[PATCH 1/2] ZERO_PAGE config

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

no changes from v4.
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Kconfig for using ZERO_PAGE or not. Using ZERO_PAGE or not is depends on
 - arch has pte_special() or not.
 - arch allows to use ZERO_PAGE or not.

In this patch, generic-config for /mm and arch-specific config for x86
is added. Other archs ?

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 arch/x86/Kconfig |    3 +++
 mm/Kconfig       |   18 ++++++++++++++++++
 2 files changed, 21 insertions(+)

Index: mmotm-2.6.31-Aug4/mm/Kconfig
===================================================================
--- mmotm-2.6.31-Aug4.orig/mm/Kconfig
+++ mmotm-2.6.31-Aug4/mm/Kconfig
@@ -225,6 +225,24 @@ config KSM
 	  saving memory until one or another app needs to modify the content.
 	  Recommended for use with KVM, or with other duplicative applications.
 
+config SUPPORT_ANON_ZERO_PAGE
+	bool "Use anon zero page"
+	default y if ARCH_SUPPORT_ANON_ZERO_PAGE
+	help
+	  In anonymous private mapping (MAP_ANONYMOUS and /dev/zero), a read
+	  page fault will allocate a new zero-cleared page. If the first page
+	  fault is write, allocating a new page is necessary. But if it is
+	  read, we can use ZERO_PAGE until a write comes. If you set this to y,
+	  the kernel use ZERO_PAGE and delays allocating new memory in private
+	  anon mapping until the first write. If applications use large mmap
+	  and most of accesses are read, this reduces memory usage and cache
+	  usage to some extent. To support this, your architecture should have
+	  _PAGE_SPECIAL bit in pte. And this will be no help to cpu cache if
+	  the arch's cache is virtually tagged.
+	  To developper:
+	  This ZERO_PAGE changes behavior of follow_page(). please check
+	  usage of follow_page() in your arch before supporting this.
+
 config DEFAULT_MMAP_MIN_ADDR
         int "Low address space to protect from user allocation"
         default 4096
Index: mmotm-2.6.31-Aug4/arch/x86/Kconfig
===================================================================
--- mmotm-2.6.31-Aug4.orig/arch/x86/Kconfig
+++ mmotm-2.6.31-Aug4/arch/x86/Kconfig
@@ -158,6 +158,9 @@ config ARCH_HIBERNATION_POSSIBLE
 config ARCH_SUSPEND_POSSIBLE
 	def_bool y
 
+config ARCH_SUPPORT_ANON_ZERO_PAGE
+	def_bool y
+
 config ZONE_DMA32
 	bool
 	default X86_64

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