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

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

On 12/19/24 18:25, 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. After marking the page for migration,
>> flushing the TLB ensures that the next page fault correctly triggers
>> do_swap_page() and waits for the migration to complete.
>>
>
> Does this reproduce with
>
> commit 536ab838a5b37b6ae3f8d53552560b7c51daeb41
> Author: Dev Jain <dev.jain@arm.com>
> Date:   Fri Aug 30 10:46:09 2024 +0530
>
>     selftests/mm: relax test to fail after 100 migration failures
>         It was recently observed at [1] that during the folio 
> unmapping stage of
>     migration, when the PTEs are cleared, a racing thread faulting on 
> that
>     folio may increase the refcount of the folio, sleep on the folio 
> lock (the
>     migration path has the lock), and migration ultimately fails when
>     asserting the actual refcount against the expected.  Thereby, the
>     migration selftest fails on shared-anon mappings.  The above 
> enforces the
>     fact that migration is a best-effort service, therefore, it is 
> wrong to
>     fail the test for just a single failure; hence, fail the test 
> after 100
>     consecutive failures (where 100 is still a subjective choice).  
> Note that,
>     this has no effect on the execution time of the test since that is
>     controlled by a timeout.
>         [1] 
> https://lore.kernel.org/all/20240801081657.1386743-1-dev.jain@arm.com/
>
> part of 6.12?
>
>
> As part of that discussion, we discussed alternatives, such as
> retrying migration more often internally.
>
I was trying with this patch and was able to recreate the issue on PowerPC.
I tried increasing the retry count, but the test was still failing.