linux-kernel - Re: [PATCH v2 0/5] add static PMD zero page support

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Message-ID: <dca5912a-cdf4-4f7e-a79a-796da8475826@lucifer.local>
Date: Tue, 15 Jul 2025 15:12:02 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Pankaj Raghav <kernel@...kajraghav.com>, Zi Yan <ziy@...dia.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>,
        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,
        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 0/5] add static PMD zero page support

On Tue, Jul 15, 2025 at 04:06:29PM +0200, David Hildenbrand wrote:
> I think at some point we discussed "when does the PMD-sized zeropage make
> *any* sense on these weird arch configs" (512MiB on arm64 64bit)
>
> No idea who wants to waste half a gig on that at runtime either.

Yeah this is a problem we _really_ need to solve. But obviously somewhat out of
scope here.

>
> But yeah, we should let the arch code opt in whether it wants it or not (in
> particular, maybe only on arm64 with CONFIG_PAGE_SIZE_4K)

I don't think this should be an ARCH_HAS_xxx.

Because that's saying 'this architecture has X', this isn't architecture
scope.

I suppose PMDs may vary in terms of how huge they are regardless of page
table size actually.

So maybe the best solution is a semantic one - just rename this to
ARCH_WANT_STATIC_PMD_ZERO_PAGE

And then put the page size selector in the arch code.

For example in arm64 we have:

	select ARCH_WANT_HUGE_PMD_SHARE if ARM64_4K_PAGES || (ARM64_16K_PAGES && !ARM64_VA_BITS_36)

So doing something similar here like:

	select ARCH_WANT_STATIC_PMD_ZERO_PAGE if ARM64_4K_PAGES

Would do thie job and sort everything out.

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

CHeers, Lorenzo