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

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Message-ID: <887fb371-409e-4dad-b4ff-38b85bfddf95@redhat.com>
Date: Wed, 7 May 2025 12:03:50 +0200
From: David Hildenbrand <david@...hat.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, peterx@...hat.com, ryan.roberts@....com,
 mingo@...nel.org, libang.li@...group.com, maobibo@...ngson.cn,
 zhengqi.arch@...edance.com, baohua@...nel.org, anshuman.khandual@....com,
 willy@...radead.org, ioworker0@...il.com, yang@...amperecomputing.com
Subject: Re: [PATCH 2/3] mm: Add generic helper to hint a large folio

On 06.05.25 07:00, 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);
> +}

So, where we want to use that is:

if (pte_present(old_pte)) {
	if ((max_nr != 1) && maybe_contiguous_pte_pfns(old_ptep, old_pte)) {
		struct folio *folio = vm_normal_folio(vma, old_addr, old_pte);

		if (folio && folio_test_large(folio))
			nr = folio_pte_batch(folio, old_addr, old_ptep,
					     old_pte, max_nr, fpb_flags, NULL, NULL, NULL);
	}
}

where we won't need the folio later. But want it all part of the same folio?


And the simpler version would be


if (pte_present(old_pte)) {
	if (max_nr != 1) {
		struct folio *folio = vm_normal_folio(vma, old_addr, old_pte);

		if (folio && folio_test_large(folio))
			nr = folio_pte_batch(folio, old_addr, old_ptep,
					     old_pte, max_nr, fpb_flags, NULL, NULL, NULL);
	}
}


Two things come to mind:

(1) Do we *really* care about the vm_normal_folio() + folio_test_large() call that much, that you
have to add this optimization ahead of times ? :)

(2) Do we really need "must be part of the same folio", or could be just batch over present
ptes that map consecutive PFNs? In that case, a helper that avoids folio_pte_batch() completely
might be better.

-- 
Cheers,

David / dhildenb