xxxxxxxxx>; liulu 00013167 > <liulu.liu@xxxxxxxxx>; hanfeng 00012985 <feng.han@xxxxxxxxx> > 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? Regards, Wangtao.
