[PATCH][BUGFIX] readahead: fix NULL filp dereference

[Wu Fengguang]

The btrfs relocate_file_extent_cluster() calls us with NULL filp:

  [ 4005.426805] BUG: unable to handle kernel NULL pointer dereference at 00000021
  [ 4005.426818] IP: [<c109a130>] page_cache_sync_readahead+0x18/0x3e

CC: Yan Zheng <yanzheng@xxxxxxxx>
Reported-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---

Andrew and Greg:

This is an obvious correct bug fix for .34 and .33-stable,
so I'm resending it directly to you without Kirill's confirmation.


--- sound-2.6.orig/mm/readahead.c	2010-03-26 11:51:57.000000000 +0800
+++ sound-2.6/mm/readahead.c	2010-03-26 11:52:11.000000000 +0800
@@ -502,7 +502,7 @@ void page_cache_sync_readahead(struct ad
 		return;
 
 	/* be dumb */
-	if (filp->f_mode & FMODE_RANDOM) {
+	if (filp && (filp->f_mode & FMODE_RANDOM)) {
 		force_page_cache_readahead(mapping, filp, offset, req_size);
 		return;
 	}
--
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