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>
Signed-off-by: Andrew Morton <akpm@osdl.org>
--- linux-2.6.19-rc4-mm1.orig/include/linux/pagemap.h
+++ linux-2.6.19-rc4-mm1/include/linux/pagemap.h
@@ -72,6 +72,8 @@ 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-2.6.19-rc4-mm1.orig/mm/filemap.c
+++ linux-2.6.19-rc4-mm1/mm/filemap.c
@@ -613,6 +613,29 @@ void fastcall __lock_page_nosync(struct 
 							TASK_UNINTERRUPTIBLE);
 }
 
+/*
+ * 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;
+}
+EXPORT_SYMBOL(probe_page);
+
 /**
  * find_get_page - find and get a page reference
  * @mapping: the address_space to search

--
-
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