Re: Using userfaultfd with KVM's async page fault handling causes processes to hung waiting for mmap_lock to be released

[Axel Rasmussen <axelrasmussen@google.com>]

Thanks for the detailed report Dimitris! I've CCed the MM mailing list and some
folks who work on userfaultfd.

I took a look at this today, but I haven't quite come up with a solution.

I thought it might be as easy as changing userfaultfd_release() to set released
*after* taking the lock. But no such luck, the ordering is what it is to deal
with another subtle case:


	WRITE_ONCE(ctx->released, true);

	if (!mmget_not_zero(mm))
		goto wakeup;

	/*
	 * Flush page faults out of all CPUs. NOTE: all page faults
	 * must be retried without returning VM_FAULT_SIGBUS if
	 * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
	 * changes while handle_userfault released the mmap_lock. So
	 * it's critical that released is set to true (above), before
	 * taking the mmap_lock for writing.
	 */
	mmap_write_lock(mm);

I think perhaps the right thing to do is to have handle_userfault() release
mmap_lock when it returns VM_FAULT_NOPAGE, and to have GUP deal with that
appropriately? But, some investigation is required to be sure that's okay to do
in the other non-GUP ways we can end up in handle_userfault().