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/20210416231531.2521383-5-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 04a34c0..d8e2624 100644 (file)
--- 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;