block/backup.c

   1 /*
   2  * QEMU backup
   3  *
   4  * Copyright (C) 2013 Proxmox Server Solutions
   5  *
   6  * Authors:
   7  *  Dietmar Maurer (dietmar@proxmox.com)
   8  *
   9  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  10  * See the COPYING file in the top-level directory.
  11  *
  12  */
  13
  14 #include "qemu/osdep.h"
  15
  16 #include "trace.h"
  17 #include "block/block.h"
  18 #include "block/block_int.h"
  19 #include "block/blockjob_int.h"
  20 #include "block/block_backup.h"
  21 #include "qapi/error.h"
  22 #include "qapi/qmp/qerror.h"
  23 #include "qemu/ratelimit.h"
  24 #include "qemu/cutils.h"
  25 #include "sysemu/block-backend.h"
  26 #include "qemu/bitmap.h"
  27 #include "qemu/error-report.h"
  28
  29 #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)
  30 #define SLICE_TIME 100000000ULL /* ns */
  31
  32 typedef struct BackupBlockJob {
  33     BlockJob common;
  34     BlockBackend *target;
  35     /* bitmap for sync=incremental */
  36     BdrvDirtyBitmap *sync_bitmap;
  37     MirrorSyncMode sync_mode;
  38     RateLimit limit;
  39     BlockdevOnError on_source_error;
  40     BlockdevOnError on_target_error;
  41     CoRwlock flush_rwlock;
  42     uint64_t bytes_read;
  43     unsigned long *done_bitmap;
  44     int64_t cluster_size;
  45     bool compress;
  46     NotifierWithReturn before_write;
  47     QLIST_HEAD(, CowRequest) inflight_reqs;
  48 } BackupBlockJob;
  49
  50 /* See if in-flight requests overlap and wait for them to complete */
  51 static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
  52                                                        int64_t start,
  53                                                        int64_t end)
  54 {
  55     CowRequest *req;
  56     bool retry;
  57
  58     do {
  59         retry = false;
  60         QLIST_FOREACH(req, &job->inflight_reqs, list) {
  61             if (end > req->start_byte && start < req->end_byte) {
  62                 qemu_co_queue_wait(&req->wait_queue, NULL);
  63                 retry = true;
  64                 break;
  65             }
  66         }
  67     } while (retry);
  68 }
  69
  70 /* Keep track of an in-flight request */
  71 static void cow_request_begin(CowRequest *req, BackupBlockJob *job,
  72                               int64_t start, int64_t end)
  73 {
  74     req->start_byte = start;
  75     req->end_byte = end;
  76     qemu_co_queue_init(&req->wait_queue);
  77     QLIST_INSERT_HEAD(&job->inflight_reqs, req, list);
  78 }
  79
  80 /* Forget about a completed request */
  81 static void cow_request_end(CowRequest *req)
  82 {
  83     QLIST_REMOVE(req, list);
  84     qemu_co_queue_restart_all(&req->wait_queue);
  85 }
  86
  87 static int coroutine_fn backup_do_cow(BackupBlockJob *job,
  88                                       int64_t offset, uint64_t bytes,
  89                                       bool *error_is_read,
  90                                       bool is_write_notifier)
  91 {
  92     BlockBackend *blk = job->common.blk;
  93     CowRequest cow_request;
  94     struct iovec iov;
  95     QEMUIOVector bounce_qiov;
  96     void *bounce_buffer = NULL;
  97     int ret = 0;
  98     int64_t start, end; /* bytes */
  99     int n; /* bytes */
 100
 101     qemu_co_rwlock_rdlock(&job->flush_rwlock);
 102
 103     start = QEMU_ALIGN_DOWN(offset, job->cluster_size);
 104     end = QEMU_ALIGN_UP(bytes + offset, job->cluster_size);
 105
 106     trace_backup_do_cow_enter(job, start, offset, bytes);
 107
 108     wait_for_overlapping_requests(job, start, end);
 109     cow_request_begin(&cow_request, job, start, end);
 110
 111     for (; start < end; start += job->cluster_size) {
 112         if (test_bit(start / job->cluster_size, job->done_bitmap)) {
 113             trace_backup_do_cow_skip(job, start);
