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

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Message-ID: <d7fa2e67-1960-4b1f-a8b7-147371e37010@redhat.com>
Date: Tue, 15 Jul 2025 16:16:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 15.07.25 16:12, Lorenzo Stoakes wrote:
> 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.

Yes.

-- 
Cheers,

David / dhildenb