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

[Jann Horn]

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.