Hi folks, I want to propose a discussion on adding zero copy to FUSE io_uring in the kernel. The source is some userspace buffer or device memory e.g. GPU VRAM. The destination is FUSE server in userspace, which will then either forward it over the network or to an underlying FS/block device. The FUSE server may want to read the data. My goal is to eliminate copies in this entire data path, including the initial hop between the userspace client and the kernel. I know Ming and Keith are working on adding ublk zero copy but it does not cover this initial hop and it does not allow the ublk/FUSE server to read the data. My idea is to use shared memory or dma-buf, i.e. the source data is encapsulated in an mmap()able fd. The client and FUSE server exchange this fd through a back channel with no kernel involvement. The FUSE server maps the fd into its address space and registers the fd with io_uring via the io_uring_register() infra. When the client does e.g. a DIO write, the pages are pinned and forwarded to FUSE kernel, which does a lookup and understands that the pages belong to the fd that was registered from the FUSE server. Then io_uring tells the FUSE server that the data is in the fd it registered, so there is no need to copy anything at all. I would like to discuss this and get feedback from the community. My top question is why do this in the kernel at all? It is entirely possible to bypass the kernel entirely by having the client and FUSE server exchange the fd and then do the I/O purely through IPC. David
