[CIFS] cifs truncate missing a fix for private map COW race

vmtruncate had added the same fix to handle the case of private pages
being Copy on writed while truncate_inode_pages is going on

Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/fs/cifs/inode.c b/fs/cifs/inode.c
index dd4167762a8e..97ccc513730f 100644
--- a/fs/cifs/inode.c
+++ b/fs/cifs/inode.c
@@ -1377,8 +1377,17 @@ static int cifs_vmtruncate(struct inode *inode, loff_t offset)
         }
         i_size_write(inode, offset);
         spin_unlock(&inode->i_lock);
+        /*
+         * unmap_mapping_range is called twice, first simply for efficiency
+         * so that truncate_inode_pages does fewer single-page unmaps. However
+         * after this first call, and before truncate_inode_pages finishes,
+         * it is possible for private pages to be COWed, which remain after
+         * truncate_inode_pages finishes, hence the second unmap_mapping_range
+         * call must be made for correctness.
+         */
         unmap_mapping_range(mapping, offset + PAGE_SIZE - 1, 0, 1);
         truncate_inode_pages(mapping, offset);
+        unmap_mapping_range(mapping, offset + PAGE_SIZE - 1, 0, 1);
         goto out_truncate;
 
 do_expand: