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

Hi Xueyuan,

On Mon, Sep 07, 2026 at 09:11:14AM +0800, Xueyuan Chen wrote:

On Sun, Sep 6, 2026 at 6:00 PM Mike Rapoport [off-list ref] wrote:

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+
+     /* 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);

Sorry, I don't remember if it was discussed previously, but why can't we
use the existing set_memory_ro() here?

Hi Mike,

We want to change the linear map here, but arm64 set_memory_ro() only
works on vmalloc addresses.

I believe this is an historical artifact. change_memory_common() already
updates the linear map when a vmalloc mapping switches to RO and the system
supports it.

On x86, set_memory_ro() does work on direct-map addresses.

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I believe arm64::set_memory_ro() can change the linear map in the general
case as well as long as can_set_direct_map() is true.

On arm64, I'm reading arch/arm64/mm/pageattr.c, and the linear-map
update in change_memory_common() is only reachable for vmalloc
addresses:

  set_memory_ro
    change_memory_common
       area = find_vm_area((void *)addr);
       if (!area || ...)
          return -EINVAL;

So set_memory_ro() on a linear-map address returns -EINVAL on arm64.

And I believe this is an historical artifact rather than necessity. For
configurations that allow set_direct_map on arm64, change_memory_common can
update direct map too, it just need to be implemented there.

-- 
Sincerely yours,
Mike.