[PATCH 4/5] block/io: fix bdrv_co_do_pwrite_zeroes head calculation

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

It's wrong to update head using num in this place, as num may be
reduced during the iteration, and we'll have wrong head value on next
iteration.

Instead update head at iteration end.

Cc: qemu-stable@nongnu.org
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/io.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/io.c b/block/io.c
index 75fd5600c2..c64566b4cf 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1785,7 +1785,6 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
              * convenience, limit this request to max_transfer even if
              * we don't need to fall back to writes.  */
             num = MIN(MIN(bytes, max_transfer), alignment - head);
-            head = (head + num) % alignment;
             assert(num < max_write_zeroes);
         } else if (tail && num > alignment) {
             /* Shorten the request to the last aligned sector.  */
@@ -1844,6 +1843,9 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
 
         offset += num;
         bytes -= num;
+        if (head) {
+            head = offset % alignment;
+        }
     }
 
 fail:
-- 
2.21.0