Re: [PATCH v7 1/3] mm: make persistent huge zero folio read-only

[Mike Rapoport <rppt@kernel.org>]

On Thu, Sep 10, 2026 at 02:22:25PM +0800, Xueyuan Chen wrote:
> On Wed, Sep 9, 2026 at 11:18 PM David Hildenbrand (Arm)
> <david@kernel.org> wrote:
> >
> > On 9/8/26 16:48, Xueyuan Chen wrote:
> > > On Tue, Sep 8, 2026 at 4:30 PM David Hildenbrand (Arm) <david@kernel.org> wrote:
> > >>
> > >> On 9/1/26 17:18, Xueyuan Chen wrote:
> > >>> The persistent huge zero folio is shared globally and should stay zero
> > >>> after initialization. As Jann Horn pointed out [1], kernel bugs have
> > >>> ended up writing to pages that were meant to be read-only, including in
> > >>> security-sensitive cases. Making the folio read-only in the direct map
> > >>> turns such writes into faults instead of silent zero-page corruption.
> > >>>
> > >>> Add a page-based helper consistent with the existing direct-map interfaces.
> > >>> Handle TLB invalidation in the architecture implementation; unsupported
> > >>> architectures retain their current behavior.
> > >>>
> > >>> Protect the folio after initialization. Skip highmem folios, which have no
> > >>> permanent direct-map mapping.
> > >>>
> > >>> Inspired by Jann Horn's read-only zero page work [1] and follow-up
> > >>> discussion [3] with Yang Shi.
> > >>>
> > >>> Link: https://lore.kernel.org/r/20260508-ro-zeropage-v1-1-9808abc20b49@google.com [1]
> > >>> Link: https://lore.kernel.org/r/0e5b23a6-4895-454a-9dfa-6dc21adc2991@kernel.org [2]
> > >>> Link: https://lore.kernel.org/r/CAHbLzkrXXe7r3n3jXgDKtwZhRqj=jDx9E6dLOULohnhBguvi9A@mail.gmail.com [3]
> > >>>
> > >>> Suggested-by: David Hildenbrand <david@kernel.org>
> > >>> Suggested-by: Usama Arif <usama.arif@linux.dev>
> > >>> Co-developed-by: Lance Yang <lance.yang@linux.dev>
> > >>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> > >>> Signed-off-by: Xueyuan Chen <xueyuan.chen21@gmail.com>
> > >>> ---
> > >>>  include/linux/set_memory.h | 17 +++++++++++++++++
> > >>>  mm/huge_memory.c           | 13 ++++++++++---
> > >>>  2 files changed, 27 insertions(+), 3 deletions(-)
> > >>>
> > >>> diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h
> > >>> index 3fe293cfed8c..ed9ce04b18a1 100644
> > >>> --- a/include/linux/set_memory.h
> > >>> +++ b/include/linux/set_memory.h
> > >>> @@ -54,6 +54,23 @@ static inline bool can_set_direct_map(void)
> > >>>  #endif
> > >>>  #endif /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
> > >>>
> > >>> +#ifndef set_direct_map_ro
> > >>> +/**
> > >>> + * set_direct_map_ro - make a direct-map range read-only
> > >>> + * @page: first page in the direct-map range
> > >>> + * @nr: number of pages in the range
> > >>> + *
> > >>> + * Make the direct-map range starting at @page read-only and invalidate stale
> > >>> + * writable translations before returning.
> > >>> + *
> > >>> + * Return: 0 on success, or a negative error code on failure.
> > >>> + */
> > >>> +static inline int set_direct_map_ro(struct page *page, unsigned int nr)
> > >>> +{
> > >>> +     return 0;
> > >>> +}
> > >>> +#endif
> > >>> +
> > >>>  #ifdef CONFIG_X86_64
> > >>>  int set_mce_nospec(unsigned long pfn);
> > >>>  int clear_mce_nospec(unsigned long pfn);
> > >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > >>> index 54494c3fa983..742283b36d74 100644
> > >>> --- a/mm/huge_memory.c
> > >>> +++ b/mm/huge_memory.c
> > >>> @@ -42,6 +42,7 @@
> > >>>  #include <linux/pgalloc_tag.h>
> > >>>  #include <linux/pagewalk.h>
> > >>>  #include <linux/cleanup.h>
> > >>> +#include <linux/set_memory.h>
> > >>>
> > >>>  #include <asm/tlb.h>
> > >>>  #include "internal.h"
> > >>> @@ -291,10 +292,16 @@ static int __init huge_zero_init(void)
> > >>>       huge_zero_folio = alloc_huge_zero_folio();
> > >>>       if (!huge_zero_folio) {
> > >>>               pr_warn("Allocating persistent huge zero folio failed\n");
> > >>> -     } else {
> > >>> -             huge_zero_pfn = folio_pfn(huge_zero_folio);
> > >>> -             count_vm_event(THP_ZERO_PAGE_ALLOC);
> > >>> +             return 0;
> > >>>       }
> > >>> +
> > >>> +     huge_zero_pfn = folio_pfn(huge_zero_folio);
> > >>> +     count_vm_event(THP_ZERO_PAGE_ALLOC);
> > >>> +
> > >>> +     /* Highmem folios have no permanent direct-map mapping to protect. */
> > >>> +     if (!folio_test_highmem(huge_zero_folio))
> > >>> +             set_direct_map_ro(folio_page(huge_zero_folio, 0), HPAGE_PMD_NR);
> > >>
> > >> Assuming we keep the page-based approach, can't we just move the highmem test in
> > >> there?
> > >
> > > Hi David,
> > >
> > > We can move the check into set_direct_map_ro(). However, on x86,
> > > set_direct_map_invalid_noflush() and set_direct_map_default_noflush()
> > > also expect callers to exclude highmem pages.
> > >
> > > Would it make sense to make highmem a documented no-op for those
> > > helpers as well, so the direct-map APIs handle it consistently?
> >
> > I think getting something consistent for now is the most important thing. :)
> >
> Hi David,
> 
> OK, I'll move the highmem check into set_direct_map_ro().

Consistent means leaving highmem check in the caller ...
 
> Should I also update the x86 set_direct_map_invalid_noflush() and
> set_direct_map_default_noflush() helpers in this series to skip
> highmem pages?

... or checking it in all set_direct_map functions.

I'd keep the check in huge_zero_init() in this patchset.

> Thanks,
> Xueyuan
> > --
> > Cheers,
> >
> > David

-- 
Sincerely yours,
Mike.