block/backup.c

   1 /*
   2  * QEMU backup
   3  *
   4  * Copyright (C) 2013 Proxmox Server Solutions
   5  *
   6  * Authors:
   7  *  Dietmar Maurer ([email protected])
   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
  31 typedef struct CowRequest {
  32     int64_t start_byte;
  33     int64_t end_byte;
  34     QLIST_ENTRY(CowRequest) list;
  35     CoQueue wait_queue; /* coroutines blocked on this request */
  36 } CowRequest;
  37
  38 typedef struct BackupBlockJob {
  39     BlockJob common;
  40     BlockBackend *target;
  41     /* bitmap for sync=incremental */
  42     BdrvDirtyBitmap *sync_bitmap;
  43     MirrorSyncMode sync_mode;
  44     BlockdevOnError on_source_error;
  45     BlockdevOnError on_target_error;
  46     CoRwlock flush_rwlock;
  47     uint64_t len;
  48     uint64_t bytes_read;
  49     int64_t cluster_size;
  50     bool compress;
  51     NotifierWithReturn before_write;
  52     QLIST_HEAD(, CowRequest) inflight_reqs;
  53
  54     HBitmap *copy_bitmap;
  55     bool use_copy_range;
  56     int64_t copy_range_size;
  57
  58     bool serialize_target_writes;
  59 } BackupBlockJob;
  60
  61 static const BlockJobDriver backup_job_driver;
  62
  63 /* See if in-flight requests overlap and wait for them to complete */
  64 static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
  65                                                        int64_t start,
  66                                                        int64_t end)
  67 {
  68     CowRequest *req;
  69     bool retry;
  70
  71     do {
  72         retry = false;
  73         QLIST_FOREACH(req, &job->inflight_reqs, list) {
  74             if (end > req->start_byte && start < req->end_byte) {
  75                 qemu_co_queue_wait(&req->wait_queue, NULL);
  76                 retry = true;
  77                 break;
  78             }
  79         }
  80     } while (retry);
  81 }
  82
  83 /* Keep track of an in-flight request */
  84 static void cow_request_begin(CowRequest *req, BackupBlockJob *job,
  85                               int64_t start, int64_t end)
  86 {
  87     req->start_byte = start;
  88     req->end_byte = end;
  89     qemu_co_queue_init(&req->wait_queue);
  90     QLIST_INSERT_HEAD(&job->inflight_reqs, req, list);
  91 }
  92
  93 /* Forget about a completed request */
  94 static void cow_request_end(CowRequest *req)
  95 {
  96     QLIST_REMOVE(req, list);
  97     qemu_co_queue_restart_all(&req->wait_queue);
  98 }
  99
 100 /* Copy range to target with a bounce buffer and return the bytes copied. If
 101  * error occurred, return a negative error number */
 102 static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
 103                                                       int64_t start,
 104                                                       int64_t end,
 105                                                       bool is_write_notifier,
 106                                                       bool *error_is_read,
 107                                                       void **bounce_buffer)
 108 {
 109     int ret;
 110     BlockBackend *blk = job->common.blk;
 111     int nbytes;
 112     int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
 113     int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
 114
 115     hbitmap_reset(job->copy_bitmap, start / job->cluster_size, 1);
 116     nbytes = MIN(job->cluster_size, job->len - start);
 117     if (!*bounce_buffer) {
 118         *bounce_buffer = blk_blockalign(blk, job->cluster_size);
 119     }
 120
 121     ret = blk_co_pread(blk, start, nbytes, *bounce_buffer, read_flags);
 122     if (ret < 0) {
 123         trace_backup_do_cow_read_fail(job, start, ret);
 124         if (error_is_read) {
 125             *error_is_read = true;
 126         }
 127         goto fail;
 128     }
 129
 130     if (buffer_is_zero(*bounce_buffer, nbytes)) {
