[PATCH] readahead: ->prev_page can overrun the ahead window

If get_next_ra_size() does not grow fast enough, ->prev_page can overrun
the ahead window.  This means the caller will read the pages from
->ahead_start + ->ahead_size to ->prev_page synchronously.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Steven Pratt <slpratt@austin.ibm.com>
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 20 insertions, 6 deletions

diff --git a/mm/readahead.c b/mm/readahead.c
index 8d6eeaaa6296..57557e294987 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -52,13 +52,24 @@ static inline unsigned long get_min_readahead(struct file_ra_state *ra)
         return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE;
 }
 
+static inline void reset_ahead_window(struct file_ra_state *ra)
+{
+        /*
+         * ... but preserve ahead_start + ahead_size value,
+         * see 'recheck:' label in page_cache_readahead().
+         * Note: We never use ->ahead_size as rvalue without
+         * checking ->ahead_start != 0 first.
+         */
+        ra->ahead_size += ra->ahead_start;
+        ra->ahead_start = 0;
+}
+
 static inline void ra_off(struct file_ra_state *ra)
 {
         ra->start = 0;
         ra->flags = 0;
         ra->size = 0;
-        ra->ahead_start = 0;
-        ra->ahead_size = 0;
+        reset_ahead_window(ra);
         return;
 }
 
@@ -426,8 +437,7 @@ static int make_ahead_window(struct address_space *mapping, struct file *filp,
                  * congestion.  The ahead window will any way be closed
                  * in case we failed due to excessive page cache hits.
                  */
-                ra->ahead_start = 0;
-                ra->ahead_size = 0;
+                reset_ahead_window(ra);
         }
 
         return ret;
@@ -520,11 +530,11 @@ page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
          * If we get here we are doing sequential IO and this was not the first
          * occurence (ie we have an existing window)
          */
-
         if (ra->ahead_start == 0) {      /* no ahead window yet */
                 if (!make_ahead_window(mapping, filp, ra, 0))
-                        goto out;
+                        goto recheck;
         }
+
         /*
          * Already have an ahead window, check if we crossed into it.
          * If so, shift windows and issue a new ahead window.
@@ -536,6 +546,10 @@ page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
                 ra->start = ra->ahead_start;
                 ra->size = ra->ahead_size;
                 make_ahead_window(mapping, filp, ra, 0);
+recheck:
+                /* prev_page shouldn't overrun the ahead window */
+                ra->prev_page = min(ra->prev_page,
+                        ra->ahead_start + ra->ahead_size - 1);
         }
 
 out: