diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 7f4f73eaca..202adb4807 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -759,31 +759,41 @@ out:
  * Check if there already is an AIO write request in flight which allocates
  * the same cluster. In this case we need to wait until the previous
  * request has completed and updated the L2 table accordingly.
+ *
+ * Returns:
+ *   0       if there was no dependency. *cur_bytes indicates the number of
+ *           bytes from guest_offset that can be read before the next
+ *           dependency must be processed (or the request is complete)
+ *
+ *   -EAGAIN if we had to wait for another request, previously gathered
+ *           information on cluster allocation may be invalid now. The caller
+ *           must start over anyway, so consider *cur_bytes undefined.
  */
 static int handle_dependencies(BlockDriverState *bs, uint64_t guest_offset,
-    unsigned int *nb_clusters)
+    uint64_t *cur_bytes)
 {
     BDRVQcowState *s = bs->opaque;
     QCowL2Meta *old_alloc;
+    uint64_t bytes = *cur_bytes;
 
     QLIST_FOREACH(old_alloc, &s->cluster_allocs, next_in_flight) {
 
-        uint64_t start = guest_offset >> s->cluster_bits;
-        uint64_t end = start + *nb_clusters;
-        uint64_t old_start = old_alloc->offset >> s->cluster_bits;
-        uint64_t old_end = old_start + old_alloc->nb_clusters;
+        uint64_t start = guest_offset;
+        uint64_t end = start + bytes;
+        uint64_t old_start = l2meta_cow_start(old_alloc);
+        uint64_t old_end = l2meta_cow_end(old_alloc);
 
         if (end <= old_start || start >= old_end) {
             /* No intersection */
         } else {
             if (start < old_start) {
                 /* Stop at the start of a running allocation */
-                *nb_clusters = old_start - start;
+                bytes = old_start - start;
             } else {
-                *nb_clusters = 0;
+                bytes = 0;
             }
 
-            if (*nb_clusters == 0) {
+            if (bytes == 0) {
                 /* Wait for the dependency to complete. We need to recheck
                  * the free/allocated clusters when we continue. */
                 qemu_co_mutex_unlock(&s->lock);
@@ -794,9 +804,9 @@ static int handle_dependencies(BlockDriverState *bs, uint64_t guest_offset,
         }
     }
 
-    if (!*nb_clusters) {
-        abort();
-    }
+    /* Make sure that existing clusters and new allocations are only used up to
+     * the next dependency if we shortened the request above */
+    *cur_bytes = bytes;
 
     return 0;
 }
@@ -875,6 +885,7 @@ int qcow2_alloc_cluster_offset(BlockDriverState *bs, uint64_t offset,
     uint64_t *l2_table;
     unsigned int nb_clusters, keep_clusters;
     uint64_t cluster_offset;
+    uint64_t cur_bytes;
 
     trace_qcow2_alloc_clusters_offset(qemu_coroutine_self(), offset,
                                       n_start, n_end);
@@ -887,6 +898,7 @@ again:
     l2_index = offset_to_l2_index(s, offset);
     nb_clusters = MIN(size_to_clusters(s, n_end << BDRV_SECTOR_BITS),
                       s->l2_size - l2_index);
+    n_end = MIN(n_end, nb_clusters * s->cluster_sectors);
 
     /*
      * Now start gathering as many contiguous clusters as possible:
@@ -911,7 +923,8 @@ again:
      * 3. If the request still hasn't completed, allocate new clusters,
      *    considering any cluster_offset of steps 1c or 2.
      */
-    ret = handle_dependencies(bs, offset, &nb_clusters);
+    cur_bytes = (n_end - n_start) * BDRV_SECTOR_SIZE;
+    ret = handle_dependencies(bs, offset, &cur_bytes);
     if (ret == -EAGAIN) {
         goto again;
     } else if (ret < 0) {
@@ -922,6 +935,9 @@ again:
          * correctly during the next loop iteration. */
     }
 
+    nb_clusters = size_to_clusters(s, offset + cur_bytes)
+                - (offset >> s->cluster_bits);
+
     /* Find L2 entry for the first involved cluster */
     ret = get_cluster_table(bs, offset, &l2_table, &l2_index);
     if (ret < 0) {
diff --git a/block/qcow2.h b/block/qcow2.h
index 0940b1b3e8..a99d51b6a1 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -307,6 +307,17 @@ static inline bool qcow2_need_accurate_refcounts(BDRVQcowState *s)
     return !(s->incompatible_features & QCOW2_INCOMPAT_DIRTY);
 }
 
+static inline uint64_t l2meta_cow_start(QCowL2Meta *m)
+{
+    return m->offset + m->cow_start.offset;
+}
+
+static inline uint64_t l2meta_cow_end(QCowL2Meta *m)
+{
+    return m->offset + m->cow_end.offset
+        + (m->cow_end.nb_sectors << BDRV_SECTOR_BITS);
+}
+
 // FIXME Need qcow2_ prefix to global functions
 
 /* qcow2.c functions */