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

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

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