linux-kernel - Re: [PATCH v2 3/5] mm: add static PMD zero page

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Message-ID: <gr6zfputin56222rjxbvnsacvuhh3ghabjbk6dgf4mcvgm2bs6@w7jak5ywgskw>
Date: Thu, 17 Jul 2025 12:34:51 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: David Hildenbrand <david@...hat.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>, 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>, 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 3/5] mm: add static PMD zero page

> > Then, we'd only need a config option to allow for that to happen.
> 
> Something incomplete and very hacky just to give an idea. It would try allocating
> it if there is actual code running that would need it, and then have it
> stick around forever.
> 
Thanks a lot for this David :) I think this is a much better idea and
reduces the amount code and reuse the existing infrastructure.

I will try this approach in the next version.

<snip>
> +       /*
> +        * Our raised reference will prevent the shrinker from ever having
> +        * success -> static.
> +        */
> +       if (atomic_read(&huge_zero_folio_is_static))
> +               return huge_zero_folio;
> +       /* TODO: memblock allocation if buddy is not up yet? Or Reject that earlier. */

Do we need memblock allocation? At least the use cases I forsee for
static pmd zero page are all after the mm is up. So I don't see why we
need to allocate it via memblock.

> +       if (!get_huge_zero_page())
> +               return NULL;
> +       if (atomic_cmpxchg(&huge_zero_folio_is_static, 0, 1) != 0)
> +               put_huge_zero_page();
> +       return huge_zero_folio;
> +
> +}
> +#endif /* CONFIG_STATIC_HUGE_ZERO_FOLIO */
> +
>  static unsigned long shrink_huge_zero_page_count(struct shrinker *shrink,
>                                         struct shrink_control *sc)
>  {
> 

--
Pankaj