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

Ryan Roberts
2024-12-17T17:22:49Z

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.

>
> /*
> @@ -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;
> }