Re: [RFC PATCH 06/12] khugepaged: Generalize __collapse_huge_page_copy_failed()

[Dev Jain <dev.jain@arm.com>]

On 17/12/24 10:52 pm, Ryan Roberts wrote:
> On 16/12/2024 16:50, Dev Jain wrote:
>> Upon failure, we repopulate the PMD in case of PMD-THP collapse. Hence, make
>> this logic specific for PMD case.
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   mm/khugepaged.c | 14 ++++++++------
>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index de044b1f83d4..886c76816963 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -766,7 +766,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
>>   					     pmd_t *pmd,
>>   					     pmd_t orig_pmd,
>>   					     struct vm_area_struct *vma,
>> -					     struct list_head *compound_pagelist)
>> +					     struct list_head *compound_pagelist, int order)
> nit: suggest putting order on its own line.
>
>>   {
>>   	spinlock_t *pmd_ptl;
>>   
>> @@ -776,14 +776,16 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
>>   	 * pages. Since pages are still isolated and locked here,
>>   	 * acquiring anon_vma_lock_write is unnecessary.
>>   	 */
>> -	pmd_ptl = pmd_lock(vma->vm_mm, pmd);
>> -	pmd_populate(vma->vm_mm, pmd, pmd_pgtable(orig_pmd));
>> -	spin_unlock(pmd_ptl);
>> +	if (order == HPAGE_PMD_ORDER) {
>> +		pmd_ptl = pmd_lock(vma->vm_mm, pmd);
>> +		pmd_populate(vma->vm_mm, pmd, pmd_pgtable(orig_pmd));
>> +		spin_unlock(pmd_ptl);
>> +	}
>>   	/*
>>   	 * Release both raw and compound pages isolated
>>   	 * in __collapse_huge_page_isolate.
>>   	 */
>> -	release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
>> +	release_pte_pages(pte, pte + (1UL << order), compound_pagelist);
>>   }
> Given this function is clearly so geared towards re-establishing the pmd, given
> that it takes the *pmd and orig_pmd as params, and given that in the
> non-pmd-order case, we only call through to release_pte_pages(), I wonder if
> it's better to make the decision at a higher level and either call this function
> or release_pte_pages() directly? No strong opinion, just looks a bit weird at
> the moment.

Makes sense, we can probably get rid of this function and let the caller call
reestablish_pmd() or something for the PMD case.

>
>>   
>>   /*
>> @@ -834,7 +836,7 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
>>   						    compound_pagelist);
>>   	else
>>   		__collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
>> -						 compound_pagelist);
>> +						 compound_pagelist, order);
>>   
>>   	return result;
>>   }