Hello, guys.

 When __generic_file_aio_read() hits an error during reading, it
reports the error iff nothing has successfully been read yet.  This is
different from how other read/write functions deal with error
condition - when an error occurs, if nothing has been read/written,
report the error code; otherwise, report the amount of bytes
successfully transferred upto that point.

 This corner case can be exposed by performing readv(2) with the
following iov.

 iov[0] = len0 @ ptr0
 iov[1] = len1 @ NULL (or any other invalid pointer)
 iov[2] = len2 @ ptr2

 When file size is enough, performing above readv(2) results in

 len0 bytes from file_pos @ ptr0
 len2 bytes from file_pos + len0 @ ptr2

 And the return value is len0 + len2.  Test program is attached to
this mail.

 This patch makes __generic_file_aio_read()'s error handling identical
to other functions.

 Thanks.

Signed-off-by: Tejun Heo <htejun@gmail.com>

diff --git a/mm/filemap.c b/mm/filemap.c
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1030,8 +1030,8 @@ __generic_file_aio_read(struct kiocb *io
 			desc.error = 0;
 			do_generic_file_read(filp,ppos,&desc,file_read_actor);
 			retval += desc.written;
-			if (!retval) {
-				retval = desc.error;
+			if (desc.error) {
+				retval = retval ?: desc.error;
 				break;
 			}
 		}