Re: [RFC PATCH] mm: Introduce new MADV_NOMOVABLE behavior

Baolin Wang
2022-10-20T07:17:20Z

On 10/19/2022 11:17 PM, David Hildenbrand wrote:
>> I observed one migration failure case (which is not easy to reproduce)
>> is that, the 'thp_migration_fail' count is 1 and the
>> 'thp_split_page_failed' count is also 1.
>>
>> That means when migrating a THP which is in CMA area, but can not
>> allocate a new THP due to memory fragmentation, so it will split the
>> THP. However THP split is also failed, probably the reason is temporary
>> reference count of this THP. And the temporary reference count can be
>> caused by dropping page caches (I observed the drop caches operation in
>> the system), but we can not drop the shmem page caches due to they are
>> already dirty at that time.
>>
>> So we can try again in migrate_pages() if THP split is failed to
>> mitigate the failure of migration, especially for the failure reason is
>> temporary reference count? Does this sound reasonable for you?
>
> It sound reasonable, and I understand that debugging these issues is
> tricky. But we really have to figure out the root cause to make these
> pages that are indeed movable (but only temporarily not movable for
> reason XYZ) movable.
>
> We'd need some indication to retry migration longer / again.

OK. Let me try this and see if there are other possible failure cases in
the products.

>>
>> However I still worried there are other possible cases to cause
>> migration failure, so no CMA allocation for our case seems more stable
>> IMO.
>
> Yes, I can understand that. But as one example, you're approach doesn't
> handle the case that a page that was allocated on !CMA/!ZONE_MOVABLE
> would get migrated to CMA/ZONE_MOVABLE just before you would try pinning
> the page (to migrate it again off CMA/ZONE_MOVABLE).

Indeed, like you said before, just helpful to minimize page migration
now. Maybe I can take MADV_PINNABLE into considering when allocating new
pages, such as alloc_migration_target().

Anyway let me try to fix the root cause first to see if it can solve our
problem.

> We really have to fix the root cause.

OK. Thanks for your input.