 114             continue; /* already copied */
 115         }
 116
 117         trace_backup_do_cow_process(job, start);
 118
 119         n = MIN(job->cluster_size, job->common.len - start);
 120
 121         if (!bounce_buffer) {
 122             bounce_buffer = blk_blockalign(blk, job->cluster_size);
 123         }
 124         iov.iov_base = bounce_buffer;
 125         iov.iov_len = n;
 126         qemu_iovec_init_external(&bounce_qiov, &iov, 1);
 127
 128         ret = blk_co_preadv(blk, start, bounce_qiov.size, &bounce_qiov,
 129                             is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0);
 130         if (ret < 0) {
 131             trace_backup_do_cow_read_fail(job, start, ret);
 132             if (error_is_read) {
 133                 *error_is_read = true;
 134             }
 135             goto out;
 136         }
 137
 138         if (buffer_is_zero(iov.iov_base, iov.iov_len)) {
 139             ret = blk_co_pwrite_zeroes(job->target, start,
 140                                        bounce_qiov.size, BDRV_REQ_MAY_UNMAP);
 141         } else {
 142             ret = blk_co_pwritev(job->target, start,
 143                                  bounce_qiov.size, &bounce_qiov,
 144                                  job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
 145         }
 146         if (ret < 0) {
 147             trace_backup_do_cow_write_fail(job, start, ret);
 148             if (error_is_read) {
 149                 *error_is_read = false;
 150             }
 151             goto out;
 152         }
 153
 154         set_bit(start / job->cluster_size, job->done_bitmap);
 155
 156         /* Publish progress, guest I/O counts as progress too.  Note that the
 157          * offset field is an opaque progress value, it is not a disk offset.
 158          */
 159         job->bytes_read += n;
 160         job->common.offset += n;
 161     }
 162
 163 out:
 164     if (bounce_buffer) {
 165         qemu_vfree(bounce_buffer);
 166     }
 167
 168     cow_request_end(&cow_request);
 169
 170     trace_backup_do_cow_return(job, offset, bytes, ret);
 171
 172     qemu_co_rwlock_unlock(&job->flush_rwlock);
 173
 174     return ret;
 175 }
 176
 177 static int coroutine_fn backup_before_write_notify(
 178         NotifierWithReturn *notifier,
 179         void *opaque)
 180 {
 181     BackupBlockJob *job = container_of(notifier, BackupBlockJob, before_write);
 182     BdrvTrackedRequest *req = opaque;
 183
 184     assert(req->bs == blk_bs(job->common.blk));
 185     assert(QEMU_IS_ALIGNED(req->offset, BDRV_SECTOR_SIZE));
 186     assert(QEMU_IS_ALIGNED(req->bytes, BDRV_SECTOR_SIZE));
 187
 188     return backup_do_cow(job, req->offset, req->bytes, NULL, true);
 189 }
 190
 191 static void backup_set_speed(BlockJob *job, int64_t speed, Error **errp)
 192 {
 193     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 194
 195     if (speed < 0) {
 196         error_setg(errp, QERR_INVALID_PARAMETER, "speed");
 197         return;
 198     }
 199     ratelimit_set_speed(&s->limit, speed, SLICE_TIME);
 200 }
 201
 202 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 203 {
 204     BdrvDirtyBitmap *bm;
 205     BlockDriverState *bs = blk_bs(job->common.blk);
 206
 207     if (ret < 0 || block_job_is_cancelled(&job->common)) {
 208         /* Merge the successor back into the parent, delete nothing. */
 209         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
 210         assert(bm);
 211     } else {
 212         /* Everything is fine, delete this bitmap and install the backup. */
 213         bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
 214         assert(bm);
 215     }
 216 }
 217
 218 static void backup_commit(BlockJob *job)
 219 {
 220     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 221     if (s->sync_bitmap) {
 222         backup_cleanup_sync_bitmap(s, 0);
 223     }
 224 }
 225
 226 static void backup_abort(BlockJob *job)
 227 {
 228     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 229     if (s->sync_bitmap) {
 230         backup_cleanup_sync_bitmap(s, -1);
 231     }
 232 }
 233
 234 static void backup_clean(BlockJob *job)
 235 {
 236     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 237     assert(s->target);
 238     blk_unref(s->target);
 239     s->target = NULL;
 240 }
 241
 242 static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
 243 {
 244     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 245
 246     blk_set_aio_context(s->target, aio_context);
 247 }
 248
 249 void backup_do_checkpoint(BlockJob *job, Error **errp)
 250 {
 251     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 252     int64_t len;
 253
 254     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);
 255
 256     if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
 257         error_setg(errp, "The backup job only supports block checkpoint in"
 258                    " sync=none mode");
 259         return;
 260     }
 261
 262     len = DIV_ROUND_UP(backup_job->common.len, backup_job->cluster_size);
 263     bitmap_zero(backup_job->done_bitmap, len);
 264 }
 265
 266 void backup_wait_for_overlapping_requests(BlockJob *job, int64_t offset,
 267                                           uint64_t bytes)
 268 {
 269     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 270     int64_t start, end;
 271
 272     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);
 273
 274     start = QEMU_ALIGN_DOWN(offset, backup_job->cluster_size);
 275     end = QEMU_ALIGN_UP(offset + bytes, backup_job->cluster_size);
 276     wait_for_overlapping_requests(backup_job, start, end);
 277 }
 278
 279 void backup_cow_request_begin(CowRequest *req, BlockJob *job,
 280                               int64_t offset, uint64_t bytes)
 281 {
 282     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 283     int64_t start, end;
 284
 285     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);
 286
 287     start = QEMU_ALIGN_DOWN(offset, backup_job->cluster_size);
 288     end = QEMU_ALIGN_UP(offset + bytes, backup_job->cluster_size);
 289     cow_request_begin(req, backup_job, start, end);
 290 }
 291
 292 void backup_cow_request_end(CowRequest *req)
 293 {
 294     cow_request_end(req);
 295 }
 296
 297 static void backup_drain(BlockJob *job)
 298 {
 299     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 300
 301     /* Need to keep a reference in case blk_drain triggers execution
 302      * of backup_complete...
 303      */
 304     if (s->target) {
 305         BlockBackend *target = s->target;
 306         blk_ref(target);
 307         blk_drain(target);
 308         blk_unref(target);
 309     }
 310 }
 311
 312 static BlockErrorAction backup_error_action(BackupBlockJob *job,
 313                                             bool read, int error)
 314 {
 315     if (read) {
 316         return block_job_error_action(&job->common, job->on_source_error,
 317                                       true, error);
 318     } else {
 319         return block_job_error_action(&job->common, job->on_target_error,
 320                                       false, error);
 321     }
 322 }
 323
 324 typedef struct {
 325     int ret;
 326 } BackupCompleteData;
 327
 328 static void backup_complete(BlockJob *job, void *opaque)
 329 {
 330     BackupCompleteData *data = opaque;
 331
 332     block_job_completed(job, data->ret);
 333     g_free(data);
 334 }
 335
 336 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 337 {
 338     if (block_job_is_cancelled(&job->common)) {
 339         return true;
 340     }
 341
 342     /* we need to yield so that bdrv_drain_all() returns.
