[PATCH 3.17 075/141] Fix thinko in iov_iter_single_seg_count

From: Greg Kroah-Hartman
Date: Wed Nov 19 2014 - 17:22:24 EST

3.17-stable review patch. If anyone has any objections, please let me know.


From: Paul Mackerras <paulus@xxxxxxxxx>

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

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

Signed-off-by: Paul Mackerras <paulus@xxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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);

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