Re: [PATCH v2 1/2] mm/huge_memory: Adjust try_to_migrate_one() and split_huge_pmd_locked()

[David Hildenbrand <david@redhat.com>]

On 25.04.25 13:10, Zi Yan wrote:
> On 25 Apr 2025, at 6:38, Gavin Guo wrote:
> 
>> The split_huge_pmd_locked function currently performs redundant checks
>> for migration entries and folio validation that are already handled by
>> the page_vma_mapped_walk mechanism in try_to_migrate_one.
>>
>> Specifically, page_vma_mapped_walk already ensures that:
>> - The folio is properly mapped in the given VMA area
>> - pmd_trans_huge, pmd_devmap, and migration entry validation are
>> performed
>>
>> To leverage page_vma_mapped_walk's work, moving TTU_SPLIT_HUGE_PMD
>> handling to the while loop checking and removing these duplicate checks
>> from split_huge_pmd_locked.
>>
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Link: https://lore.kernel.org/all/98d1d195-7821-4627-b518-83103ade56c0@redhat.com/
>> Link: https://lore.kernel.org/all/91599a3c-e69e-4d79-bac5-5013c96203d7@redhat.com/
>> Signed-off-by: Gavin Guo <gavinguo@igalia.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> ---

> 
> I wonder if we need a WARN here to make sure when THP migration support is not
> present all PMDs are split in try_to_migrate_one().

Can you elaborate on the condition you have in mind?

If we have TTU_SPLIT_HUGE_PMD set, we'll never reach that point.

Without CONFIG_ARCH_ENABLE_THP_MIGRATION, we should be running into the
	VM_BUG_ON_FOLIO(!pvmw.pte, folio);

right?

-- 
Cheers,

David / dhildenb