 343      * (without, VM does not reboot)
 344      */
 345     if (job->common.speed) {
 346         uint64_t delay_ns = ratelimit_calculate_delay(&job->limit,
 347                                                       job->bytes_read);
 348         job->bytes_read = 0;
 349         block_job_sleep_ns(&job->common, QEMU_CLOCK_REALTIME, delay_ns);
 350     } else {
 351         block_job_sleep_ns(&job->common, QEMU_CLOCK_REALTIME, 0);
 352     }
 353
 354     if (block_job_is_cancelled(&job->common)) {
 355         return true;
 356     }
 357
 358     return false;
 359 }
 360
 361 static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
 362 {
 363     bool error_is_read;
 364     int ret = 0;
 365     int clusters_per_iter;
 366     uint32_t granularity;
 367     int64_t offset;
 368     int64_t cluster;
 369     int64_t end;
 370     int64_t last_cluster = -1;
 371     BdrvDirtyBitmapIter *dbi;
 372
 373     granularity = bdrv_dirty_bitmap_granularity(job->sync_bitmap);
 374     clusters_per_iter = MAX((granularity / job->cluster_size), 1);
 375     dbi = bdrv_dirty_iter_new(job->sync_bitmap, 0);
 376
 377     /* Find the next dirty sector(s) */
 378     while ((offset = bdrv_dirty_iter_next(dbi) * BDRV_SECTOR_SIZE) >= 0) {
 379         cluster = offset / job->cluster_size;
 380
 381         /* Fake progress updates for any clusters we skipped */
 382         if (cluster != last_cluster + 1) {
 383             job->common.offset += ((cluster - last_cluster - 1) *
 384                                    job->cluster_size);
 385         }
 386
 387         for (end = cluster + clusters_per_iter; cluster < end; cluster++) {
 388             do {
 389                 if (yield_and_check(job)) {
 390                     goto out;
 391                 }
 392                 ret = backup_do_cow(job, cluster * job->cluster_size,
 393                                     job->cluster_size, &error_is_read,
 394                                     false);
 395                 if ((ret < 0) &&
 396                     backup_error_action(job, error_is_read, -ret) ==
 397                     BLOCK_ERROR_ACTION_REPORT) {
 398                     goto out;
 399                 }
 400             } while (ret < 0);
 401         }
 402
 403         /* If the bitmap granularity is smaller than the backup granularity,
 404          * we need to advance the iterator pointer to the next cluster. */
 405         if (granularity < job->cluster_size) {
 406             bdrv_set_dirty_iter(dbi,
 407                                 cluster * job->cluster_size / BDRV_SECTOR_SIZE);
 408         }
 409
 410         last_cluster = cluster - 1;
 411     }
 412
 413     /* Play some final catchup with the progress meter */
 414     end = DIV_ROUND_UP(job->common.len, job->cluster_size);
 415     if (last_cluster + 1 < end) {
 416         job->common.offset += ((end - last_cluster - 1) * job->cluster_size);
 417     }
 418
 419 out:
 420     bdrv_dirty_iter_free(dbi);
 421     return ret;
 422 }
 423
 424 static void coroutine_fn backup_run(void *opaque)
 425 {
 426     BackupBlockJob *job = opaque;
 427     BackupCompleteData *data;
 428     BlockDriverState *bs = blk_bs(job->common.blk);
 429     int64_t offset;
 430     int ret = 0;
 431
 432     QLIST_INIT(&job->inflight_reqs);
 433     qemu_co_rwlock_init(&job->flush_rwlock);
 434
 435     job->done_bitmap = bitmap_new(DIV_ROUND_UP(job->common.len,
 436                                                job->cluster_size));
 437
 438     job->before_write.notify = backup_before_write_notify;
 439     bdrv_add_before_write_notifier(bs, &job->before_write);
 440
 441     if (job->sync_mode == MIRROR_SYNC_MODE_NONE) {
 442         while (!block_job_is_cancelled(&job->common)) {
 443             /* Yield until the job is cancelled.  We just let our before_write
 444              * notify callback service CoW requests. */
 445             block_job_yield(&job->common);
 446         }
 447     } else if (job->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 448         ret = backup_run_incremental(job);
 449     } else {
 450         /* Both FULL and TOP SYNC_MODE's require copying.. */
 451         for (offset = 0; offset < job->common.len;
 452              offset += job->cluster_size) {
 453             bool error_is_read;
 454             int alloced = 0;
 455
 456             if (yield_and_check(job)) {
 457                 break;
 458             }
 459
 460             if (job->sync_mode == MIRROR_SYNC_MODE_TOP) {
 461                 int i;
 462                 int64_t n;
 463
 464                 /* Check to see if these blocks are already in the
 465                  * backing file. */
 466
 467                 for (i = 0; i < job->cluster_size;) {
 468                     /* bdrv_is_allocated() only returns true/false based
 469                      * on the first set of sectors it comes across that
 470                      * are are all in the same state.
