Re: [PATCHv7 13/18] mm: Drop fake head checks

[Kiryl Shutsemau]

On Wed, Oct 28, 2026 at 02:34:39PM +0100, Peter Zijlstra wrote:
> > Only architectures with HVO matter here: x86-64, riscv and loongarch.
> > arm64 has had it disabled since 060a2c92d1b6.
> 
> Per that commit, could it be re-enabled now that its no longer marked RO?

No. The problem there is the PFN change on a live vmemmap PTE, which
needs break-before-make on arm64. Dropping RO only removes the
permission change.

James Houghton is working on it with an Access Flag trick:

https://lore.kernel.org/r/20261003002123.505555-1-jthoughton@xxxxxxxxxx

arm64 will need the writable tail too: its atomic_fetch_add_unless() is
a CAS loop.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov