readahead: fix NULL filp dereference

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

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Yan Zheng <yanzheng@21cn.com>
Reported-by: Kirill A. Shutemov <kirill@shutemov.name>
Tested-by: Kirill A. Shutemov <kirill@shutemov.name>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/mm/readahead.c b/mm/readahead.c
index 999b54bb462f..dfa9a1a03a11 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -503,7 +503,7 @@ void page_cache_sync_readahead(struct address_space *mapping,
                 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;
         }