Re: [PATCH] mm: make zeropage read-only

[Lance Yang <lance.yang@linux.dev>]

On 2026/5/14 07:10, Yang Shi wrote:
> On Tue, May 12, 2026 at 7:12 PM Lance Yang <lance.yang@linux.dev> wrote:
>>
>>
>> On Tue, May 12, 2026 at 10:14:01AM -0700, Yang Shi wrote:
>>> On Tue, May 12, 2026 at 6:48 AM Jann Horn <jannh@google.com> wrote:
>>>>
>>>> On Tue, May 12, 2026 at 4:31 AM Lance Yang <lance.yang@linux.dev> wrote:
>>>>> Would it makes sense to apply a similar treatment to huge_zero_folio
>>>>> as well?
>>>>>
>>>>> with CONFIG_PERSISTENT_HUGE_ZERO_FOLIO=y, it is allocated at boot and
>>>>> never freed, so it should never be written after initialization either :)
>>>>
>>>> Oh, neat, I didn't realize that that feature exists.
>>>>
>>>> I guess there are two aspects of making the huge zero folio RO that
>>>> could be problematic:
>>>>
>>>> 1. If the huge zero folio comes from the page allocator, making it
>>>> read-only might require splitting a huge PUD, which could have
>>>> performance implications.
>>>> 2. I vaguely remember arm64 has rules about how PUD/PMD entries in the
>>>> linear mapping can't be split at runtime at all depending on hardware
>>>> capabilities, meaning the entire linear mapping may need to be mapped
>>>> without any huge PUD/PMD entries - IDK if thp_shrinker_init() runs
>>>> early enough to be excepted from that. See can_set_direct_map() and
>>>> force_pte_mapping() in arch/arm64/.
>>>
>>> Yes. First of all, this relies rodata mode. If rodata=on (used to be
>>> called full), the linear mapping may be mapped by PUD/PMD if the
>>> hardware can support BBML2_NOABORT, otherwise it is mapped at PTE
>>> level all the time. But how huge zero folio is mapped in linear
>>> mapping should not matter, you just need to change the linear mapping
>>> permission to RO anyway.
>>>
>>> If the rodata mode is off or noalias (used to be called on), the
>>> linear mapping may be mapped by PUD/PMD, but basically changing linear
>>> mapping permission is not expected by kernel.
>>
>> Ah, right. So for huge_zero_folio the hard part is not just making the
>> backing memory read-only, but also whether we can change the linear
>> mapping permission for that range. That depends on the arm64 rodata mode
>> / direct-map setup.
> 
> An alternative is you can allocate huge zero folio in the early stage
> of boot before linear mapping is set up. Then when setting up linear
> mapping, you can just make that PMD RO. It should work regardless of
> rodata mode.

Cool. Good call. That would avoid changing the linear map permissions at
runtime.

Thanks for pointing that out! I'll take a look when I get a chance :)

Cheers, Lance

> Thanks,
> Yang
> 
>>
>> Thanks Jann and Yang for the explanations!
>> Lance
>>
>>> Thanks,
>>> Yang
>>>
>>>>
>>>> So making the huge zero folio RO in the linear map would probably
>>>> require adding a new config flag, connecting that to
>>>> ARCH_HAS_SET_DIRECT_MAP, and changing one or two places in arm64
>>>> memory management.
>>>>
>>>