[PATCH 1/2] ZERO PAGE config

[KAMEZAWA Hiroyuki]

Only x86 config is inculded because I can test only x86...
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

Kconfig for using ZERO_PAGE. 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 ? AFAIK, powerpc and s390 has pte_special().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 arch/x86/Kconfig |    3 +++
 mm/Kconfig       |   15 +++++++++++++++
 2 files changed, 18 insertions(+)

Index: zeropage-trialv3/mm/Kconfig
===================================================================
--- zeropage-trialv3.orig/mm/Kconfig
+++ zeropage-trialv3/mm/Kconfig
@@ -214,6 +214,21 @@ config HAVE_MLOCKED_PAGE_BIT
 config MMU_NOTIFIER
 	bool
 
+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.
+
 config DEFAULT_MMAP_MIN_ADDR
         int "Low address space to protect from user allocation"
         default 4096
Index: zeropage-trialv3/arch/x86/Kconfig
===================================================================
--- zeropage-trialv3.orig/arch/x86/Kconfig
+++ zeropage-trialv3/arch/x86/Kconfig
@@ -161,6 +161,9 @@ 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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