linux-kernel - Re: [PATCH v2 4/5] mm: add largest_zero

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Message-ID: <0793154d-a6ca-43b7-a0c3-01532d9cccc8@redhat.com>
Date: Tue, 15 Jul 2025 16:16:09 +0200
From: David Hildenbrand <david@...hat.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Borislav Petkov <bp@...en8.de>,
 Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
 Vlastimil Babka <vbabka@...e.cz>, Zi Yan <ziy@...dia.com>,
 Mike Rapoport <rppt@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Michal Hocko <mhocko@...e.com>, Lorenzo Stoakes
 <lorenzo.stoakes@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>,
 Dev Jain <dev.jain@....com>, "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Jens Axboe <axboe@...nel.dk>
Cc: linux-kernel@...r.kernel.org, willy@...radead.org, linux-mm@...ck.org,
 x86@...nel.org, linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 "Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org,
 gost.dev@...sung.com, hch@....de, Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v2 4/5] mm: add largest_zero_folio() routine

On 07.07.25 16:23, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@...sung.com>
> 
> Add largest_zero_folio() routine so that huge_zero_folio can be
> used without the need to pass any mm struct. This will return ZERO_PAGE
> folio if CONFIG_STATIC_PMD_ZERO_PAGE is disabled or if we failed to
> allocate a PMD page from memblock.
> 
> This routine can also be called even if THP is disabled.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@...sung.com>
> ---
>   include/linux/mm.h | 28 ++++++++++++++++++++++++++--
>   1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 428fe6d36b3c..d5543cf7b8e9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4018,17 +4018,41 @@ static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
>   
>   #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
>   
> +extern struct folio *huge_zero_folio;
> +extern unsigned long huge_zero_pfn;

No need for "extern".

> +
>   #ifdef CONFIG_STATIC_PMD_ZERO_PAGE
>   extern void __init static_pmd_zero_init(void);
> +
> +/*
> + * largest_zero_folio - Get the largest zero size folio available
> + *
> + * This function will return a PMD sized zero folio if CONFIG_STATIC_PMD_ZERO_PAGE
> + * is enabled. Otherwise, a ZERO_PAGE folio is returned.
> + *
> + * Deduce the size of the folio with folio_size instead of assuming the
> + * folio size.
> + */
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	if(!huge_zero_folio)

Nit: if (!huge_zero_folio), but see below

I assume this check is only in place to handle whether static allocation 
failed, correct?

> +		return page_folio(ZERO_PAGE(0));
> +
> +	return READ_ONCE(huge_zero_folio);

READ_ONCE should not be required if it cannot get freed.

> +}
> +
>   #else
>   static inline void __init static_pmd_zero_init(void)
>   {
>   	return;
>   }
> +
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	return page_folio(ZERO_PAGE(0));
> +}
>   #endif

Could we do:

static inline struct folio *largest_zero_folio(void)
{
	if (IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE) &&
  	    likely(huge_zero_folio))
		return huge_zero_folio;
	return page_folio(ZERO_PAGE(0));
}

-- 
Cheers,

David / dhildenb