Re: Race condition observed between page migration and page fault handling on arm64 machines

[David Hildenbrand <david@redhat.com>]

On 01.08.24 15:13, David Hildenbrand wrote:
>>>> To dampen the tradeoff, we could do this in shmem_fault() instead? But
>>>> then, this would mean that we do this in all
>>>>
>>>> kinds of vma->vm_ops->fault, only when we discover another reference
>>>> count race condition :) Doing this in do_fault()
>>>>
>>>> should solve this once and for all. In fact, do_pte_missing() may call
>>>> do_anonymous_page() or do_fault(), and I just
>>>>
>>>> noticed that the former already checks this using vmf_pte_changed().
>>>
>>> What I am still missing is why this is (a) arm64 only; and (b) if this
>>> is something we should really worry about. There are other reasons
>>> (e.g., speculative references) why migration could temporarily fail,
>>> does it happen that often that it is really something we have to worry
>>> about?
>>
>>
>> (a) See discussion at [1]; I guess it passes on x86, which is quite
>> strange since the race is clearly arch-independent.
> 
> Yes, I think this is what we have to understand. Is the race simply less
> likely to trigger on x86?
> 
> I would assume that it would trigger on any arch.
> 
> I just ran it on a x86 VM with 2 NUMA nodes and it also seems to work here.
> 
> Is this maybe related to deferred flushing? Such that the other CPU will
> by accident just observe the !pte_none a little less likely?
> 
> But arm64 also usually defers flushes, right? At least unless
> ARM64_WORKAROUND_REPEAT_TLBI is around. With that we never do deferred
> flushes.

Bingo!

diff --git a/mm/rmap.c b/mm/rmap.c
index e51ed44f8b53..ce94b810586b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -718,10 +718,7 @@ static void set_tlb_ubc_flush_pending(struct 
mm_struct *mm, pte_t pteval,
   */
  static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
  {
-       if (!(flags & TTU_BATCH_FLUSH))
-               return false;
-
-       return arch_tlbbatch_should_defer(mm);
+       return false;
  }


On x86:

# ./migration
TAP version 13
1..1
# Starting 1 tests from 1 test cases.
#  RUN           migration.shared_anon ...
Didn't migrate 1 pages
# migration.c:170:shared_anon:Expected migrate(ptr, self->n1, self->n2) 
(-2) == 0 (0)
# shared_anon: Test terminated by assertion
#          FAIL  migration.shared_anon
not ok 1 migration.shared_anon


It fails all of the time!

-- 
Cheers,

David / dhildenb