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Date: Mon, 9 Apr 2007 11:09:40 -0700 (PDT)
From: Christoph Lameter <clameter@....com>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: Hugh Dickins <hugh@...itas.com>, Nick Piggin <npiggin@...e.de>,
dgc@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] Optimize compound_head() by avoiding a shared page
flag
Add PageTail / PageHead in order to avoid multiple branches when compound
pages are checked.
The patch adds PageTail(page) and PageHead(page) to check if a page is the
head or the tail of a compound page. This is done by masking the two
bits describing the state of a compound page and then comparing them. So
one comparision and a branch instead of two bit checks and two branches.
---
include/linux/mm.h | 11 ++---------
include/linux/page-flags.h | 28 +++++++++++++++++-----------
mm/page_alloc.c | 10 ++++------
3 files changed, 23 insertions(+), 26 deletions(-)
Index: linux-2.6.21-rc5-mm4/include/linux/mm.h
===================================================================
--- linux-2.6.21-rc5-mm4.orig/include/linux/mm.h 2007-04-09 11:04:15.000000000 -0700
+++ linux-2.6.21-rc5-mm4/include/linux/mm.h 2007-04-09 11:04:31.000000000 -0700
@@ -299,14 +299,7 @@ static inline int get_page_unless_zero(s
static inline struct page *compound_head(struct page *page)
{
- /*
- * We could avoid the PageCompound(page) check if
- * we would not overload PageTail().
- *
- * This check has to be done in several performance critical
- * paths of the slab etc. IMHO PageTail deserves its own flag.
- */
- if (unlikely(PageCompound(page) && PageTail(page)))
+ if (unlikely(PageTail(page)))
return page->first_page;
return page;
}
@@ -363,7 +356,7 @@ static inline compound_page_dtor *get_co
static inline int compound_order(struct page *page)
{
- if (!PageCompound(page) || PageTail(page))
+ if (!PageHead(page))
return 0;
return (unsigned long)page[1].lru.prev;
}
Index: linux-2.6.21-rc5-mm4/include/linux/page-flags.h
===================================================================
--- linux-2.6.21-rc5-mm4.orig/include/linux/page-flags.h 2007-04-09 11:04:10.000000000 -0700
+++ linux-2.6.21-rc5-mm4/include/linux/page-flags.h 2007-04-09 11:04:31.000000000 -0700
@@ -95,12 +95,6 @@
/* PG_owner_priv_1 users should have descriptive aliases */
#define PG_checked PG_owner_priv_1 /* Used by some filesystems */
-/*
- * Marks tail portion of a compound page. We currently do not reclaim
- * compound pages so we can reuse a flag only used for reclaim here.
- */
-#define PG_tail PG_reclaim
-
#if (BITS_PER_LONG > 32)
/*
* 64-bit-only flags build down from bit 31
@@ -221,12 +215,24 @@ static inline void SetPageUptodate(struc
#define __ClearPageCompound(page) __clear_bit(PG_compound, &(page)->flags)
/*
- * Note: PG_tail is an alias of another page flag. The result of PageTail()
- * is only valid if PageCompound(page) is true.
+ * PG_reclaim is used in combination with PG_compound to mark the
+ * head and tail of a compound page
+ *
+ * PG_compound & PG_reclaim => Tail page
+ * PG_compound & ~PG_reclaim => Head page
*/
-#define PageTail(page) test_bit(PG_tail, &(page)->flags)
-#define __SetPageTail(page) __set_bit(PG_tail, &(page)->flags)
-#define __ClearPageTail(page) __clear_bit(PG_tail, &(page)->flags)
+
+#define PG_head_tail_mask ((1L << PG_compound) | (1L << PG_reclaim))
+
+#define PageTail(page) ((page->flags & PG_head_tail_mask) \
+ == PG_head_tail_mask)
+#define __SetPageTail(page) page->flags |= PG_head_tail_mask
+#define __ClearPageTail(page) page->flags ~= PG_head_tail_mask
+
+#define PageHead(page) ((page->flags & PG_head_tail_mask) \
+ == (1L << PG_compound))
+#define __SetPageHead(page) __SetCompoundPage(page)
+#define __ClearPageHead(page) __ClearCompoundPage(page)
#ifdef CONFIG_SWAP
#define PageSwapCache(page) test_bit(PG_swapcache, &(page)->flags)
Index: linux-2.6.21-rc5-mm4/mm/page_alloc.c
===================================================================
--- linux-2.6.21-rc5-mm4.orig/mm/page_alloc.c 2007-04-09 11:04:10.000000000 -0700
+++ linux-2.6.21-rc5-mm4/mm/page_alloc.c 2007-04-09 11:04:31.000000000 -0700
@@ -300,12 +300,11 @@ static void prep_compound_page(struct pa
set_compound_page_dtor(page, free_compound_page);
set_compound_order(page, order);
- __SetPageCompound(page);
+ __SetPageHead(page);
for (i = 1; i < nr_pages; i++) {
struct page *p = page + i;
__SetPageTail(p);
- __SetPageCompound(p);
p->first_page = page;
}
}
@@ -318,17 +317,16 @@ static void destroy_compound_page(struct
if (unlikely(compound_order(page) != order))
bad_page(page);
- if (unlikely(!PageCompound(page)))
+ if (unlikely(!PageHead(page)))
bad_page(page);
- __ClearPageCompound(page);
+ __ClearPageHead(page);
for (i = 1; i < nr_pages; i++) {
struct page *p = page + i;
- if (unlikely(!PageCompound(p) | !PageTail(p) |
+ if (unlikely(!PageTail(p) |
(p->first_page != page)))
bad_page(page);
__ClearPageTail(p);
- __ClearPageCompound(p);
}
}
-
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