[PATCH v2 1/4] x86: hugetlb: disable default CMA reservations on 32-bit kernel

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>

A 32-bit x86 kernel can run on a 64-bit-capable CPU advertising GBPAGES.
arch_hugetlb_cma_order() checks only this feature and returns a nonzero
order, although gigantic HugeTLB support on x86 requires CONFIG_X86_64.
Specifying hugetlb_cma= can therefore reserve a CMA area that cannot
serve the intended gigantic HugeTLB allocations.

On non-PAE kernels, the folded PUD_SHIFT yields a 4 MiB order, which is
equal to MAX_PAGE_ORDER. With CONFIG_DEBUG_VM enabled, this triggers the
VM_WARN_ON(order <= MAX_PAGE_ORDER) check in hugetlb_cma_reserve(),
warning that hugetlb_cma is attempting to reserve a non-gigantic order.

Require CONFIG_X86_64 as well as GBPAGES when selecting the default CMA
order to avoid these ineffective reservations.

Fixes: cf11e85fc08c ("mm: hugetlb: optionally allocate gigantic hugepages using cma")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Acked-by: Muchun Song <muchun.song@xxxxxxxxx>
Reviewed-by: Jose A. Perez de Azpillaga <azpijr@xxxxxxxxx>
---
 arch/x86/mm/hugetlbpage.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
index 3b26621c91286..70726a99ede7b 100644
--- a/arch/x86/mm/hugetlbpage.c
+++ b/arch/x86/mm/hugetlbpage.c
@@ -45,7 +45,7 @@ arch_initcall(gigantic_pages_init);
 
 unsigned int __init arch_hugetlb_cma_order(void)
 {
-	if (boot_cpu_has(X86_FEATURE_GBPAGES))
+	if (IS_ENABLED(CONFIG_X86_64) && boot_cpu_has(X86_FEATURE_GBPAGES))
 		return PUD_SHIFT - PAGE_SHIFT;
 
 	return 0;
-- 
2.54.0