linux-kernel - Re: [PATCH v2 1/5] mm: move huge_zero_page declaration from huge

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Message-ID: <a0233f30-b04d-461e-a662-b6f20dca02c5@lucifer.local>
Date: Tue, 15 Jul 2025 15:08:40 +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 1/5] mm: move huge_zero_page declaration from
 huge_mm.h to mm.h

On Mon, Jul 07, 2025 at 04:23:15PM +0200, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@...sung.com>
>
> Move the declaration associated with huge_zero_page from huge_mm.h to
> mm.h. This patch is in preparation for adding static PMD zero page as we
> will be reusing some of the huge_zero_page infrastructure.

Hmm this is really iffy.

The whole purpose of huge_mm.h is to handle huge page stuff, and now you're
moving it to a general header... not a fan of this - now we have _some_
huge stuff in mm.h and some stuff here.

Yes this might be something we screwed up already, but that's not a recent
to perpetuate mistakes.

Surely you don't _need_ to do this and this is a question of fixing up
header includes right?

Or is them some horrible cyclical header issue here?

Also your commit message doesn't give any reason as to why you _need_ to do
this also. For something like this where you're doing something that at
face value seems to contradict the purpose of these headers, you need to
explain why.

>
> No functional changes.
>
> Signed-off-by: Pankaj Raghav <p.raghav@...sung.com>
> ---
>  include/linux/huge_mm.h | 31 -------------------------------
>  include/linux/mm.h      | 34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 34 insertions(+), 31 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 2f190c90192d..3e887374892c 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -478,22 +478,6 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>
>  vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf);
>
> -extern struct folio *huge_zero_folio;
> -extern unsigned long huge_zero_pfn;
> -
> -static inline bool is_huge_zero_folio(const struct folio *folio)
> -{
> -	return READ_ONCE(huge_zero_folio) == folio;
> -}
> -
> -static inline bool is_huge_zero_pmd(pmd_t pmd)
> -{
> -	return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd);
> -}
> -
> -struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
> -void mm_put_huge_zero_folio(struct mm_struct *mm);
> -
>  static inline bool thp_migration_supported(void)
>  {
>  	return IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);
> @@ -631,21 +615,6 @@ static inline vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>  	return 0;
>  }
>
> -static inline bool is_huge_zero_folio(const struct folio *folio)
> -{
> -	return false;
> -}
> -
> -static inline bool is_huge_zero_pmd(pmd_t pmd)
> -{
> -	return false;
> -}
> -
> -static inline void mm_put_huge_zero_folio(struct mm_struct *mm)
> -{
> -	return;
> -}
> -
>  static inline struct page *follow_devmap_pmd(struct vm_area_struct *vma,
>  	unsigned long addr, pmd_t *pmd, int flags, struct dev_pagemap **pgmap)
>  {
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0ef2ba0c667a..c8fbeaacf896 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4018,6 +4018,40 @@ static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
>
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
>
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +extern struct folio *huge_zero_folio;
> +extern unsigned long huge_zero_pfn;
> +
> +static inline bool is_huge_zero_folio(const struct folio *folio)
> +{
> +	return READ_ONCE(huge_zero_folio) == folio;
> +}
> +
> +static inline bool is_huge_zero_pmd(pmd_t pmd)
> +{
> +	return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd);
> +}
> +
> +struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
> +void mm_put_huge_zero_folio(struct mm_struct *mm);
> +
> +#else
> +static inline bool is_huge_zero_folio(const struct folio *folio)
> +{
> +	return false;
> +}
> +
> +static inline bool is_huge_zero_pmd(pmd_t pmd)
> +{
> +	return false;
> +}
> +
> +static inline void mm_put_huge_zero_folio(struct mm_struct *mm)
> +{
> +	return;
> +}
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +
>  #if MAX_NUMNODES > 1
>  void __init setup_nr_node_ids(void);
>  #else
> --
> 2.49.0
>