[1/7] mm: Store compound_nr as well as compound_order
diff mbox series

Message ID 20200629151959.15779-2-willy@infradead.org
State New
Headers show
Series
  • THP prep patches
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Commit Message

Matthew Wilcox June 29, 2020, 3:19 p.m. UTC
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(-)

Comments

Kirill A. Shutemov July 6, 2020, 10:29 a.m. UTC | #1
On Mon, Jun 29, 2020 at 04:19:53PM +0100, Matthew Wilcox (Oracle) wrote:
> 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.

I'm not a fan of redundant data in struct page, even if it's less busy
tail page. We tend to find more use of the space over time.

Any numbers on what it gives for typical kernel? Does it really worth it?

Patch
diff mbox series

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 48eb0f1410d4..c7beb5f13193 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);