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

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Message-ID: <bdlp7psevt6qqtssknhp55b65sfmbrnz373hudn3i4hqkoxa7u@oabcqpvc3z5k>
Date: Tue, 15 Jul 2025 17:25:29 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: David Hildenbrand <david@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 04:16:44PM +0200, David Hildenbrand wrote:
> 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.

Actually I had something similar in one of my earlier versions[1] where we
can opt in from arch specific Kconfig with *WANT* instead *HAS*.

For starters, I will enable this only from x86. We can probably extend
this once we get the base patches up.

[1] https://lore.kernel.org/linux-mm/20250522090243.758943-2-p.raghav@samsung.com/
--
Pankaj