linux-kernel - Re: [PATCH 3/5] mm: add static huge zero folio

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Message-ID: <bmngjssdvffqvnfcoledenlxefdqesvfv7l6os5lfpurmczfw5@mn7jouglo72s>
Date: Wed, 6 Aug 2025 14:18:05 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: David Hildenbrand <david@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.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>, 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, 
	linux-mm@...ck.org, willy@...radead.org, x86@...nel.org, linux-block@...r.kernel.org, 
	Ritesh Harjani <ritesh.list@...il.com>, 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 3/5] mm: add static huge zero folio

> We could go one step further and special case in mm_get_huge_zero_folio() +
> mm_put_huge_zero_folio() on CONFIG_STATIC_HUGE_ZERO_FOLIO.
> 

Hmm, but we could have also failed to allocate even though the option
was enabled.

2 options:
- we could do: 
if (IS_ENABLED(CONFIG_STATIC_HUGE_ZERO_FOLIO) && huge_zero_folio)
  return huge_zero_folio;

or
- As dave suggested, we could do a static key and enable it when the
  option is enabled and the allocation succeeded. The same static key
  can also be used in get_static_huge_zero_folio().

I think the latter option will look more clean and slightly more
optimal?

> Something like
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9c38a95e9f091..9b87884e5f299 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -248,6 +248,9 @@ static void put_huge_zero_page(void)
> 
>  struct folio *mm_get_huge_zero_folio(struct mm_struct *mm)
>  {
> +       if (IS_ENABLED(CONFIG_STATIC_HUGE_ZERO_FOLIO))
> +               return huge_zero_folio;
> +
>         if (test_bit(MMF_HUGE_ZERO_PAGE, &mm->flags))
>                 return READ_ONCE(huge_zero_folio);
> 
> @@ -262,6 +265,9 @@ struct folio *mm_get_huge_zero_folio(struct mm_struct
> *mm)
> 
>  void mm_put_huge_zero_folio(struct mm_struct *mm)
>  {
> +       if (IS_ENABLED(CONFIG_STATIC_HUGE_ZERO_FOLIO))
> +               return huge_zero_folio;
> +
>         if (test_bit(MMF_HUGE_ZERO_PAGE, &mm->flags))
>                 put_huge_zero_page();
>  }
> 
--
Pankaj