 471                      * For that reason we must verify each sector in the
 472                      * backup cluster length.  We end up copying more than
 473                      * needed but at some point that is always the case. */
 474                     alloced =
 475                         bdrv_is_allocated(bs, offset + i,
 476                                           job->cluster_size - i, &n);
 477                     i += n;
 478
 479                     if (alloced || n == 0) {
 480                         break;
 481                     }
 482                 }
 483
 484                 /* If the above loop never found any sectors that are in
 485                  * the topmost image, skip this backup. */
 486                 if (alloced == 0) {
 487                     continue;
 488                 }
 489             }
 490             /* FULL sync mode we copy the whole drive. */
 491             if (alloced < 0) {
 492                 ret = alloced;
 493             } else {
 494                 ret = backup_do_cow(job, offset, job->cluster_size,
 495                                     &error_is_read, false);
 496             }
 497             if (ret < 0) {
 498                 /* Depending on error action, fail now or retry cluster */
 499                 BlockErrorAction action =
 500                     backup_error_action(job, error_is_read, -ret);
 501                 if (action == BLOCK_ERROR_ACTION_REPORT) {
 502                     break;
 503                 } else {
 504                     offset -= job->cluster_size;
 505                     continue;
 506                 }
 507             }
 508         }
 509     }
 510
 511     notifier_with_return_remove(&job->before_write);
 512
 513     /* wait until pending backup_do_cow() calls have completed */
 514     qemu_co_rwlock_wrlock(&job->flush_rwlock);
 515     qemu_co_rwlock_unlock(&job->flush_rwlock);
 516     g_free(job->done_bitmap);
 517
 518     data = g_malloc(sizeof(*data));
 519     data->ret = ret;
 520     block_job_defer_to_main_loop(&job->common, backup_complete, data);
 521 }
 522
 523 static const BlockJobDriver backup_job_driver = {
 524     .instance_size          = sizeof(BackupBlockJob),
 525     .job_type               = BLOCK_JOB_TYPE_BACKUP,
 526     .start                  = backup_run,
 527     .set_speed              = backup_set_speed,
 528     .commit                 = backup_commit,
 529     .abort                  = backup_abort,
 530     .clean                  = backup_clean,
 531     .attached_aio_context   = backup_attached_aio_context,
 532     .drain                  = backup_drain,
 533 };
 534
 535 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
 536                   BlockDriverState *target, int64_t speed,
 537                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
 538                   bool compress,
 539                   BlockdevOnError on_source_error,
 540                   BlockdevOnError on_target_error,
 541                   int creation_flags,
 542                   BlockCompletionFunc *cb, void *opaque,
 543                   BlockJobTxn *txn, Error **errp)
 544 {
 545     int64_t len;
 546     BlockDriverInfo bdi;
 547     BackupBlockJob *job = NULL;
 548     int ret;
 549
 550     assert(bs);
 551     assert(target);
 552
 553     if (bs == target) {
 554         error_setg(errp, "Source and target cannot be the same");
 555         return NULL;
 556     }
 557
 558     if (!bdrv_is_inserted(bs)) {
 559         error_setg(errp, "Device is not inserted: %s",
 560                    bdrv_get_device_name(bs));
 561         return NULL;
 562     }
 563
 564     if (!bdrv_is_inserted(target)) {
 565         error_setg(errp, "Device is not inserted: %s",
 566                    bdrv_get_device_name(target));
 567         return NULL;
 568     }
 569
 570     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
 571         error_setg(errp, "Compression is not supported for this drive %s",
 572                    bdrv_get_device_name(target));
 573         return NULL;
 574     }
 575
 576     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
 577         return NULL;
 578     }
 579
 580     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
 581         return NULL;
