linux-kernel - Re: [PATCH 2/3] mm: Add generic helper to hint a large folio

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Message-ID: <f960f7c2-465f-4b18-b0b6-3db4a12104d2@arm.com>
Date: Tue, 6 May 2025 14:40:44 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org
Cc: Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com, vbabka@...e.cz,
 jannh@...gle.com, pfalcato@...e.de, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, david@...hat.com, peterx@...hat.com,
 ryan.roberts@....com, mingo@...nel.org, libang.li@...group.com,
 maobibo@...ngson.cn, zhengqi.arch@...edance.com, baohua@...nel.org,
 willy@...radead.org, ioworker0@...il.com, yang@...amperecomputing.com
Subject: Re: [PATCH 2/3] mm: Add generic helper to hint a large folio

On 5/6/25 10:30, Dev Jain wrote:
> To use PTE batching, we want to determine whether the folio mapped by
> the PTE is large, thus requiring the use of vm_normal_folio(). We want
> to avoid the cost of vm_normal_folio() if the code path doesn't already
> require the folio. For arm64, pte_batch_hint() does the job. To generalize
> this hint, add a helper which will determine whether two consecutive PTEs
> point to consecutive PFNs, in which case there is a high probability that
> the underlying folio is large.
> 
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
>  include/linux/pgtable.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index b50447ef1c92..28e21fcc7837 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -369,6 +369,22 @@ static inline pgd_t pgdp_get(pgd_t *pgdp)
>  }
>  #endif
>  
> +/* Caller must ensure that ptep + 1 exists */
> +static inline bool maybe_contiguous_pte_pfns(pte_t *ptep, pte_t pte)
> +{
> +	pte_t *next_ptep, next_pte;
> +
> +	if (pte_batch_hint(ptep, pte) != 1)
> +		return true;
> +
> +	next_ptep = ptep + 1;
> +	next_pte = ptep_get(next_ptep);
> +	if (!pte_present(next_pte))
> +		return false;
> +
> +	return unlikely(pte_pfn(next_pte) - pte_pfn(pte) == PAGE_SIZE);
> +}
> +
>  #ifndef __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
>  static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
>  					    unsigned long address,

As mentioned earlier, this new helper maybe_contiguous_pte_pfns() does not
have a proposed caller. Hence please do consider folding this forward with
the next patch where move_ptes() starts using the helper. Besides, it is
also difficult to review this patch without a proper caller context.