[PATCH 3.17 075/141] Fix thinko in iov_iter_single_seg_count

Greg Kroah-Hartman
2014-11-19T22:22:20Z

From: Paul Mackerras <paulus@samba.org>

commit ad0eab9293485d1c06237e9249f6d4dfa3d93d4d upstream.

The branches of the if (i->type & ITER_BVEC) statement in
iov_iter_single_seg_count() are the wrong way around; if ITER_BVEC is
clear then we use i->bvec, when we should be using i->iov. This fixes
it.

In my case, the symptom that this caused was that a KVM guest doing
filesystem operations on a virtual disk would result in one of qemu's
threads on the host going into an infinite loop in
generic_perform_write(). The loop would hit the copied == 0 case and
call iov_iter_single_seg_count() to reduce the number of bytes to try
to process, but because of the error, iov_iter_single_seg_count()
would just return i->count and the loop made no progress and continued
forever.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/iov_iter.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/iov_iter.c
+++ b/mm/iov_iter.c
@@ -699,9 +699,9 @@ size_t iov_iter_single_seg_count(const s
if (i->nr_segs == 1)
return i->count;
else if (i->type & ITER_BVEC)
- return min(i->count, i->iov->iov_len - i->iov_offset);
- else
return min(i->count, i->bvec->bv_len - i->iov_offset);
+ else
+ return min(i->count, i->iov->iov_len - i->iov_offset);
}
EXPORT_SYMBOL(iov_iter_single_seg_count);