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mm: optimize compound_head() by avoiding a shared page flag 

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.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Christoph Lameter 2007-05-06 14:49:40 -07:00 committed by Linus Torvalds
parent d85f33855c
commit 6d7779538f
3 changed files with 32 additions and 26 deletions
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11
include/linux/mm.h
View File

@ -269,14 +269,7 @@ static inline int get_page_unless_zero(struct page *page)
static inline struct page *compound_head(struct page *page) static inline struct page *compound_head(struct page *page)
{ {
/* if (unlikely(PageTail(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)))
return page->first_page; return page->first_page;
return page; return page;
} }
@ -327,7 +320,7 @@ static inline compound_page_dtor *get_compound_page_dtor(struct page *page)
static inline int compound_order(struct page *page) static inline int compound_order(struct page *page)
{ {
if (!PageCompound(page) || PageTail(page)) if (!PageHead(page))
return 0; return 0;
return (unsigned long)page[1].lru.prev; return (unsigned long)page[1].lru.prev;
} }

37
include/linux/page-flags.h
View File

@ -6,6 +6,7 @@
#define PAGE_FLAGS_H #define PAGE_FLAGS_H
#include <linux/types.h> #include <linux/types.h>
#include <linux/mm_types.h>
/* /*
* Various page->flags bits: * Various page->flags bits:
@ -94,12 +95,6 @@
/* PG_owner_priv_1 users should have descriptive aliases */ /* PG_owner_priv_1 users should have descriptive aliases */
#define PG_checked PG_owner_priv_1 /* Used by some filesystems */ #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) #if (BITS_PER_LONG > 32)
/* /*
* 64-bit-only flags build down from bit 31 * 64-bit-only flags build down from bit 31
@ -248,12 +243,32 @@ static inline void SetPageUptodate(struct page *page)
#define __ClearPageCompound(page) __clear_bit(PG_compound, &(page)->flags) #define __ClearPageCompound(page) __clear_bit(PG_compound, &(page)->flags)
/* /*
* Note: PG_tail is an alias of another page flag. The result of PageTail() * PG_reclaim is used in combination with PG_compound to mark the
* is only valid if PageCompound(page) is true. * 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 PG_head_tail_mask ((1L << PG_compound) | (1L << PG_reclaim))
#define __ClearPageTail(page) __clear_bit(PG_tail, &(page)->flags)
#define PageTail(page) ((page->flags & PG_head_tail_mask) \
== PG_head_tail_mask)
static inline void __SetPageTail(struct page *page)
{
page->flags |= PG_head_tail_mask;
}
static inline void __ClearPageTail(struct page *page)
{
page->flags &= ~PG_head_tail_mask;
}
#define PageHead(page) ((page->flags & PG_head_tail_mask) \
== (1L << PG_compound))
#define __SetPageHead(page) __SetPageCompound(page)
#define __ClearPageHead(page) __ClearPageCompound(page)
#ifdef CONFIG_SWAP #ifdef CONFIG_SWAP
#define PageSwapCache(page) test_bit(PG_swapcache, &(page)->flags) #define PageSwapCache(page) test_bit(PG_swapcache, &(page)->flags)

10
mm/page_alloc.c
View File

@ -235,12 +235,11 @@ static void prep_compound_page(struct page *page, unsigned long order)
set_compound_page_dtor(page, free_compound_page); set_compound_page_dtor(page, free_compound_page);
set_compound_order(page, order); set_compound_order(page, order);
__SetPageCompound(page); __SetPageHead(page);
for (i = 1; i < nr_pages; i++) { for (i = 1; i < nr_pages; i++) {
struct page *p = page + i; struct page *p = page + i;
__SetPageTail(p); __SetPageTail(p);
__SetPageCompound(p);
p->first_page = page; p->first_page = page;
} }
} }
@ -253,17 +252,16 @@ static void destroy_compound_page(struct page *page, unsigned long order)
if (unlikely(compound_order(page) != order)) if (unlikely(compound_order(page) != order))
bad_page(page); bad_page(page);
if (unlikely(!PageCompound(page))) if (unlikely(!PageHead(page)))
bad_page(page); bad_page(page);
__ClearPageCompound(page); __ClearPageHead(page);
for (i = 1; i < nr_pages; i++) { for (i = 1; i < nr_pages; i++) {
struct page *p = page + i; struct page *p = page + i;
if (unlikely(!PageCompound(p) | !PageTail(p) | if (unlikely(!PageTail(p) |
(p->first_page != page))) (p->first_page != page)))
bad_page(page); bad_page(page);
__ClearPageTail(p); __ClearPageTail(p);
__ClearPageCompound(p);
} }
} }