linux-kernel - Re: [RFC PATCH 3/3] mm/mremap: Use can_pte_batch_count() instead of folio_pte

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Message-ID: <177420bf-ba51-4841-8703-632622935afd@lucifer.local>
Date: Mon, 27 Oct 2025 19:57:13 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Zhang Qilong <zhangqilong3@...wei.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, Liam.Howlett@...cle.com,
        vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
        jannh@...gle.com, pfalcato@...e.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, wangkefeng.wang@...wei.com,
        sunnanyong@...wei.com
Subject: Re: [RFC PATCH 3/3] mm/mremap: Use can_pte_batch_count() instead of
 folio_pte_batch() for pte batch

On Mon, Oct 27, 2025 at 10:03:15PM +0800, Zhang Qilong wrote:
> In current mremap_folio_pte_batch(), 1) pte_batch_hint() always
> return one pte in non-ARM64 machine, it is not efficient. 2) Next,

Err... but there's basically no benefit for non-arm64 machines?

The key benefit is the mTHP side of things and making the underlying
arch-specific code more efficient right?

And again you need to get numbers to demonstrate you don't regress non-arm64.

> it need to acquire a folio to call the folio_pte_batch().
>
> Due to new added can_pte_batch_count(), we just call it instead of
> folio_pte_batch(). And then rename mremap_folio_pte_batch() to
> mremap_pte_batch().
>
> Signed-off-by: Zhang Qilong <zhangqilong3@...wei.com>
> ---
>  mm/mremap.c | 16 +++-------------
>  1 file changed, 3 insertions(+), 13 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index bd7314898ec5..d11f93f1622f 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -169,27 +169,17 @@ static pte_t move_soft_dirty_pte(pte_t pte)
>  		pte = pte_swp_mksoft_dirty(pte);
>  #endif
>  	return pte;
>  }
>
> -static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr,
> +static int mremap_pte_batch(struct vm_area_struct *vma, unsigned long addr,
>  		pte_t *ptep, pte_t pte, int max_nr)
>  {
> -	struct folio *folio;
> -
>  	if (max_nr == 1)
>  		return 1;
>
> -	/* Avoid expensive folio lookup if we stand no chance of benefit. */
> -	if (pte_batch_hint(ptep, pte) == 1)
> -		return 1;

Why are we eliminating an easy exit here and instead always invoking the
more involved function?

Again this has to be tested against non-arm architectures.

> -
> -	folio = vm_normal_folio(vma, addr, pte);
> -	if (!folio || !folio_test_large(folio))
> -		return 1;
> -
> -	return folio_pte_batch(folio, ptep, pte, max_nr);
> +	return can_pte_batch_count(vma, ptep, &pte, max_nr, 0);

This is very silly to have this function now ust return another function + a
trivial check that your function should be doing...

>  }
>
>  static int move_ptes(struct pagetable_move_control *pmc,
>  		unsigned long extent, pmd_t *old_pmd, pmd_t *new_pmd)
>  {
> @@ -278,11 +268,11 @@ static int move_ptes(struct pagetable_move_control *pmc,
>  		 * make sure the physical page stays valid until
>  		 * the TLB entry for the old mapping has been
>  		 * flushed.
>  		 */
>  		if (pte_present(old_pte)) {
> -			nr_ptes = mremap_folio_pte_batch(vma, old_addr, old_ptep,
> +			nr_ptes = mremap_pte_batch(vma, old_addr, old_ptep,
>  							 old_pte, max_nr_ptes);
>  			force_flush = true;
>  		}
>  		pte = get_and_clear_ptes(mm, old_addr, old_ptep, nr_ptes);
>  		pte = move_pte(pte, old_addr, new_addr);
> --
> 2.43.0
>