Re: [RFC PATCH 1/3] mm: make persistent huge zero folio read-only

[Yang Shi]

On 5/27/26 9:20 AM, Jann Horn wrote:
> On Wed, May 27, 2026 at 5:55 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> > On 5/26/26 20:56, Xueyuan chen wrote:
> > > +config READONLY_HUGE_ZERO_FOLIO
> > > +     bool "Map the huge zero folio read-only in the direct map"
> > > +     depends on PERSISTENT_HUGE_ZERO_FOLIO
> > > +     depends on ARCH_HAS_READONLY_HUGE_ZERO_FOLIO
> > > +     help
> > > +       The persistent huge zero folio is shared globally, and nothing
> > > +       should ever change its contents after initialization.
> > > +
> > > +       When supported, mark the folio read-only in the direct map so such
> > > +       writes trigger a fault instead of silently corrupting the zero contents.
> > > +
> > > +       If the permission change is not supported, the kernel keeps using
> > > +       the writable persistent huge zero folio.
> > I vote for no Kconfig options here. Why? This adds "security" with
> > _basically_ no extra runtime cost. The runtime cost is, what, usually
> > one kernel TLB invalidation during boot?
> Plus potentially a bit more TLB pressure from losing a huge PUD in the
> linear map, IDK how much we care about that.

This shouldn't be a big issue on ARM64. The most ARM64 machines have 
linear mapping mapped with PTE if rodata is on. Some machines with 
BBML2_NOABORT support have linear mapping mapped with PUD/PMD, but those 
machines typically have large memory, having 512 PMDs instead of 1 PUD 
shouldn't be a noticeable issue IMHO.

Thanks,
Yang