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From:   "Matthew Wilcox (Oracle)" <willy@infradead.org>
To:     linux-mm@kvack.org
Cc:     "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: [PATCH 4/6] mm: Make compound_head const-preserving
Date:   Sat, 17 Apr 2021 00:15:29 +0100
Message-Id: <20210416231531.2521383-5-willy@infradead.org>
In-Reply-To: <20210416231531.2521383-1-willy@infradead.org>
References: <20210416231531.2521383-1-willy@infradead.org>
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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.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/page-flags.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 04a34c08e0a6..d8e26243db25 100644
--- 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;
-- 
2.30.2