I see the improvements that Facebook have been making to the nbd driver, and I think that's a wonderful thing. Maybe the outcome of this topic is simply: "Shut up, Matthew, this is good enough". It's clear that there's an appetite for userspace block devices; not for swap devices or the root device, but for accessing data that's stored in that silo over there, and I really don't want to bring that entire mess of CORBA / Go / Rust / whatever into the kernel to get to it, but it would be really handy to present it as a block device. I've looked at a few block-driver-in-userspace projects that exist, and they all seem pretty bad. For example, one API maps a few gigabytes of address space and plays games with vm_insert_page() to put page cache pages into the address space of the client process. Of course, the TLB flush overhead of that solution is criminal. I've looked at pipes, and they're not an awful solution. We've almost got enough syscalls to treat other objects as pipes. The problem is that they're not seekable. So essentially you're looking at having one pipe per outstanding command. If yu want to make good use of a modern NAND device, you want a few hundred outstanding commands, and that's a bit of a shoddy interface. Right now, I'm leaning towards combining these two approaches; adding a VM_NOTLB flag so the mmaped bits of the page cache never make it into the process's address space, so the TLB shootdown can be safely skipped. Then check it in follow_page_mask() and return the appropriate struct page. As long as the userspace process does everything using O_DIRECT, I think this will work. It's either that or make pipes seekable ...
