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

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

@ -6,6 +6,7 @@
#define PAGE_FLAGS_H
#include <linux/types.h>
#include <linux/mm_types.h>
/*
* Various page->flags bits:
@ -94,12 +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
@ -248,12 +243,32 @@ static inline void SetPageUptodate(struct page *page)
#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)
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
#define PageSwapCache(page) test_bit(PG_swapcache, &(page)->flags)