diff --git a/block.c b/block.c
index 493c75f..784ff07 100644
--- a/block.c
+++ b/block.c
@@ -2231,6 +2231,8 @@
         .is_write       = is_write,
         .co             = qemu_coroutine_self(),
         .serialising    = false,
+        .overlap_offset = offset,
+        .overlap_bytes  = bytes,
     };
 
     qemu_co_queue_init(&req->wait_queue);
@@ -2238,12 +2240,19 @@
     QLIST_INSERT_HEAD(&bs->tracked_requests, req, list);
 }
 
-static void mark_request_serialising(BdrvTrackedRequest *req)
+static void mark_request_serialising(BdrvTrackedRequest *req, size_t align)
 {
+    int64_t overlap_offset = req->offset & ~(align - 1);
+    int overlap_bytes = ROUND_UP(req->offset + req->bytes, align)
+                      - overlap_offset;
+
     if (!req->serialising) {
         req->bs->serialising_in_flight++;
         req->serialising = true;
     }
+
+    req->overlap_offset = MIN(req->overlap_offset, overlap_offset);
+    req->overlap_bytes = MAX(req->overlap_bytes, overlap_bytes);
 }
 
 /**
@@ -2267,20 +2276,16 @@
     }
 }
 
-static void round_bytes_to_clusters(BlockDriverState *bs,
-                                    int64_t offset, unsigned int bytes,
-                                    int64_t *cluster_offset,
-                                    unsigned int *cluster_bytes)
+static int bdrv_get_cluster_size(BlockDriverState *bs)
 {
     BlockDriverInfo bdi;
+    int ret;
 
-    if (bdrv_get_info(bs, &bdi) < 0 || bdi.cluster_size == 0) {
-        *cluster_offset = offset;
-        *cluster_bytes = bytes;
+    ret = bdrv_get_info(bs, &bdi);
+    if (ret < 0 || bdi.cluster_size == 0) {
+        return bs->request_alignment;
     } else {
-        *cluster_offset = QEMU_ALIGN_DOWN(offset, bdi.cluster_size);
-        *cluster_bytes = QEMU_ALIGN_UP(offset - *cluster_offset + bytes,
-                                       bdi.cluster_size);
+        return bdi.cluster_size;
     }
 }
 
@@ -2288,11 +2293,11 @@
                                      int64_t offset, unsigned int bytes)
 {
     /*        aaaa   bbbb */
-    if (offset >= req->offset + req->bytes) {
+    if (offset >= req->overlap_offset + req->overlap_bytes) {
         return false;
     }
     /* bbbb   aaaa        */
-    if (req->offset >= offset + bytes) {
+    if (req->overlap_offset >= offset + bytes) {
         return false;
     }
     return true;
@@ -2302,30 +2307,21 @@
 {
     BlockDriverState *bs = self->bs;
     BdrvTrackedRequest *req;
-    int64_t cluster_offset;
-    unsigned int cluster_bytes;
     bool retry;
 
     if (!bs->serialising_in_flight) {
         return;
     }
 
-    /* If we touch the same cluster it counts as an overlap.  This guarantees
-     * that allocating writes will be serialized and not race with each other
-     * for the same cluster.  For example, in copy-on-read it ensures that the
-     * CoR read and write operations are atomic and guest writes cannot
-     * interleave between them.
-     */
-    round_bytes_to_clusters(bs, self->offset, self->bytes,
-                            &cluster_offset, &cluster_bytes);
-
     do {
         retry = false;
         QLIST_FOREACH(req, &bs->tracked_requests, list) {
             if (req == self || (!req->serialising && !self->serialising)) {
                 continue;
             }
-            if (tracked_request_overlaps(req, cluster_offset, cluster_bytes)) {
+            if (tracked_request_overlaps(req, self->overlap_offset,
+                                         self->overlap_bytes))
+            {
                 /* Hitting this means there was a reentrant request, for
                  * example, a block driver issuing nested requests.  This must
                  * never happen since it means deadlock.
@@ -2941,7 +2937,12 @@
 
     /* Handle Copy on Read and associated serialisation */
     if (flags & BDRV_REQ_COPY_ON_READ) {
-        mark_request_serialising(req);
+        /* If we touch the same cluster it counts as an overlap.  This
+         * guarantees that allocating writes will be serialized and not race
+         * with each other for the same cluster.  For example, in copy-on-read
+         * it ensures that the CoR read and write operations are atomic and
+         * guest writes cannot interleave between them. */
+        mark_request_serialising(req, bdrv_get_cluster_size(bs));
     }
 
     wait_serialising_requests(req);