Re: [PATCH 1/1] mm: disable CONFIG_PER_VMA_LOCK by default until its fixed

[David Hildenbrand <david@redhat.com>]

On 04.07.23 08:50, Suren Baghdasaryan wrote:
> On Mon, Jul 3, 2023 at 10:39 PM Suren Baghdasaryan <surenb@google.com> wrote:
>>
>> On Mon, Jul 3, 2023 at 8:30 PM David Hildenbrand <david@redhat.com> wrote:
>>>
>>> On 03.07.23 20:21, Suren Baghdasaryan wrote:
>>>> A memory corruption was reported in [1] with bisection pointing to the
>>>> patch [2] enabling per-VMA locks for x86.
>>>> Disable per-VMA locks config to prevent this issue while the problem is
>>>> being investigated. This is expected to be a temporary measure.
>>>>
>>>> [1] https://bugzilla.kernel.org/show_bug.cgi?id=217624
>>>> [2] https://lore.kernel.org/all/20230227173632.3292573-30-surenb@google.com
>>>>
>>>> Reported-by: Jiri Slaby <jirislaby@kernel.org>
>>>> Reported-by: Jacob Young <jacobly.alt@gmail.com>
>>>> Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first")
>>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>>> ---
>>>>    mm/Kconfig | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>>> index 09130434e30d..de94b2497600 100644
>>>> --- a/mm/Kconfig
>>>> +++ b/mm/Kconfig
>>>> @@ -1224,7 +1224,7 @@ config ARCH_SUPPORTS_PER_VMA_LOCK
>>>>           def_bool n
>>>>
>>>>    config PER_VMA_LOCK
>>>> -     def_bool y
>>>> +     bool "Enable per-vma locking during page fault handling."
>>>>        depends on ARCH_SUPPORTS_PER_VMA_LOCK && MMU && SMP
>>>>        help
>>>>          Allow per-vma locking during page fault handling.
>>>
>>> As raised at LSF/MM, I was "surprised" that we can now handle page faults
>>> concurrent to fork() and was expecting something to be broken already.
>>>
>>> What probably happens is that we wr-protected the page in the parent process and
>>> COW-shared an anon page with the child using copy_present_pte().
>>>
>>> But we only flush the parent MM tlb before we drop the parent MM lock in
>>> dup_mmap().
>>>
>>>
>>> If we get a write-fault before that TLB flush in the parent, and we end up
>>> replacing that anon page in the parent process in do_wp_page() [because, COW-shared with the child],
>>> this might be problematic: some stale writable TLB entries can target the wrong (old) page.
>>
>> Hi David,
>> Thanks for the detailed explanation. Let me check if this is indeed
>> what's happening here. If that's indeed the cause, I think we can
>> write-lock the VMAs being dup'ed until the TLB is flushed and
>> mmap_write_unlock(oldmm) unlocks them all and lets page faults to
>> proceed. If that works we at least will know the reason for the memory
>> corruption.
> 
> Yep, locking the VMAs being copied inside dup_mmap() seems to fix the issue:
> 
>          for_each_vma(old_vmi, mpnt) {
>                  struct file *file;
> 
> +               vma_start_write(mpnt);
>                 if (mpnt->vm_flags & VM_DONTCOPY) {
>                         vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt));
>                          continue;
>                 }
> 
> At least the reproducer at
> https://bugzilla.kernel.org/show_bug.cgi?id=217624 is working now. But
> I wonder if that's the best way to fix this. It's surely simple but
> locking every VMA is not free and doing that on every fork might
> regress performance.


That would mean that we can possibly still get page faults concurrent to 
fork(), on the yet unprocessed part. While that fixes the issue at hand, 
I cannot reliably tell if this doesn't mess with some other fork() 
corner case.

I'd suggest write-locking all VMAs upfront, before doing any kind of 
fork-mm operation. Just like the old code did. See below.

> 
>> Thanks,
>> Suren.
>>
>>>
>>>
>>> We had similar issues in the past with userfaultfd, see the comment at the beginning of do_wp_page():
>>>
>>>
>>>          if (likely(!unshare)) {
>>>                  if (userfaultfd_pte_wp(vma, *vmf->pte)) {
>>>                          pte_unmap_unlock(vmf->pte, vmf->ptl);
>>>                          return handle_userfault(vmf, VM_UFFD_WP);
>>>                  }
>>>
>>>                  /*
>>>                   * Userfaultfd write-protect can defer flushes. Ensure the TLB
>>>                   * is flushed in this case before copying.
>>>                   */
>>>                  if (unlikely(userfaultfd_wp(vmf->vma) &&
>>>                               mm_tlb_flush_pending(vmf->vma->vm_mm)))
>>>                          flush_tlb_page(vmf->vma, vmf->address);
>>>          }
> 
> If do_wp_page() could identify that vmf->vma is being copied, we could
> simply return VM_FAULT_RETRY and retry the page fault under mmap_lock,
> which would block until dup_mmap() is done... Maybe we could use
> mm_tlb_flush_pending() for that? WDYT?

I'm not convinced that we should be making that code more complicated 
simply to speed up fork() with concurrent page faults.

My gut feeling is that many operations that could possible take the VMA 
lock in the future (page pinning triggering unsharing) should not run 
concurrent with fork().

So IMHO, keep the old behavior of fork() -- i.e., no concurrent page 
faults -- and unlock that eventually in the future when deemed really 
required (but people should really avoid fork() in performance-sensitive 
applications if not absolutely required).

-- 
Cheers,

David / dhildenb