powerpc/mm/64s: Restrict THP to Radix or HPT w/64K pages

Transparent hugepages (THP) are not supported when using the Hash Page
Table (HPT) MMU with 4K pages.

Currently a HPT-only 4K kernel still allows THP to be enabled, which
is misleading.

Add restrictions to the PPC_THP symbol so that if the kernel is
configured with 4K pages and only the HPT MMU (no Radix), then THP is
disabled.

Note that it's still possible to build a combined Radix/HPT kernel with 4K
pages, which does allow THP to be enabled at build time. As such the HPT
code still needs to provide some THP related symbols, to allow the build
to succeed, but those code paths are never run. See the stubs in
arch/powerpc/include/asm/book3s/64/hash-4k.h.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240823032911.1238471-2-mpe@ellerman.id.au

Author: mpe

arch/powerpc/platforms/Kconfig.cputype

@@ -108,6 +108,7 @@ endchoice
 config PPC_THP
        def_bool y
        depends on PPC_BOOK3S_64
+       depends on PPC_RADIX_MMU || (PPC_64S_HASH_MMU && PAGE_SIZE_64KB)
        select HAVE_ARCH_TRANSPARENT_HUGEPAGE
        select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
        select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE