[PATCH v6 06/51] mm: Store compound_nr as well as compound_order

[Matthew Wilcox <willy@infradead.org>]

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>

This removes a few instructions from functions which need to know how many
pages are in a compound page.  The storage used is either page->mapping
on 64-bit or page->index on 32-bit.  Both of these are fine to overlay
on tail pages.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/mm.h       | 5 ++++-
 include/linux/mm_types.h | 1 +
 mm/page_alloc.c          | 5 +++--
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dc7b87310c10..af0305ad090f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -911,12 +911,15 @@ static inline int compound_pincount(struct page *page)
 static inline void set_compound_order(struct page *page, unsigned int order)
 {
 	page[1].compound_order = order;
+	page[1].compound_nr = 1U << order;
 }
 
 /* Returns the number of pages in this potentially compound page. */
 static inline unsigned long compound_nr(struct page *page)
 {
-	return 1UL << compound_order(page);
+	if (!PageHead(page))
+		return 1;
+	return page[1].compound_nr;
 }
 
 /* Returns the number of bytes in this potentially compound page. */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 64ede5f150dc..561ed987ab44 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -134,6 +134,7 @@ struct page {
 			unsigned char compound_dtor;
 			unsigned char compound_order;
 			atomic_t compound_mapcount;
+			unsigned int compound_nr; /* 1 << compound_order */
 		};
 		struct {	/* Second tail page of compound page */
 			unsigned long _compound_pad_1;	/* compound_head */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 727751219003..3fb61ef4c3a4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -673,8 +673,6 @@ void prep_compound_page(struct page *page, unsigned int order)
 	int i;
 	int nr_pages = 1 << order;
 
-	set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
-	set_compound_order(page, order);
 	__SetPageHead(page);
 	for (i = 1; i < nr_pages; i++) {
 		struct page *p = page + i;
@@ -682,6 +680,9 @@ void prep_compound_page(struct page *page, unsigned int order)
 		p->mapping = TAIL_MAPPING;
 		set_compound_head(p, page);
 	}
+
+	set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
+	set_compound_order(page, order);
 	atomic_set(compound_mapcount_ptr(page), -1);
 	if (hpage_pincount_available(page))
 		atomic_set(compound_pincount_ptr(page), 0);
-- 
2.26.2