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

Nadav Amit
2020-12-21T19:53:01Z

> 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?