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

[Ryan Roberts]

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@xxxxxxx>
> ---
>  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.

>  
>  /*
> @@ -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;
>  }