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

[Nadav Amit <namit@vmware.com>]

> On Dec 21, 2020, at 11:28 AM, Peter Xu <peterx@redhat.com> wrote:
> 
> 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?

But what happens if the copy completed before the enqueuing? Assume
the page is write-protected during UFFDIO_COPY:


cpu0					cpu1		
----					----			
handle_userfault
					UFFDIO_COPY
					[ write-protected ]
				 	 fill in the hole
				 	 wake up threads
				 	 [nothing to wake]
							
					UFFD_WP (unprotect)
					 logically marks as unprotected
					 [nothing to wake]

 spin_lock(fault_pending_wqh)
  enqueue()
  set_current_state(INTERRUPTIBLE)
  spin_unlock(fault_pending_wqh)
  must_wait()

					[ #PF on the same PTE
					 due to write-protection ]

					...
					 wp_page_copy()
					  ptep_clear_flush_notify()
					  [ PTE is clear ]
					
   lockless walk page table
    pte_none(*pte) -> must wait

Note that additional scenarios are possible. For instance, instead of
wp_page_copy(), we can have other change_pte_range() (due to worker’s
mprotect() or NUMA balancing), calling ptep_modify_prot_start() and clearing
the PTE.

Am I missing something?