mm: make compound_head const-preserving

mainline inclusion
from mainline-v5.14-rc1
commit 0f2317e3
category: feature
bugzilla: 187198, https://gitee.com/openeuler/kernel/issues/I5GVFO
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0f2317e34e2c7b97efd4600122115410795ebeea

--------------------------------

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.orgSigned-off-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: NVlastimil Babka <vbabka@suse.cz>
Reviewed-by: NAnshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: NWilliam Kucharski <william.kucharski@oracle.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NLiu Shixin <liushixin2@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

include/linux/page-flags.h

@@ -194,17 +194,17 @@ 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;