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

David Hildenbrand
2024-12-20T10:12:02Z

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.

--
Cheers,

David / dhildenb