 131         ret = blk_co_pwrite_zeroes(job->target, start,
 132                                    nbytes, write_flags | BDRV_REQ_MAY_UNMAP);
 133     } else {
 134         ret = blk_co_pwrite(job->target, start,
 135                             nbytes, *bounce_buffer, write_flags |
 136                             (job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0));
 137     }
 138     if (ret < 0) {
 139         trace_backup_do_cow_write_fail(job, start, ret);
 140         if (error_is_read) {
 141             *error_is_read = false;
 142         }
 143         goto fail;
 144     }
 145
 146     return nbytes;
 147 fail:
 148     hbitmap_set(job->copy_bitmap, start / job->cluster_size, 1);
 149     return ret;
 150
 151 }
 152
 153 /* Copy range to target and return the bytes copied. If error occurred, return a
 154  * negative error number. */
 155 static int coroutine_fn backup_cow_with_offload(BackupBlockJob *job,
 156                                                 int64_t start,
 157                                                 int64_t end,
 158                                                 bool is_write_notifier)
 159 {
 160     int ret;
 161     int nr_clusters;
 162     BlockBackend *blk = job->common.blk;
 163     int nbytes;
 164     int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
 165     int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
 166
 167     assert(QEMU_IS_ALIGNED(job->copy_range_size, job->cluster_size));
 168     nbytes = MIN(job->copy_range_size, end - start);
 169     nr_clusters = DIV_ROUND_UP(nbytes, job->cluster_size);
 170     hbitmap_reset(job->copy_bitmap, start / job->cluster_size,
 171                   nr_clusters);
 172     ret = blk_co_copy_range(blk, start, job->target, start, nbytes,
 173                             read_flags, write_flags);
 174     if (ret < 0) {
 175         trace_backup_do_cow_copy_range_fail(job, start, ret);
 176         hbitmap_set(job->copy_bitmap, start / job->cluster_size,
 177                     nr_clusters);
 178         return ret;
 179     }
 180
 181     return nbytes;
 182 }
 183
 184 static int coroutine_fn backup_do_cow(BackupBlockJob *job,
 185                                       int64_t offset, uint64_t bytes,
 186                                       bool *error_is_read,
 187                                       bool is_write_notifier)
 188 {
 189     CowRequest cow_request;
 190     int ret = 0;
 191     int64_t start, end; /* bytes */
 192     void *bounce_buffer = NULL;
 193
 194     qemu_co_rwlock_rdlock(&job->flush_rwlock);
 195
 196     start = QEMU_ALIGN_DOWN(offset, job->cluster_size);
 197     end = QEMU_ALIGN_UP(bytes + offset, job->cluster_size);
 198
 199     trace_backup_do_cow_enter(job, start, offset, bytes);
 200
 201     wait_for_overlapping_requests(job, start, end);
 202     cow_request_begin(&cow_request, job, start, end);
 203
 204     while (start < end) {
 205         if (!hbitmap_get(job->copy_bitmap, start / job->cluster_size)) {
 206             trace_backup_do_cow_skip(job, start);
 207             start += job->cluster_size;
 208             continue; /* already copied */
 209         }
 210
 211         trace_backup_do_cow_process(job, start);
 212
 213         if (job->use_copy_range) {
 214             ret = backup_cow_with_offload(job, start, end, is_write_notifier);
 215             if (ret < 0) {
 216                 job->use_copy_range = false;
 217             }
 218         }
 219         if (!job->use_copy_range) {
 220             ret = backup_cow_with_bounce_buffer(job, start, end, is_write_notifier,
 221                                                 error_is_read, &bounce_buffer);
 222         }
 223         if (ret < 0) {
 224             break;
 225         }
 226
 227         /* Publish progress, guest I/O counts as progress too.  Note that the
 228          * offset field is an opaque progress value, it is not a disk offset.
