linux-kernel - Re: [PATCH v4 2/3] khugepaged: Optimize __collapse_huge_page_copy

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Message-ID: <kjveoiyczkjlm65d3agasjbtplezshwwjyudq4jbxhygcc7plm@dre33kse3y6g>
Date: Thu, 24 Jul 2025 18:32:24 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
        baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
        npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] khugepaged: Optimize
 __collapse_huge_page_copy_succeeded() by PTE batching

Message-ID: <32843cfb-a70b-4dfb-965c-4e1b0623a1b4@...ifer.local>
Reply-To:
In-Reply-To: <20250724052301.23844-3-dev.jain@....com>

NIT: Please don't capitalise 'Optimize' here.

I think Andrew fixed this for you actually in the repo though :P

On Thu, Jul 24, 2025 at 10:53:00AM +0530, Dev Jain wrote:
> Use PTE batching to batch process PTEs mapping the same large folio. An
> improvement is expected due to batching refcount-mapcount manipulation on
> the folios, and for arm64 which supports contig mappings, the number of
> TLB flushes is also reduced.
>
> Acked-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
>  mm/khugepaged.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index a55fb1dcd224..f23e943506bc 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -700,12 +700,15 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>  						spinlock_t *ptl,
>  						struct list_head *compound_pagelist)
>  {
> +	unsigned long end = address + HPAGE_PMD_SIZE;
>  	struct folio *src, *tmp;
> -	pte_t *_pte;
>  	pte_t pteval;
> +	pte_t *_pte;
> +	unsigned int nr_ptes;
>
> -	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> -	     _pte++, address += PAGE_SIZE) {
> +	for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
> +	     address += nr_ptes * PAGE_SIZE) {
> +		nr_ptes = 1;
>  		pteval = ptep_get(_pte);
>  		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>  			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> @@ -722,18 +725,26 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>  			struct page *src_page = pte_page(pteval);
>
>  			src = page_folio(src_page);
> -			if (!folio_test_large(src))
> +
> +			if (folio_test_large(src)) {
> +				unsigned int max_nr_ptes = (end - address) >> PAGE_SHIFT;
> +
> +				nr_ptes = folio_pte_batch(src, _pte, pteval, max_nr_ptes);
> +			} else {
>  				release_pte_folio(src);
> +			}
> +
>  			/*
>  			 * ptl mostly unnecessary, but preempt has to
>  			 * be disabled to update the per-cpu stats
>  			 * inside folio_remove_rmap_pte().
>  			 */
>  			spin_lock(ptl);
> -			ptep_clear(vma->vm_mm, address, _pte);
> -			folio_remove_rmap_pte(src, src_page, vma);
> +			clear_ptes(vma->vm_mm, address, _pte, nr_ptes);
> +			folio_remove_rmap_ptes(src, src_page, nr_ptes, vma);
>  			spin_unlock(ptl);
> -			free_folio_and_swap_cache(src);
> +			free_swap_cache(src);
> +			folio_put_refs(src, nr_ptes);

Hm one thing here though is the free_folio_and_swap_cache() does:

	free_swap_cache(folio);
	if (!is_huge_zero_folio(folio))
		folio_put(folio);

Whereas here you unconditionally reduce the reference count. Might this
cause issues with the shrinker version of the huge zero folio?

Should this be:

			if (!is_huge_zero_folio(src))
				folio_put_refs(src, nr_ptes);

Or do we otherwise avoid issues with this?

>  		}
>  	}
>
> --
> 2.30.2
>