 582     }
 583
 584     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 585         if (!sync_bitmap) {
 586             error_setg(errp, "must provide a valid bitmap name for "
 587                              "\"incremental\" sync mode");
 588             return NULL;
 589         }
 590
 591         /* Create a new bitmap, and freeze/disable this one. */
 592         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
 593             return NULL;
 594         }
 595     } else if (sync_bitmap) {
 596         error_setg(errp,
 597                    "a sync_bitmap was provided to backup_run, "
 598                    "but received an incompatible sync_mode (%s)",
 599                    MirrorSyncMode_lookup[sync_mode]);
 600         return NULL;
 601     }
 602
 603     len = bdrv_getlength(bs);
 604     if (len < 0) {
 605         error_setg_errno(errp, -len, "unable to get length for '%s'",
 606                          bdrv_get_device_name(bs));
 607         goto error;
 608     }
 609
 610     /* job->common.len is fixed, so we can't allow resize */
 611     job = block_job_create(job_id, &backup_job_driver, bs,
 612                            BLK_PERM_CONSISTENT_READ,
 613                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 614                            BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD,
 615                            speed, creation_flags, cb, opaque, errp);
 616     if (!job) {
 617         goto error;
 618     }
 619
 620     /* The target must match the source in size, so no resize here either */
 621     job->target = blk_new(BLK_PERM_WRITE,
 622                           BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 623                           BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD);
 624     ret = blk_insert_bs(job->target, target, errp);
 625     if (ret < 0) {
 626         goto error;
 627     }
 628
 629     job->on_source_error = on_source_error;
 630     job->on_target_error = on_target_error;
 631     job->sync_mode = sync_mode;
 632     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
 633                        sync_bitmap : NULL;
 634     job->compress = compress;
 635
 636     /* If there is no backing file on the target, we cannot rely on COW if our
 637      * backup cluster size is smaller than the target cluster size. Even for
 638      * targets with a backing file, try to avoid COW if possible. */
 639     ret = bdrv_get_info(target, &bdi);
 640     if (ret == -ENOTSUP && !target->backing) {
 641         /* Cluster size is not defined */
 642         warn_report("The target block device doesn't provide "
 643                     "information about the block size and it doesn't have a "
 644                     "backing file. The default block size of %u bytes is "
 645                     "used. If the actual block size of the target exceeds "
 646                     "this default, the backup may be unusable",
 647                     BACKUP_CLUSTER_SIZE_DEFAULT);
 648         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 649     } else if (ret < 0 && !target->backing) {
 650         error_setg_errno(errp, -ret,
 651             "Couldn't determine the cluster size of the target image, "
 652             "which has no backing file");
 653         error_append_hint(errp,
 654             "Aborting, since this may create an unusable destination image\n");
 655         goto error;
 656     } else if (ret < 0 && target->backing) {
 657         /* Not fatal; just trudge on ahead. */
 658         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 659     } else {
 660         job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
 661     }
 662
 663     /* Required permissions are already taken with target's blk_new() */
 664     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
 665                        &error_abort);
 666     job->common.len = len;
 667     block_job_txn_add_job(txn, &job->common);
 668
 669     return &job->common;
 670
 671  error:
 672     if (sync_bitmap) {
 673         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
 674     }
 675     if (job) {
 676         backup_clean(&job->common);
 677         block_job_early_fail(&job->common);
 678     }
 679
 680     return NULL;
 681 }