mm: make compound_head const-preserving

If you pass a const pointer to compound_head(), you get a const pointer back; if you pass a mutable pointer, you get a mutable pointer back. Also remove an unnecessary forward definition of struct page; we're about to dereference page->compound_head, so it must already have been defined. Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: Matthew Wilcox (Oracle) <[email protected]> Reviewed-by: Vlastimil Babka <[email protected]> Reviewed-by: Anshuman Khandual <[email protected]> Reviewed-by: William Kucharski <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
sm00th · Jun 29, 2021 · 0f2317e · 0f2317e
1 parent 8bf6f45
commit 0f2317e
Showing 1 changed file with 5 additions and 5 deletions.
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
@@ -177,17 +177,17 @@ enum pageflags {
 
 #ifndef __GENERATING_BOUNDS_H
 
-struct page;	/* forward declaration */
-
-static inline struct page *compound_head(struct page *page)
+static inline unsigned long _compound_head(const struct page *page)
 {
 	unsigned long head = READ_ONCE(page->compound_head);
 
 	if (unlikely(head & 1))
-		return (struct page *) (head - 1);
-	return page;
+		return head - 1;
+	return (unsigned long)page;
 }
 
+#define compound_head(page)	((typeof(page))_compound_head(page))
+
 static __always_inline int PageTail(struct page *page)
 {
 	return READ_ONCE(page->compound_head) & 1;