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[PATCH] GFP_ZONETYPES: add commentry on how to calculate 

GFP_ZONETYPES define using GFP_ZONEMASK and add commentry

Add commentry explaining the optimisation that we can apply to GFP_ZONETYPES
when the leftmost bit is a 'loaner', it can only be set in isolation.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Andy Whitcroft 2006-02-01 03:05:26 -08:00 committed by Linus Torvalds
parent 42c722d4cb
commit 79046ae07a
1 changed files with 8 additions and 0 deletions
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8
include/linux/mmzone.h
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@ -91,6 +91,14 @@ struct per_cpu_pageset {
* be 8 (2 ** 3) zonelists. GFP_ZONETYPES defines the number of possible
* combinations of zone modifiers in "zone modifier space".
*
* As an optimisation any zone modifier bits which are only valid when
* no other zone modifier bits are set (loners) should be placed in
* the highest order bits of this field. This allows us to reduce the
* extent of the zonelists thus saving space. For example in the case
* of three zone modifier bits, we could require up to eight zonelists.
* If the left most zone modifier is a "loner" then the highest valid
* zonelist would be four allowing us to allocate only five zonelists.
*
* NOTE! Make sure this matches the zones in <linux/gfp.h>
*/
#define GFP_ZONEMASK 0x07