Introduce a pair of functions to probe the existence of file page.
	- int __probe_page(mapping, offset)
	- int probe_page(mapping, offset)

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---


--- linux.orig/include/linux/pagemap.h
+++ linux/include/linux/pagemap.h
@@ -68,6 +68,8 @@ static inline struct page *page_cache_al
 
 typedef int filler_t(void *, struct page *);
 
+extern int __probe_page(struct address_space *mapping, pgoff_t offset);
+extern int probe_page(struct address_space *mapping, pgoff_t offset);
 extern struct page * find_get_page(struct address_space *mapping,
 				unsigned long index);
 extern struct page * find_lock_page(struct address_space *mapping,
--- linux.orig/mm/filemap.c
+++ linux/mm/filemap.c
@@ -548,6 +548,28 @@ void fastcall __lock_page(struct page *p
 EXPORT_SYMBOL(__lock_page);
 
 /*
+ * Probing page existence.
+ */
+int __probe_page(struct address_space *mapping, pgoff_t offset)
+{
+	return !! radix_tree_lookup(&mapping->page_tree, offset);
+}
+
+/*
+ * Here we just do not bother to grab the page, it's meaningless anyway.
+ */
+int probe_page(struct address_space *mapping, pgoff_t offset)
+{
+	int exists;
+
+	read_lock_irq(&mapping->tree_lock);
+	exists = __probe_page(mapping, offset);
+	read_unlock_irq(&mapping->tree_lock);
+
+	return exists;
+}
+
+/*
  * a rather lightweight function, finding and getting a reference to a
  * hashed page atomically.
  */

--