On 2026/5/14 07:10, Yang Shi wrote:
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.
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.
