From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> Subject: 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@xxxxxxxxxxxxx Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx> Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx> Reviewed-by: William Kucharski <william.kucharski@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/page-flags.h | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) --- a/include/linux/page-flags.h~mm-make-compound_head-const-preserving +++ a/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; _
