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

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Message-ID: <7060b3f3-3849-45d0-979f-733f29d883c8@lucifer.local>
Date: Tue, 15 Jul 2025 17:13:32 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: 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>, David Hildenbrand <david@...hat.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>, 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

Nit on subject - this is a function not a routine :)

On Mon, Jul 07, 2025 at 04:23:18PM +0200, 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.

This is pretty much implicit in the series as a whole though, so probably
not worth mentioning :)

>
> 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;

I mean this should be i huge_mm.h again imo, but at any rate I don't know why
you're moving them up here.

But maybe diff algorithm being weird?

> +
>  #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)
> +		return page_folio(ZERO_PAGE(0));
> +
> +	return READ_ONCE(huge_zero_folio);
> +}
> +
>  #else
>  static inline void __init static_pmd_zero_init(void)
>  {
>  	return;

No need to return in void functions.

>  }
> +
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	return page_folio(ZERO_PAGE(0));
> +}
>  #endif
>
> -extern struct folio *huge_zero_folio;
> -extern unsigned long huge_zero_pfn;
>
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  static inline bool is_huge_zero_folio(const struct folio *folio)
> --
> 2.49.0
>