[PATCH 02/13] block/preallocate: keep zero_start above written data

["Denis V. Lunev" <den@openvz.org>]

From: Denis V. Lunev <den@openvz.org>

The filter records where the trailing preallocated zeroes begin, and
skips a write zeroes request that starts at or above that point,
because the range is zero already. A plain write below data_end returns
before that bookkeeping, so a write into the zero region leaves the
mark behind it.

The guest then loses an acknowledged write zeroes over the range: it is
skipped as already zero, and the plain write's data stays. Reachable
from qcow2 when a cluster is allocated again after a discard and the
allocating write zeroes straddles data_end.

Move the mark past any write that reaches into the region it covers and
leaves data there. want_merge_zero is what the function already uses to
tell those apart when the write extends the file: a request that writes
zeroes leaves the region zero and the mark where it is.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Cc: qemu-stable@nongnu.org
CC: Kevin Wolf <kwolf@redhat.com>
CC: Hanna Reitz <hreitz@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
 block/preallocate.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/preallocate.c b/block/preallocate.c
index d215bc5d6d..99bd6b1ae7 100644
--- a/block/preallocate.c
+++ b/block/preallocate.c
@@ -347,6 +347,10 @@ handle_write(BlockDriverState *bs, int64_t offset, int64_t bytes,
         }
     }
 
+    if (!want_merge_zero && s->zero_start >= 0 && end > s->zero_start) {
+        s->zero_start = end;
+    }
+
     if (end <= s->data_end) {
         return false;
     }
-- 
2.53.0