On Tue, May 12, 2026 at 7:12 PM Lance Yang <[email protected]> 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 <[email protected]> wrote: > >> > >> On Tue, May 12, 2026 at 4:31 AM Lance Yang <[email protected]> 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. 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. > >> > >
