Re: [PATCH] mm: migration :shared anonymous migration test is failing

[Donet Tom <donettom@linux.ibm.com>]

On 12/20/24 15:41, David Hildenbrand wrote:
> On 20.12.24 03:55, Donet Tom wrote:
>>
>> On 12/19/24 18:28, David Hildenbrand wrote:
>>> On 19.12.24 13:47, Donet Tom wrote:
>>>> The migration selftest is currently failing for shared anonymous
>>>> mappings due to a race condition.
>>>>
>>>> During migration, the source folio's PTE is unmapped by nuking the
>>>> PTE, flushing the TLB,and then marking the page for migration
>>>> (by creating the swap entries). The issue arises when, immediately
>>>> after the PTE is nuked and the TLB is flushed, but before the page
>>>> is marked for migration, another thread accesses the page. This
>>>> triggers a page fault, and the page fault handler invokes
>>>> do_pte_missing() instead of do_swap_page(), as the page is not yet
>>>> marked for migration.
>>>>
>>>> In the fault handling path, do_pte_missing() calls __do_fault()
>>>> ->shmem_fault() -> shmem_get_folio_gfp() -> filemap_get_entry().
>>>> This eventually calls folio_try_get(), incrementing the reference
>>>> count of the folio undergoing migration. The thread then blocks
>>>> on folio_lock(), as the migration path holds the lock. This
>>>> results in the migration failing in __migrate_folio(), which expects
>>>> the folio's reference count to be 2. However, the reference count is
>>>> incremented by the fault handler, leading to the failure.
>>>>
>>>> The issue arises because, after nuking the PTE and before marking the
>>>> page for migration, the page is accessed. To address this, we have
>>>> updated the logic to first nuke the PTE, then mark the page for
>>>> migration, and only then flush the TLB. With this patch, If the 
>>>> page is
>>>> accessed immediately after nuking the PTE, the TLB entry is still
>>>> valid, so no fault occurs.
>>>
>>> But what about if the PTE is not in the TLB yet, and you get an access
>>> from another CPU just after clearing the PTE (but not flushing the
>>> TLB)? The other CPU will still observe PTE=none, trigger a fault etc.
>>>
>> Yes, in this scenario, the migration will fail. Do you think the
>> migration test
>> failure, even after a retry, should be considered a major issue that
>> must be fixed?
>
> I think it is something we should definitely improve, but I think our 
> page migration should handle this in a better way, not the unmap 
> logic. I recall we discussed with Dev some ideas on how to improve that?
>
>
> I'm pretty sure one can trigger similar case using a tmpfs file and 
> using read/write in a loop instead of memory access -> page faults. So 
> where racing with page faults is completely out of the picture.

Thank you David. I will try this scenario as well and come back with 
some ideas for improvement.