qcow2: Allow requests with multiple l2metas

Instead of expecting a single l2meta, have a list of them. This allows
to still have a single I/O request for the guest data, even though
multiple l2meta may be needed in order to describe both a COW overwrite
and a new cluster allocation (typical sequential write case).

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

block/qcow2-cluster.c

@@ -1069,10 +1069,13 @@ static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
     int alloc_n_start = offset_into_cluster(s, guest_offset)
                         >> BDRV_SECTOR_BITS;
     int nb_sectors = MIN(requested_sectors, avail_sectors);
+    QCowL2Meta *old_m = *m;
 
     *m = g_malloc0(sizeof(**m));
 
     **m = (QCowL2Meta) {
+        .next           = old_m,
+
         .alloc_offset   = alloc_cluster_offset,
         .offset         = start_of_cluster(s, guest_offset),
         .nb_clusters    = nb_clusters,

block/qcow2.c

@@ -858,7 +858,9 @@ static coroutine_fn int qcow2_co_writev(BlockDriverState *bs,
             goto fail;
         }
 
-        if (l2meta != NULL) {
+        while (l2meta != NULL) {
+            QCowL2Meta *next;
+
             ret = qcow2_alloc_cluster_link_l2(bs, l2meta);
             if (ret < 0) {
                 goto fail;
@@ -871,8 +873,9 @@ static coroutine_fn int qcow2_co_writev(BlockDriverState *bs,
 
             qemu_co_queue_restart_all(&l2meta->dependent_requests);
 
+            next = l2meta->next;
             g_free(l2meta);
-            l2meta = NULL;
+            l2meta = next;
         }
 
         remaining_sectors -= cur_nr_sectors;
@@ -885,12 +888,17 @@ static coroutine_fn int qcow2_co_writev(BlockDriverState *bs,
 fail:
     qemu_co_mutex_unlock(&s->lock);
 
-    if (l2meta != NULL) {
+    while (l2meta != NULL) {
+        QCowL2Meta *next;
+
         if (l2meta->nb_clusters != 0) {
             QLIST_REMOVE(l2meta, next_in_flight);
         }
         qemu_co_queue_restart_all(&l2meta->dependent_requests);
+
+        next = l2meta->next;
         g_free(l2meta);
+        l2meta = next;
     }
 
     qemu_iovec_destroy(&hd_qiov);

block/qcow2.h

@@ -250,6 +250,9 @@ typedef struct QCowL2Meta
      */
     Qcow2COWRegion cow_end;
 
+    /** Pointer to next L2Meta of the same write request */
+    struct QCowL2Meta *next;
+
     QLIST_ENTRY(QCowL2Meta) next_in_flight;
 } QCowL2Meta;