Re: [PATCH v3 1/3] mm/migrate_device.c: Flush TLB while holding PTL

[David Hildenbrand <david@redhat.com>]

On 24.08.22 14:26, Alistair Popple wrote:
> 
> David Hildenbrand <david@redhat.com> writes:
> 
>> On 24.08.22 05:03, Alistair Popple wrote:
>>> When clearing a PTE the TLB should be flushed whilst still holding the
>>> PTL to avoid a potential race with madvise/munmap/etc. For example
>>> consider the following sequence:
>>>
>>>   CPU0                          CPU1
>>>   ----                          ----
>>>
>>>   migrate_vma_collect_pmd()
>>>   pte_unmap_unlock()
>>>                                 madvise(MADV_DONTNEED)
>>>                                 -> zap_pte_range()
>>>                                 pte_offset_map_lock()
>>>                                 [ PTE not present, TLB not flushed ]
>>>                                 pte_unmap_unlock()
>>>                                 [ page is still accessible via stale TLB ]
>>>   flush_tlb_range()
>>>
>>> In this case the page may still be accessed via the stale TLB entry
>>> after madvise returns. Fix this by flushing the TLB while holding the
>>> PTL.
>>>
>>> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>>> Reported-by: Nadav Amit <nadav.amit@gmail.com>
>>> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
>>> Cc: stable@vger.kernel.org
>>>
>>> ---
>>>
>>> Changes for v3:
>>>
>>>  - New for v3
>>> ---
>>>  mm/migrate_device.c | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>> index 27fb37d..6a5ef9f 100644
>>> --- a/mm/migrate_device.c
>>> +++ b/mm/migrate_device.c
>>> @@ -254,13 +254,14 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>  		migrate->dst[migrate->npages] = 0;
>>>  		migrate->src[migrate->npages++] = mpfn;
>>>  	}
>>> -	arch_leave_lazy_mmu_mode();
>>> -	pte_unmap_unlock(ptep - 1, ptl);
>>>
>>>  	/* Only flush the TLB if we actually modified any entries */
>>>  	if (unmapped)
>>>  		flush_tlb_range(walk->vma, start, end);
>>>
>>> +	arch_leave_lazy_mmu_mode();
>>> +	pte_unmap_unlock(ptep - 1, ptl);
>>> +
>>>  	return 0;
>>>  }
>>>
>>>
>>> base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136
>>
>> I'm not a TLB-flushing expert, but this matches my understanding (and a
>> TLB flushing Linux documentation I stumbled over some while ago but
>> cannot quickly find).
>>
>> In the ordinary try_to_migrate_one() path, flushing would happen via
>> ptep_clear_flush() (just like we do for the anon_exclusive case here as
>> well), correct?
> 
> Correct.
> 

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb