mm: do not map the shadow stack as THP

mm: do not map the shadow stack as THP

The default shadow stack 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 (for arm64).  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.  It didn't make sense for the normal stacks either, see commit
c4608d1bf7c6 ("mm: mmap: map MAP_STACK to VM_NOHUGEPAGE").

Force VM_NOHUGEPAGE when allocating/mapping the shadow stack.  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.

Link: https://lkml.kernel.org/r/20260225161404.3157851-6-catalin.marinas@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: "Borislav Petkov (AMD)" <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <pjw@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>