Re: [RESEND v7 10/29] mm: make PMD migration-entry splitting explicit

[Usama Arif <usama.arif@linux.dev>]

On 23/09/2026 12:04, David Hildenbrand (Arm) wrote:
> Having another look ...
> 
> 
>> +
>> +/*
>> + * Split a present PMD into PTE migration entries, for the rmap migration
>> + * walker.  
> 
> I assume pmd_is_device_private_entry() is also allowed?
> 
> "present or device-private PMDs"
> 

Yes, ack.

> 
>> Like split_huge_pmd_locked(), the caller must hold the PMD lock and
>> + * must already be inside an mmu_notifier invalidate range.
>> + */
>> +void split_pmd_to_migration_entries(struct vm_area_struct *vma,
>> +				    unsigned long address, pmd_t *pmd)
>> +{
>> +	VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
>> +	if (pmd_trans_huge(*pmd) || pmd_is_valid_softleaf(*pmd))
> 
> pmd_is_valid_softleaf() would include migration entries and swap entries?
> 
> I'd assume that try_to_migrate_one() will never call us with something that is
> not a present or device-private PMD?
> 
> Shouldn't we instead just
> 
> 
> VM_WARN_ON_ONCE(!pmd_present(*pmd) && !pmd_is_device_private_entry(*pmd));
> __split_huge_pmd_locked(vma, pmd, address, true);
> 
> And not offer some weird fallback where we silently don't split?


Yes, that sounds much better. The function right now is:


/**
 * split_pmd_to_migration_entries() - Split a present or device private PMD into
 * PTE migration entries.
 * @vma: The VMA containing the PMD.
 * @address: The PMD-aligned address the PMD maps.
 * @pmd: A pointer to the leaf PMD entry.
 *
 * For the rmap migration walker, which only ever hands back those two entry
 * types.  Like split_huge_pmd_locked(), the caller must hold the PMD lock and
 * must already be inside an mmu_notifier invalidate range.
 */
void split_pmd_to_migration_entries(struct vm_area_struct *vma,
		unsigned long address, pmd_t *pmd)
{
	VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
	VM_WARN_ON_ONCE(!pmd_present(*pmd) &&
			!pmd_is_device_private_entry(*pmd));
	__split_huge_pmd_locked(vma, pmd, address, true);
}

> 
>> +		__split_huge_pmd_locked(vma, pmd, address, true);
>>  }
>