linux-kernel - Re: [PATCH v2 1/2] mm/huge_memory: Adjust try_to_migrate_one() and split_huge_pmd

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Message-ID: <96738f27-25bc-4d9e-9c4e-709e840563eb@redhat.com>
Date: Fri, 25 Apr 2025 13:23:26 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, Gavin Guo <gavinguo@...lia.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org, gshan@...hat.com,
 willy@...radead.org, linmiaohe@...wei.com, hughd@...gle.com,
 revest@...gle.com, kernel-dev@...lia.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/huge_memory: Adjust try_to_migrate_one() and
 split_huge_pmd_locked()

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@...hat.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@...lia.com>
>> Acked-by: David Hildenbrand <david@...hat.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