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

[Axel Rasmussen]

On Wed, Jul 19, 2023 at 2:16 PM Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote:
>
> Thanks for the detailed report Dimitris! I've CCed the MM mailing list and some
> folks who work on userfaultfd.

Apologies, I should have quoted the original message for the others I
added to CC: https://lore.kernel.org/lkml/79375b71-db2e-3e66-346b-254c90d915e2@xxxxxxxxxxxxxxxxx/T/#u

>
> 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().