[PATCH v2] mm/hugetlb: fix validation of unsupported HugeTLB orders

[Muchun Song <songmuchun@bytedance.com>]

Commit 7ce6048d3a7d ("loongarch: drop ranges for definition of
ARCH_FORCE_MAX_ORDER") removed the ARCH_FORCE_MAX_ORDER ranges for
LoongArch. Besides dropping unreasonable upper limits, this allowed
values below the architecture defaults.

With 16KB pages, LoongArch defaults ARCH_FORCE_MAX_ORDER to 11,
matching HUGETLB_PAGE_ORDER. Forcing it to 10 while enabling HugeTLB
permits order-11 HugeTLB folios even though MAX_FOLIO_ORDER is 10.
LoongArch does not select ARCH_HAS_GIGANTIC_PAGE, so this violates the
generic MM assumption that folios cannot exceed MAX_PAGE_ORDER.

This invalid configuration is accepted by Kconfig.
BUILD_BUG_ON_INVALID() does not reject it either because it only checks
whether the expression is valid and never evaluates the comparison.
Detection is left to the runtime warning in hugetlb_add_hstate(), after
earlier boot paths may already have used the invalid configuration.

When an invalid HugeTLB order can be identified at build time, reject it
there. Otherwise, reject it at runtime before the mismatch propagates
further. Use MAYBE_BUILD_BUG_ON() so constant violations fail the build
while dynamically determined orders are checked at runtime.

Fixes: 7ce6048d3a7d ("loongarch: drop ranges for definition of ARCH_FORCE_MAX_ORDER")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/sashiko-outbox-192874@kernel.org/
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Suggested-by: Huacai Chen <chenhuacai@kernel.org>
---
v2:
- Move the order check after hugepages_supported() to avoid a false
  PowerPC BUG_ON() (suggested by Sashiko)
---
 mm/hugetlb.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 779b1de33c78..b49130eb7ad8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4197,7 +4197,6 @@ static int __init hugetlb_init(void)
 
 	BUILD_BUG_ON(sizeof_field(struct page, private) * BITS_PER_BYTE <
 			__NR_HPAGEFLAGS);
-	BUILD_BUG_ON_INVALID(HUGETLB_PAGE_ORDER > MAX_FOLIO_ORDER);
 
 	if (!hugepages_supported()) {
 		if (hugetlb_max_hstate || default_hstate_max_huge_pages)
@@ -4205,6 +4204,8 @@ static int __init hugetlb_init(void)
 		return 0;
 	}
 
+	MAYBE_BUILD_BUG_ON(HUGETLB_PAGE_ORDER > MAX_FOLIO_ORDER);
+
 	/*
 	 * Make sure HPAGE_SIZE (HUGETLB_PAGE_ORDER) hstate exists.  Some
 	 * architectures depend on setup being done here.

base-commit: 74d73ffcffe03804f97e9fb82301dadf75671282
-- 
2.54.0