[PATCH mm-unstable] mm: clarify folio_set_compound_order() zero support

[Sidhartha Kumar]

Document hugetlb's use of a zero compound order so support for zero
orders is not removed from folio_set_compound_order().

Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
Suggested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Suggested-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
This can be folded into f2b67a51d0ef6871d4fb0c3e8199f278112bd108
mm: add folio dtor and order setter functions

 include/linux/mm.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 443d496949a8..cd8508d728f1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -999,9 +999,16 @@ static inline void set_compound_order(struct page *page, unsigned int order)
 #endif
 }
 
+/*
+ * folio_set_compound_order is generally passed a non-zero order to
+ * initialize a large folio.  However, hugetlb code abuses this by
+ * passing in zero when 'dissolving' a large folio.
+ */
 static inline void folio_set_compound_order(struct folio *folio,
 		unsigned int order)
 {
+	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
+
 	folio->_folio_order = order;
 #ifdef CONFIG_64BIT
 	folio->_folio_nr_pages = order ? 1U << order : 0;
-- 
2.38.1