[PATCH 3/3] arm64: gcs: Do not map the guarded control stack as THP

[David Hildenbrand (Arm)]

On 2/20/26 15:05, Catalin Marinas wrote:
> The default GCS size allocated on first prctl() for the main thread or
> subsequently on clone() is either half of RLIMIT_STACK or half of a
> thread's stack size. Both of these are likely to be suitable for a THP
> allocation and the kernel is more aggressive in creating such mappings.
> However, it does not make much sense to use a huge page as it didn't
> make sense for the normal stacks either. See commit c4608d1bf7c6 ("mm:
> mmap: map MAP_STACK to VM_NOHUGEPAGE").

Agreed. At least when it comes to PMD THPs.

> 
> Force VM_NOHUGEPAGE when allocating/mapping the GCS. As per commit
> 7190b3c8bd2b ("mm: mmap: map MAP_STACK to VM_NOHUGEPAGE only if THP is
> enabled"), only pass this flag if TRANSPARENT_HUGEPAGE is enabled as not
> to confuse CRIU tools.

I was briefly concerned that we wouldn't even have PMD support to handle 
shadow stack, but turns out we do have pmd_mkwrite() that consumes a VMA 
to handle it.

> 
> While at it, use the PROT_WRITE prot argument rather than the VM_WRITE
> flag when calling do_mmap().

That LGTM as well.


Agreed with trying to let common code deal with that.

I'll note that madvise() would be able to turn THPs back on; I assume 
that's okay.

-- 
Cheers,

David