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

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

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.

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.
>>
>