readahead: split ondemand readahead interface into two functions

Split ondemand readahead interface into two functions.  I think this makes it
a little clearer for non-readahead experts (like Rusty).

Internally they both call ondemand_readahead(), but the page argument is
changed to an obvious boolean flag.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3d0d7d2..50a0ed1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1138,12 +1138,20 @@
 			pgoff_t offset, unsigned long nr_to_read);
 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 			pgoff_t offset, unsigned long nr_to_read);
-unsigned long page_cache_readahead_ondemand(struct address_space *mapping,
-			  struct file_ra_state *ra,
-			  struct file *filp,
-			  struct page *page,
-			  pgoff_t offset,
-			  unsigned long size);
+
+void page_cache_sync_readahead(struct address_space *mapping,
+			       struct file_ra_state *ra,
+			       struct file *filp,
+			       pgoff_t offset,
+			       unsigned long size);
+
+void page_cache_async_readahead(struct address_space *mapping,
+				struct file_ra_state *ra,
+				struct file *filp,
+				struct page *pg,
+				pgoff_t offset,
+				unsigned long size);
+
 unsigned long max_sane_readahead(unsigned long nr);
 
 /* Do stack extension */