[RFC][PATCH 1/4] introduce pte_zero()

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

initializng zero_page_pfn is not clean yet...

==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Add some helper functions for supporing zero-page again.
This patch itself adss some tiny functions but no behavior change.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 include/linux/mm.h |   20 ++++++++++++++++++++
 mm/memory.c        |    8 ++++++++
 2 files changed, 28 insertions(+)

Index: zeropage-trial/include/linux/mm.h
===================================================================
--- zeropage-trial.orig/include/linux/mm.h
+++ zeropage-trial/include/linux/mm.h
@@ -822,6 +822,26 @@ static inline int handle_mm_fault(struct
 }
 #endif
 
+/*
+ * ZERO page is used for read-only(never write) private page mapping. It's not
+ * usually used but sometimes useful at maping /dev/zero or at scanning
+ * sparsely used big private memory or at calculation.with sparse matrix where
+ * most of entries are zero. ZERO page is not refcounted and exists as
+ * PG_reserved page. zero_page_pfn is pfn of ZERO_PAGE(0).
+ */
+
+extern unsigned long zero_page_pfn;
+static inline int pte_zero(pte_t pte)
+{
+	return (pte_pfn(pte) == zero_page_pfn);
+}
+
+static inline int page_is_zero(struct page *page)
+{
+	return page == ZERO_PAGE(0);
+}
+
+
 extern int make_pages_present(unsigned long addr, unsigned long end);
 extern int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, int len, int write);
 
Index: zeropage-trial/mm/memory.c
===================================================================
--- zeropage-trial.orig/mm/memory.c
+++ zeropage-trial/mm/memory.c
@@ -106,6 +106,14 @@ static int __init disable_randmaps(char 
 }
 __setup("norandmaps", disable_randmaps);
 
+unsigned long zero_page_pfn __read_mostly;
+static int __init zeropage_init(void)
+{
+	zero_page_pfn = page_to_pfn(ZERO_PAGE(0));
+	return 0;
+}
+__initcall(zeropage_init);
+
 
 /*
  * If a p?d_bad entry is found while walking page tables, report

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