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

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Message-ID: <057eb808-1820-4e70-b13c-a020cc6efcd1@lucifer.local>
Date: Tue, 5 Aug 2025 14:40:38 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: "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>,
        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

On Tue, Aug 05, 2025 at 02:10:39PM +0200, David Hildenbrand wrote:
> On 05.08.25 13:40, Pankaj Raghav (Samsung) wrote:
> > Thanks a lot Lorenzo and David for the feedback and quick iteration on
> > the patchset. I really like the number of lines of code has been
> > steadily reducing since the first version :)
> >
> > I will fold the changes in the next series.
> >
> > <snip>
> > > > > @@ -866,9 +866,14 @@ static int __init thp_shrinker_init(void)
> > > > >    	huge_zero_folio_shrinker->scan_objects = shrink_huge_zero_folio_scan;
> > > > >    	shrinker_register(huge_zero_folio_shrinker);
> > > > > -	deferred_split_shrinker->count_objects = deferred_split_count;
> > > > > -	deferred_split_shrinker->scan_objects = deferred_split_scan;
> > > > > -	shrinker_register(deferred_split_shrinker);
> > > > > +	if (IS_ENABLED(CONFIG_STATIC_HUGE_ZERO_FOLIO)) {
> > > > > +		if (!get_huge_zero_folio())
> > > > > +			pr_warn("Allocating static huge zero folio failed\n");
> > > > > +	} else {
> > > > > +		deferred_split_shrinker->count_objects = deferred_split_count;
> > > > > +		deferred_split_shrinker->scan_objects = deferred_split_scan;
> > > > > +		shrinker_register(deferred_split_shrinker);
> > > > > +	}
> > > > >    	return 0;
> > > > >    }
> > > > > --
> > > > > 2.50.1
> > > > >
> > > > >
> > > > > Now, one thing I do not like is that we have "ARCH_WANTS_STATIC_HUGE_ZERO_FOLIO" but
> > > > > then have a user-selectable option.
> > > > >
> > > > > Should we just get rid of ARCH_WANTS_STATIC_HUGE_ZERO_FOLIO?
> > > >
> >
> > One of the early feedbacks from Lorenzo was that there might be some
> > architectures that has PMD size > 2M might enable this by mistake. So
> > the ARCH_WANTS_STATIC_HUGE_ZERO_FOLIO was introduced as an extra
> > precaution apart from user selectable CONFIG_STATIC_HUGE_ZERO_FOLIO.
>

Oh yeah so I did :P forgot I said that.

> People will find creative ways to mis-configure their system no matter what
> you try :)

I think with an explicit config flag, this won't be _broken_ so much as
wasteful, so therefore not really an issue.

>
> So I think best we can do is document it carefully.

Yup, well always :)

>
> --
> Cheers,
>
> David / dhildenb
>

Cheers, Lorenzo