[PATCH mm-unstable v2] mm: move folio_set_compound_order() to mm/internal.h

[Sidhartha Kumar]

folio_set_compound_order() is moved to an mm-internal location so external
folio users cannot misuse this function. Change the name of the function
to folio_set_order() and use WARN_ON_ONCE() rather than BUG_ON. Also,
handle the case if a non-large folio is passed and add clarifying comments
to the function.

Link: https://lore.kernel.org/lkml/20221207223731.32784-1-sidhartha.kumar@xxxxxxxxxx/T/
Fixes: 9fd330582b2f ("mm: add folio dtor and order setter functions")

Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
Suggested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Suggested-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Suggested-by: John Hubbard <jhubbard@xxxxxxxxxx>
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
v1 -> v2:
	- include WARN_ON_ONCE() inside the if statement
	- add r-b

 include/linux/mm.h | 16 ----------------
 mm/hugetlb.c       |  6 +++---
 mm/internal.h      | 19 +++++++++++++++++++
 3 files changed, 22 insertions(+), 19 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7dc376052d40..300d92d2b49d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1019,22 +1019,6 @@ 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;
-#endif
-}
-
 /* Returns the number of pages in this potentially compound page. */
 static inline unsigned long compound_nr(struct page *page)
 {
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 7cdbcc22587b..810e840bb4f1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1344,7 +1344,7 @@ static void __destroy_compound_gigantic_folio(struct folio *folio,
 			set_page_refcounted(p);
 	}
 
-	folio_set_compound_order(folio, 0);
+	folio_set_order(folio, 0);
 	__folio_clear_head(folio);
 }
 
@@ -1808,7 +1808,7 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
 	__folio_clear_reserved(folio);
 	__folio_set_head(folio);
 	/* we rely on prep_new_hugetlb_folio to set the destructor */
-	folio_set_compound_order(folio, order);
+	folio_set_order(folio, order);
 	for (i = 0; i < nr_pages; i++) {
 		p = folio_page(folio, i);
 
@@ -1872,7 +1872,7 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
 		p = folio_page(folio, j);
 		__ClearPageReserved(p);
 	}
-	folio_set_compound_order(folio, 0);
+	folio_set_order(folio, 0);
 	__folio_clear_head(folio);
 	return false;
 }
diff --git a/mm/internal.h b/mm/internal.h
index bcf75a8b032d..1d6f4e168510 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -378,6 +378,25 @@ extern void *memmap_alloc(phys_addr_t size, phys_addr_t align,
 int split_free_page(struct page *free_page,
 			unsigned int order, unsigned long split_pfn_offset);
 
+/*
+ * This will have no effect, other than possibly generating a warning, if the
+ * caller passes in a non-large folio.
+ */
+static inline void folio_set_order(struct folio *folio, unsigned int order)
+{
+	if (WARN_ON_ONCE(!folio_test_large(folio)))
+		return;
+
+	folio->_folio_order = order;
+#ifdef CONFIG_64BIT
+	/*
+	 * When hugetlb dissolves a folio, we need to clear the tail
+	 * page, rather than setting nr_pages to 1.
+	 */
+	folio->_folio_nr_pages = order ? 1U << order : 0;
+#endif
+}
+
 #if defined CONFIG_COMPACTION || defined CONFIG_CMA
 
 /*
-- 
2.38.1