On 6/3/25 15:19, Christoph Hellwig wrote: > 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? It unfortunately does, see the numbers on patch 3 and 4. I'm not very keen about it either, but I don't see much other way to do this. > 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. That would work for me as well. But if splice or copy_file_range is used is not that important to me. My question is rather if it's ok to call f_op->write_iter() and f_op->read_iter() with pages allocated by alloc_pages(), e.g. where drivers potentially ignore the page count and just re-use pages as they like? Regards, Christian.
