qcow2: Fix avail_sectors in cluster allocation code

avail_sectors should really be the number of sectors from the start of
the allocation, not from the start of the write request.

We're lucky enough that this mistake didn't cause any real bug.
avail_sectors is only used in the intialiser of QCowL2Meta:

  .nb_available   = MIN(requested_sectors, avail_sectors),

m->nb_available in turn is only used for COW at the end of the
allocation. A COW occurs only if the request wasn't cluster aligned,
which in turn would imply that requested_sectors was less than
avail_sectors (both in the original and in the fixed version). In this
case avail_sectors is ignored and therefore the mistake doesn't cause
any misbehaviour.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 98fba71fd7555139bf630cb8c33a1c2156464d0e..d7e0e19d9c0063838a53e8ad676b11131160e3be 100644 (file)
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -947,8 +947,16 @@ again:
 
         /* save info needed for meta data update */
         if (nb_clusters > 0) {
+            /*
+             * requested_sectors: Number of sectors from the start of the first
+             * newly allocated cluster to the end of the (possibly shortened
+             * before) write request.
+             *
+             * avail_sectors: Number of sectors from the start of the first
+             * newly allocated to the end of the last newly allocated cluster.
+             */
             int requested_sectors = n_end - keep_clusters * s->cluster_sectors;
-            int avail_sectors = (keep_clusters + nb_clusters)
+            int avail_sectors = nb_clusters
                                 << (s->cluster_bits - BDRV_SECTOR_BITS);
 
             *m = (QCowL2Meta) {