Re: [RFC PATCH 03/13] selftests/vm/userfaultfd: wake after copy failure

[Peter Xu <peterx@redhat.com>]

On Sat, Nov 28, 2020 at 04:45:38PM -0800, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> When userfaultfd copy-ioctl fails since the PTE already exists, an
> -EEXIST error is returned and the faulting thread is not woken. The
> current userfaultfd test does not wake the faulting thread in such case.
> The assumption is presumably that another thread set the PTE through
> copy/wp ioctl and would wake the faulting thread or that alternatively
> the fault handler would realize there is no need to "must_wait" and
> continue. This is not necessarily true.
> 
> There is an assumption that the "must_wait" tests in handle_userfault()
> are sufficient to provide definitive answer whether the offending PTE is
> populated or not. However, userfaultfd_must_wait() test is lockless.
> Consequently, concurrent calls to ptep_modify_prot_start(), for
> instance, can clear the PTE and can cause userfaultfd_must_wait()
> to wrongly assume it is not populated and a wait is needed.

Yes userfaultfd_must_wait() is lockless, however my understanding is that we'll
enqueue before reading the page table, which seems to me that we'll always get
notified even the race happens.  Should apply to either UFFDIO_WRITEPROTECT or
UFFDIO_COPY, iiuc, as long as we follow the order of (1) modify pgtable (2)
wake sleeping threads.  Then it also means that when must_wait() returned true,
it should always get waked up when fault resolved.

Taking UFFDIO_COPY as example, even if UFFDIO_COPY happen right before
must_wait() calls:

       worker thread                       uffd thread
       -------------                       -----------

   handle_userfault
    spin_lock(fault_pending_wqh)
    enqueue()
    set_current_state(INTERRUPTIBLE)
    spin_unlock(fault_pending_wqh)
    must_wait()
      lockless walk page table
                                           UFFDIO_COPY
                                             fill in the hole
                                             wake up threads
                                               (this will wake up worker thread too?)
    schedule()
      (which may return immediately?)

While here fault_pending_wqh is lock protected. I just feel like there's some
other reason to cause the thread to stall.  Or did I miss something?

Thanks,

-- 
Peter Xu