linux-kernel - [PATCH] mm: remove redundant compound

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Date:   Wed, 11 Aug 2021 18:14:31 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     akpm@...ux-foundation.org, willy@...radead.org,
        william.kucharski@...cle.com, kirill.shutemov@...ux.intel.com,
        dhowells@...hat.com, hannes@...xchg.org
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Muchun Song <songmuchun@...edance.com>
Subject: [PATCH] mm: remove redundant compound_head() calling

There is a READ_ONCE() in the macro of compound_head(), which will
prevent compiler from optimizing the code when there are more than
once calling of it in a function. Remove the redundant calling of
compound_head() from page_to_index() and page_add_file_rmap() for
better code generation.

Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
 include/linux/pagemap.h | 7 +++----
 mm/rmap.c               | 6 ++++--
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 79ec90e97e94..03b9a957ef10 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -608,18 +608,17 @@ static inline struct page *read_mapping_page(struct address_space *mapping,
  */
 static inline pgoff_t page_to_index(struct page *page)
 {
-	pgoff_t pgoff;
+	struct page *head;
 
 	if (likely(!PageTransTail(page)))
 		return page->index;
 
+	head = compound_head(page);
 	/*
 	 *  We don't initialize ->index for tail pages: calculate based on
 	 *  head page
 	 */
-	pgoff = compound_head(page)->index;
-	pgoff += page - compound_head(page);
-	return pgoff;
+	return head->index + page - head;
 }
 
 extern pgoff_t hugetlb_basepage_index(struct page *page);
diff --git a/mm/rmap.c b/mm/rmap.c
index 232494888628..2e216835f07c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1231,11 +1231,13 @@ void page_add_file_rmap(struct page *page, bool compound)
 						nr_pages);
 	} else {
 		if (PageTransCompound(page) && page_mapping(page)) {
+			struct page *head = compound_head(page);
+
 			VM_WARN_ON_ONCE(!PageLocked(page));
 
-			SetPageDoubleMap(compound_head(page));
+			SetPageDoubleMap(head);
 			if (PageMlocked(page))
-				clear_page_mlock(compound_head(page));
+				clear_page_mlock(head);
 		}
 		if (!atomic_inc_and_test(&page->_mapcount))
 			goto out;
-- 
2.11.0