Backward prefetching is vital to structural analysis and some other
scientific applications. Comment this use case.

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


--- linux-2.6.17-rc4-mm3.orig/mm/readahead.c
+++ linux-2.6.17-rc4-mm3/mm/readahead.c
@@ -1540,6 +1540,8 @@ initial_readahead(struct address_space *
  * Backward prefetching.
  *
  * No look-ahead and thrashing safety guard: should be unnecessary.
+ *
+ * Important for certain scientific arenas(i.e. structural analysis).
  */
 static int
 try_read_backward(struct file_ra_state *ra, pgoff_t begin_index,

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