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 persistent huge zero folio read-only
in the direct map turns such writes into faults instead of silent zero-page
corruption.

This series adds set_direct_map_ro_noflush() so mm code can make a
direct-map range read-only, then uses it for persistent huge zero folio.
The helper is direct-map specific, takes an address-based range as
discussed for set_direct_map* helpers[2], and does not flush TLBs.

Persistent huge zero folio setup happens during early boot, so no explicit
TLB flush is needed here.

Inspired by Jann Horn's read-only zero page work[1] and follow-up
discussion[3] with Yang Shi.

Patches 2 and 3 add arm64 and x86 implementations.

[1] https://lore.kernel.org/linux-mm/20260508-ro-zeropage-v1-1-9808abc20b49@google.com/ (local)
[2] https://lore.kernel.org/linux-mm/0e5b23a6-4895-454a-9dfa-6dc21adc2991@kernel.org/ (local)
[3] https://lore.kernel.org/linux-mm/CAHbLzkrXXe7r3n3jXgDKtwZhRqj=jDx9E6dLOULohnhBguvi9A@mail.gmail.com/ (local)

RFC v2 -> RFC v3:
- Patch #01: Replace arch_make_pages_readonly() with
  set_direct_map_ro_noflush() in the existing set_direct_map* family
  (per Mike and David, thanks!).
- Patch #01: Use a direct-map address and number of pages, and document the
  direct-map-only and no-TLB-flush semantics (per David, thanks!).
- Patch #02 and #03: Update the arm64 and x86 implementations for
  set_direct_map_ro_noflush().
- https://lore.kernel.org/linux-mm/20260609143801.7917-1-xueyuan.chen21@gmail.com/ (local)

RFC v1 -> RFC v2:
- Patch #01: Drop the READONLY_HUGE_ZERO_FOLIO Kconfig option
  (per Dave, thanks!).
- Patch #01: Replace the huge-zero-folio-specific hook with a generic
  page-range hook (per David, thanks!)
- Patch #02 and #03: Update the arm64 and x86 implementations for the new
  hook.
- https://lore.kernel.org/linux-mm/20260527035607.14919-1-xueyuan.chen21@gmail.com/ (local)

Xueyuan Chen (3):
  mm: make persistent huge zero folio read-only
  arm64/mm: add set_direct_map_ro_noflush()
  x86/mm: add set_direct_map_ro_noflush()

 arch/arm64/include/asm/set_memory.h |  2 ++
 arch/arm64/mm/pageattr.c            | 10 ++++++++++
 arch/x86/include/asm/set_memory.h   |  2 ++
 arch/x86/mm/pat/set_memory.c        | 15 +++++++++++++++
 include/linux/set_memory.h          | 26 ++++++++++++++++++++++++++
 mm/huge_memory.c                    |  9 ++++++++-
 6 files changed, 63 insertions(+), 1 deletion(-)

-- 
2.49.0