qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

If BDRV_REQ_ZERO_WRITE is set and we're extending the image, calling
qcow2_cluster_zeroize() with flags=0 does the right thing: It doesn't
undo any previous preallocation, but just adds the zero flag to all
relevant L2 entries. If an external data file is in use, a write_zeroes
request to the data file is made instead.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200424125448.63318-5-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -1795,7 +1795,7 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
     /* Caller must pass aligned values, except at image end */
     assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
     assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
-           end_offset == bs->total_sectors << BDRV_SECTOR_BITS);
+           end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
 
     /* The zero flag is only supported by version 3 and newer */
     if (s->qcow_version < 3) {

block/qcow2.c

@@ -1726,6 +1726,7 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
 
     bs->supported_zero_flags = header.version >= 3 ?
                                BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK : 0;
+    bs->supported_truncate_flags = BDRV_REQ_ZERO_WRITE;
 
     /* Repair image if dirty */
     if (!(flags & (BDRV_O_CHECK | BDRV_O_INACTIVE)) && !bs->read_only &&
@@ -4214,6 +4215,39 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         g_assert_not_reached();
     }
 
+    if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
+        uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->cluster_size);
+
+        /*
+         * Use zero clusters as much as we can. qcow2_cluster_zeroize()
+         * requires a cluster-aligned start. The end may be unaligned if it is
+         * at the end of the image (which it is here).
+         */
+        ret = qcow2_cluster_zeroize(bs, zero_start, offset - zero_start, 0);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "Failed to zero out new clusters");
+            goto fail;
+        }
+
+        /* Write explicit zeros for the unaligned head */
+        if (zero_start > old_length) {
+            uint64_t len = zero_start - old_length;
+            uint8_t *buf = qemu_blockalign0(bs, len);
+            QEMUIOVector qiov;
+            qemu_iovec_init_buf(&qiov, buf, len);
+
+            qemu_co_mutex_unlock(&s->lock);
+            ret = qcow2_co_pwritev_part(bs, old_length, len, &qiov, 0, 0);
+            qemu_co_mutex_lock(&s->lock);
+
+            qemu_vfree(buf);
+            if (ret < 0) {
+                error_setg_errno(errp, -ret, "Failed to zero out the new area");
+                goto fail;
+            }
+        }
+    }
+
     if (prealloc != PREALLOC_MODE_OFF) {
         /* Flush metadata before actually changing the image size */
         ret = qcow2_write_caches(bs);