 229          */
 230         start += ret;
 231         job->bytes_read += ret;
 232         job_progress_update(&job->common.job, ret);
 233         ret = 0;
 234     }
 235
 236     if (bounce_buffer) {
 237         qemu_vfree(bounce_buffer);
 238     }
 239
 240     cow_request_end(&cow_request);
 241
 242     trace_backup_do_cow_return(job, offset, bytes, ret);
 243
 244     qemu_co_rwlock_unlock(&job->flush_rwlock);
 245
 246     return ret;
 247 }
 248
 249 static int coroutine_fn backup_before_write_notify(
 250         NotifierWithReturn *notifier,
 251         void *opaque)
 252 {
 253     BackupBlockJob *job = container_of(notifier, BackupBlockJob, before_write);
 254     BdrvTrackedRequest *req = opaque;
 255
 256     assert(req->bs == blk_bs(job->common.blk));
 257     assert(QEMU_IS_ALIGNED(req->offset, BDRV_SECTOR_SIZE));
 258     assert(QEMU_IS_ALIGNED(req->bytes, BDRV_SECTOR_SIZE));
 259
 260     return backup_do_cow(job, req->offset, req->bytes, NULL, true);
 261 }
 262
 263 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 264 {
 265     BdrvDirtyBitmap *bm;
 266     BlockDriverState *bs = blk_bs(job->common.blk);
 267
 268     if (ret < 0) {
 269         /* Merge the successor back into the parent, delete nothing. */
 270         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
 271         assert(bm);
 272     } else {
 273         /* Everything is fine, delete this bitmap and install the backup. */
 274         bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
 275         assert(bm);
 276     }
 277 }
 278
 279 static void backup_commit(Job *job)
 280 {
 281     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 282     if (s->sync_bitmap) {
 283         backup_cleanup_sync_bitmap(s, 0);
 284     }
 285 }
 286
 287 static void backup_abort(Job *job)
 288 {
 289     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 290     if (s->sync_bitmap) {
 291         backup_cleanup_sync_bitmap(s, -1);
 292     }
 293 }
 294
 295 static void backup_clean(Job *job)
 296 {
 297     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 298     assert(s->target);
 299     blk_unref(s->target);
 300     s->target = NULL;
 301 }
 302
 303 static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
 304 {
 305     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 306
 307     blk_set_aio_context(s->target, aio_context);
 308 }
 309
 310 void backup_do_checkpoint(BlockJob *job, Error **errp)
 311 {
 312     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 313     int64_t len;
 314
 315     assert(block_job_driver(job) == &backup_job_driver);
 316
 317     if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
 318         error_setg(errp, "The backup job only supports block checkpoint in"
 319                    " sync=none mode");
 320         return;
 321     }
 322
 323     len = DIV_ROUND_UP(backup_job->len, backup_job->cluster_size);
 324     hbitmap_set(backup_job->copy_bitmap, 0, len);
 325 }
 326
 327 static void backup_drain(BlockJob *job)
 328 {
 329     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 330
 331     /* Need to keep a reference in case blk_drain triggers execution
 332      * of backup_complete...
 333      */
 334     if (s->target) {
 335         BlockBackend *target = s->target;
 336         blk_ref(target);
 337         blk_drain(target);
 338         blk_unref(target);
 339     }
 340 }
 341
 342 static BlockErrorAction backup_error_action(BackupBlockJob *job,
 343                                             bool read, int error)
 344 {
 345     if (read) {
 346         return block_job_error_action(&job->common, job->on_source_error,
 347                                       true, error);
 348     } else {
 349         return block_job_error_action(&job->common, job->on_target_error,
 350                                       false, error);
 351     }
 352 }
 353
 354 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 355 {
 356     uint64_t delay_ns;
 357
