linux-kernel - Re: [PATCH v3 2/3] khugepaged: Optimize __collapse_huge_page_copy

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Message-ID: <2765db16-431d-4c77-8091-889a45c18e35@redhat.com>
Date: Tue, 22 Jul 2025 18:03:35 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 lorenzo.stoakes@...cle.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 v3 2/3] khugepaged: Optimize
 __collapse_huge_page_copy_succeeded() by PTE batching

On 22.07.25 17:05, Dev Jain wrote:
> Use PTE batching to optimize __collapse_huge_page_copy_succeeded().
> 
> On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
> Then, calling ptep_clear() for every pte will cause a TLB flush for every
> contpte block. Instead, clear_ptes() does a contpte_try_unfold_partial()
> which will flush the TLB only for the (if any) starting and ending contpte
> block, if they partially overlap with the range khugepaged is looking at.

I suggest not talking so much about arm specifics.

Simply say that batching reduced the number of TLB flushes, especially 
on architectures that support cont-pte optimizations.

> 
> For all arches, there should be a benefit due to batching atomic operations
> on mapcounts due to folio_remove_rmap_ptes() and saving some calls.

I would rephrase that to "Independent of that, batching PTE unmapping 
has known performance benfits (i.e., less refcount and mapcount updates)".

> 
> 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..63517ef7eafb 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;
> +	int nr_ptes;

Nit: I guess we should switch to "unsigned int" here now for consistency 
with folio_pte_batch().

>   
> -	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)) {
> +				int max_nr_ptes = (end - address) >> PAGE_SHIFT;

Dito.

> +
> +				nr_ptes = folio_pte_batch(src, _pte, pteval, max_nr_ptes);
> +			} else {
>   				release_pte_folio(src);
> +			}
> +

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb