On Wed, Jan 17, 2018 at 01:21:44PM -0800, Matthew Wilcox wrote: > On Wed, Jan 17, 2018 at 10:49:24AM +0800, Ming Lei wrote: > > Userfaultfd might be another choice: > > > > 1) map the block LBA space into a range of process vm space > > That would limit the size of a block device to ~200TB (with my laptop's > CPU). That's probably OK for most users, but I suspect there are some > who would chafe at such a restriction (before the 57-bit CPUs arrive). > > > 2) when READ/WRITE req comes, convert it to page fault on the > > mapped range, and let userland to take control of it, and meantime > > kernel req context is slept > > You don't want to sleep the request; you want it to be able to submit > more I/O. But we have infrastructure in place to inform the submitter > when I/Os have completed. It's possible to queue IO requests and have a kthread that will convert those requests to page faults. The thread indeed will sleep on each page fault, though. > > 3) IO req context in kernel side is waken up after userspace completed > > the IO request via userfaultfd > > > > 4) kernel side continue to complete the IO, such as copying page from > > storage range to req(bio) pages. > > > > Seems READ should be fine since it is very similar with the use case > > of QEMU postcopy live migration, WRITE can be a bit different, and > > maybe need some change on userfaultfd. > > I like this idea, and maybe extending UFFD is the way to solve this > problem. Perhaps I should explain a little more what the requirements > are. At the point the driver gets the I/O, pages to copy data into (for > a read) or copy data from (for a write) have already been allocated. > At all costs, we need to avoid playing VM tricks (because TLB flushes > are expensive). So one copy is probably OK, but we'd like to avoid it > if reasonable. > > Let's assume that the userspace program looks at the request metadata and > decides that it needs to send a network request. Ideally, it would find > a way to have the data from the response land in the pre-allocated pages > (for a read) or send the data straight from the pages in the request > (for a write). I'm not sure UFFD helps us with that part of the problem. As of now it does not. UFFD allocates pages when userland asks to copy the data into UFFD controlled VMA. In your example, after the data had arrives from the network userland it can be copied into a page UFFD will allocate. Unrelated to block device, I've been thinking of implementing splice for userfaultfd... -- Sincerely yours, Mike.