 358     if (job_is_cancelled(&job->common.job)) {
 359         return true;
 360     }
 361
 362     /* We need to yield even for delay_ns = 0 so that bdrv_drain_all() can
 363      * return. Without a yield, the VM would not reboot. */
 364     delay_ns = block_job_ratelimit_get_delay(&job->common, job->bytes_read);
 365     job->bytes_read = 0;
 366     job_sleep_ns(&job->common.job, delay_ns);
 367
 368     if (job_is_cancelled(&job->common.job)) {
 369         return true;
 370     }
 371
 372     return false;
 373 }
 374
 375 static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
 376 {
 377     int ret;
 378     bool error_is_read;
 379     int64_t cluster;
 380     HBitmapIter hbi;
 381
 382     hbitmap_iter_init(&hbi, job->copy_bitmap, 0);
 383     while ((cluster = hbitmap_iter_next(&hbi)) != -1) {
 384         do {
 385             if (yield_and_check(job)) {
 386                 return 0;
 387             }
 388             ret = backup_do_cow(job, cluster * job->cluster_size,
 389                                 job->cluster_size, &error_is_read, false);
 390             if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
 391                            BLOCK_ERROR_ACTION_REPORT)
 392             {
 393                 return ret;
 394             }
 395         } while (ret < 0);
 396     }
 397
 398     return 0;
 399 }
 400
 401 /* init copy_bitmap from sync_bitmap */
 402 static void backup_incremental_init_copy_bitmap(BackupBlockJob *job)
 403 {
 404     BdrvDirtyBitmapIter *dbi;
 405     int64_t offset;
 406     int64_t end = DIV_ROUND_UP(bdrv_dirty_bitmap_size(job->sync_bitmap),
 407                                job->cluster_size);
 408
 409     dbi = bdrv_dirty_iter_new(job->sync_bitmap);
 410     while ((offset = bdrv_dirty_iter_next(dbi)) != -1) {
 411         int64_t cluster = offset / job->cluster_size;
 412         int64_t next_cluster;
 413
 414         offset += bdrv_dirty_bitmap_granularity(job->sync_bitmap);
 415         if (offset >= bdrv_dirty_bitmap_size(job->sync_bitmap)) {
 416             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 417             break;
 418         }
 419
 420         offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset,
 421                                              UINT64_MAX);
 422         if (offset == -1) {
 423             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 424             break;
 425         }
 426
 427         next_cluster = DIV_ROUND_UP(offset, job->cluster_size);
 428         hbitmap_set(job->copy_bitmap, cluster, next_cluster - cluster);
 429         if (next_cluster >= end) {
 430             break;
 431         }
 432
 433         bdrv_set_dirty_iter(dbi, next_cluster * job->cluster_size);
 434     }
 435
 436     /* TODO job_progress_set_remaining() would make more sense */
 437     job_progress_update(&job->common.job,
 438         job->len - hbitmap_count(job->copy_bitmap) * job->cluster_size);
 439
 440     bdrv_dirty_iter_free(dbi);
 441 }
 442
 443 static int coroutine_fn backup_run(Job *job, Error **errp)
 444 {
 445     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 446     BlockDriverState *bs = blk_bs(s->common.blk);
 447     int64_t offset, nb_clusters;
 448     int ret = 0;
 449
 450     QLIST_INIT(&s->inflight_reqs);
 451     qemu_co_rwlock_init(&s->flush_rwlock);
 452
 453     nb_clusters = DIV_ROUND_UP(s->len, s->cluster_size);
 454     job_progress_set_remaining(job, s->len);
 455
 456     s->copy_bitmap = hbitmap_alloc(nb_clusters, 0);
 457     if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 458         backup_incremental_init_copy_bitmap(s);
 459     } else {
 460         hbitmap_set(s->copy_bitmap, 0, nb_clusters);
 461     }
 462
 463
 464     s->before_write.notify = backup_before_write_notify;
 465     bdrv_add_before_write_notifier(bs, &s->before_write);
 466
 467     if (s->sync_mode == MIRROR_SYNC_MODE_NONE) {
 468         /* All bits are set in copy_bitmap to allow any cluster to be copied.
