[PATCH v2] mm/debug_vm_pgtable: Fix kernel crash by checking for THP support

Architectures can have CONFIG_TRANSPARENT_HUGEPAGE enabled but
no THP support enabled based on platforms. For ex: with 4K
PAGE_SIZE ppc64 supports THP only with radix translation.

This results in below crash when running with hash translation and
4K PAGE_SIZE.

kernel BUG at arch/powerpc/include/asm/book3s/64/hash-4k.h:140!
cpu 0x61: Vector: 700 (Program Check) at [c000000ff948f860]
    pc: c0000000018810f8: debug_vm_pgtable+0x480/0x8b0
    lr: c0000000018810ec: debug_vm_pgtable+0x474/0x8b0
...
[c000000ff948faf0] c000000001880fec debug_vm_pgtable+0x374/0x8b0 (unreliable)
[c000000ff948fbf0] c000000000011648 do_one_initcall+0x98/0x4f0
[c000000ff948fcd0] c000000001843928 kernel_init_freeable+0x330/0x3fc
[c000000ff948fdb0] c0000000000122ac kernel_init+0x24/0x148
[c000000ff948fe20] c00000000000cc44 ret_from_kernel_thread+0x5c/0x78

Check for THP support correctly

Cc: anshuman.khandual@arm.com
Fixes: 399145f9eb6c ("mm/debug: add tests validating architecture page table helpers")
Signed-off-by: Aneesh Kumar K.V <redacted>
---
 mm/debug_vm_pgtable.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index 188c18908964..df3a3a08f4f8 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c

@@ -61,6 +61,9 @@ static void __init pmd_basic_tests(unsigned long pfn, pgprot_t prot)
 {
 	pmd_t pmd = pfn_pmd(pfn, prot);
 
+	if (!has_transparent_hugepage())
+		return;
+
 	WARN_ON(!pmd_same(pmd, pmd));
 	WARN_ON(!pmd_young(pmd_mkyoung(pmd_mkold(pmd))));
 	WARN_ON(!pmd_dirty(pmd_mkdirty(pmd_mkclean(pmd))));

@@ -80,6 +83,9 @@ static void __init pud_basic_tests(unsigned long pfn, pgprot_t prot)
 {
 	pud_t pud = pfn_pud(pfn, prot);
 
+	if (!has_transparent_hugepage())
+		return;
+
 	WARN_ON(!pud_same(pud, pud));
 	WARN_ON(!pud_young(pud_mkyoung(pud_mkold(pud))));
 	WARN_ON(!pud_write(pud_mkwrite(pud_wrprotect(pud))));