 469          * This does not actually require them to be copied. */
 470         while (!job_is_cancelled(job)) {
 471             /* Yield until the job is cancelled.  We just let our before_write
 472              * notify callback service CoW requests. */
 473             job_yield(job);
 474         }
 475     } else if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 476         ret = backup_run_incremental(s);
 477     } else {
 478         /* Both FULL and TOP SYNC_MODE's require copying.. */
 479         for (offset = 0; offset < s->len;
 480              offset += s->cluster_size) {
 481             bool error_is_read;
 482             int alloced = 0;
 483
 484             if (yield_and_check(s)) {
 485                 break;
 486             }
 487
 488             if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
 489                 int i;
 490                 int64_t n;
 491
 492                 /* Check to see if these blocks are already in the
 493                  * backing file. */
 494
 495                 for (i = 0; i < s->cluster_size;) {
 496                     /* bdrv_is_allocated() only returns true/false based
 497                      * on the first set of sectors it comes across that
 498                      * are are all in the same state.
 499                      * For that reason we must verify each sector in the
 500                      * backup cluster length.  We end up copying more than
 501                      * needed but at some point that is always the case. */
 502                     alloced =
 503                         bdrv_is_allocated(bs, offset + i,
 504                                           s->cluster_size - i, &n);
 505                     i += n;
 506
 507                     if (alloced || n == 0) {
 508                         break;
 509                     }
 510                 }
 511
 512                 /* If the above loop never found any sectors that are in
 513                  * the topmost image, skip this backup. */
 514                 if (alloced == 0) {
 515                     continue;
 516                 }
 517             }
 518             /* FULL sync mode we copy the whole drive. */
 519             if (alloced < 0) {
 520                 ret = alloced;
 521             } else {
 522                 ret = backup_do_cow(s, offset, s->cluster_size,
 523                                     &error_is_read, false);
 524             }
 525             if (ret < 0) {
 526                 /* Depending on error action, fail now or retry cluster */
 527                 BlockErrorAction action =
 528                     backup_error_action(s, error_is_read, -ret);
 529                 if (action == BLOCK_ERROR_ACTION_REPORT) {
 530                     break;
 531                 } else {
 532                     offset -= s->cluster_size;
 533                     continue;
 534                 }
 535             }
 536         }
 537     }
 538
 539     notifier_with_return_remove(&s->before_write);
 540
 541     /* wait until pending backup_do_cow() calls have completed */
 542     qemu_co_rwlock_wrlock(&s->flush_rwlock);
 543     qemu_co_rwlock_unlock(&s->flush_rwlock);
 544     hbitmap_free(s->copy_bitmap);
 545
 546     return ret;
 547 }
 548
 549 static const BlockJobDriver backup_job_driver = {
 550     .job_driver = {
 551         .instance_size          = sizeof(BackupBlockJob),
 552         .job_type               = JOB_TYPE_BACKUP,
 553         .free                   = block_job_free,
 554         .user_resume            = block_job_user_resume,
 555         .drain                  = block_job_drain,
 556         .run                    = backup_run,
 557         .commit                 = backup_commit,
 558         .abort                  = backup_abort,
 559         .clean                  = backup_clean,
 560     },
 561     .attached_aio_context   = backup_attached_aio_context,
 562     .drain                  = backup_drain,
 563 };
 564
 565 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
 566                   BlockDriverState *target, int64_t speed,
 567                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
 568                   bool compress,
 569                   BlockdevOnError on_source_error,
 570                   BlockdevOnError on_target_error,
 571                   int creation_flags,
 572                   BlockCompletionFunc *cb, void *opaque,
 573                   JobTxn *txn, Error **errp)
 574 {
 575     int64_t len;
 576     BlockDriverInfo bdi;
 577     BackupBlockJob *job = NULL;
 578     int ret;
 579
 580     assert(bs);
 581     assert(target);
 582
 583     if (bs == target) {
 584         error_setg(errp, "Source and target cannot be the same");
 585         return NULL;
 586     }
 587
 588     if (!bdrv_is_inserted(bs)) {
 589         error_setg(errp, "Device is not inserted: %s",
 590                    bdrv_get_device_name(bs));
 591         return NULL;
 592     }
 593
 594     if (!bdrv_is_inserted(target)) {
 595         error_setg(errp, "Device is not inserted: %s",
 596                    bdrv_get_device_name(target));
 597         return NULL;
 598     }
 599
 600     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
 601         error_setg(errp, "Compression is not supported for this drive %s",
 602                    bdrv_get_device_name(target));
 603         return NULL;
 604     }
 605
 606     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
 607         return NULL;
 608     }
 609
 610     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
 611         return NULL;
 612     }
 613
 614     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 615         if (!sync_bitmap) {
 616             error_setg(errp, "must provide a valid bitmap name for "
 617                              "\"incremental\" sync mode");
 618             return NULL;
 619         }
 620
 621         /* Create a new bitmap, and freeze/disable this one. */
 622         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
 623             return NULL;
 624         }
 625     } else if (sync_bitmap) {
 626         error_setg(errp,
 627                    "a sync_bitmap was provided to backup_run, "
 628                    "but received an incompatible sync_mode (%s)",
 629                    MirrorSyncMode_str(sync_mode));
 630         return NULL;
 631     }
 632
 633     len = bdrv_getlength(bs);
 634     if (len < 0) {
 635         error_setg_errno(errp, -len, "unable to get length for '%s'",
 636                          bdrv_get_device_name(bs));
 637         goto error;
 638     }
 639
 640     /* job->len is fixed, so we can't allow resize */
 641     job = block_job_create(job_id, &backup_job_driver, txn, bs,
 642                            BLK_PERM_CONSISTENT_READ,
 643                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 644                            BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD,
 645                            speed, creation_flags, cb, opaque, errp);
 646     if (!job) {
 647         goto error;
 648     }
 649
 650     /* The target must match the source in size, so no resize here either */
 651     job->target = blk_new(BLK_PERM_WRITE,
 652                           BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 653                           BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD);
 654     ret = blk_insert_bs(job->target, target, errp);
 655     if (ret < 0) {
 656         goto error;
 657     }
 658
 659     job->on_source_error = on_source_error;
 660     job->on_target_error = on_target_error;
 661     job->sync_mode = sync_mode;
 662     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
 663                        sync_bitmap : NULL;
 664     job->compress = compress;
 665
 666     /* Detect image-fleecing (and similar) schemes */
 667     job->serialize_target_writes = bdrv_chain_contains(target, bs);
 668
 669     /* If there is no backing file on the target, we cannot rely on COW if our
 670      * backup cluster size is smaller than the target cluster size. Even for
 671      * targets with a backing file, try to avoid COW if possible. */
 672     ret = bdrv_get_info(target, &bdi);
 673     if (ret == -ENOTSUP && !target->backing) {
 674         /* Cluster size is not defined */
 675         warn_report("The target block device doesn't provide "
 676                     "information about the block size and it doesn't have a "
 677                     "backing file. The default block size of %u bytes is "
 678                     "used. If the actual block size of the target exceeds "
 679                     "this default, the backup may be unusable",
 680                     BACKUP_CLUSTER_SIZE_DEFAULT);
 681         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 682     } else if (ret < 0 && !target->backing) {
 683         error_setg_errno(errp, -ret,
 684             "Couldn't determine the cluster size of the target image, "
 685             "which has no backing file");
 686         error_append_hint(errp,
 687             "Aborting, since this may create an unusable destination image\n");
 688         goto error;
 689     } else if (ret < 0 && target->backing) {
 690         /* Not fatal; just trudge on ahead. */
 691         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 692     } else {
 693         job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
 694     }
 695     job->use_copy_range = true;
 696     job->copy_range_size = MIN_NON_ZERO(blk_get_max_transfer(job->common.blk),
 697                                         blk_get_max_transfer(job->target));
 698     job->copy_range_size = MAX(job->cluster_size,
 699                                QEMU_ALIGN_UP(job->copy_range_size,
 700                                              job->cluster_size));
 701
 702     /* Required permissions are already taken with target's blk_new() */
 703     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
 704                        &error_abort);
 705     job->len = len;
 706
 707     return &job->common;
 708
 709  error:
 710     if (sync_bitmap) {
 711         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
 712     }
 713     if (job) {
 714         backup_clean(&job->common.job);
 715         job_early_fail(&job->common.job);
 716     }
 717
 718     return NULL;
 719 }