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
  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     QEMUIOVector qiov;
 111     BlockBackend *blk = job->common.blk;
 112     int nbytes;
 113     int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
 114     int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
 115
 116     hbitmap_reset(job->copy_bitmap, start / job->cluster_size, 1);
 117     nbytes = MIN(job->cluster_size, job->len - start);
 118     if (!*bounce_buffer) {
 119         *bounce_buffer = blk_blockalign(blk, job->cluster_size);
 120     }
 121     qemu_iovec_init_buf(&qiov, *bounce_buffer, nbytes);
 122
 123     ret = blk_co_preadv(blk, start, qiov.size, &qiov, read_flags);
 124     if (ret < 0) {
 125         trace_backup_do_cow_read_fail(job, start, ret);
 126         if (error_is_read) {
 127             *error_is_read = true;
 128         }
 129         goto fail;
 130     }
 131
 132     if (qemu_iovec_is_zero(&qiov)) {
 133         ret = blk_co_pwrite_zeroes(job->target, start,
 134                                    qiov.size, write_flags | BDRV_REQ_MAY_UNMAP);
 135     } else {
 136         ret = blk_co_pwritev(job->target, start,
 137                              qiov.size, &qiov, write_flags |
 138                              (job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0));
 139     }
 140     if (ret < 0) {
 141         trace_backup_do_cow_write_fail(job, start, ret);
 142         if (error_is_read) {
 143             *error_is_read = false;
 144         }
 145         goto fail;
 146     }
 147
 148     return nbytes;
 149 fail:
 150     hbitmap_set(job->copy_bitmap, start / job->cluster_size, 1);
 151     return ret;
 152
 153 }
 154
 155 /* Copy range to target and return the bytes copied. If error occurred, return a
 156  * negative error number. */
 157 static int coroutine_fn backup_cow_with_offload(BackupBlockJob *job,
 158                                                 int64_t start,
 159                                                 int64_t end,
 160                                                 bool is_write_notifier)
 161 {
 162     int ret;
 163     int nr_clusters;
 164     BlockBackend *blk = job->common.blk;
 165     int nbytes;
 166     int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
 167     int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
 168
 169     assert(QEMU_IS_ALIGNED(job->copy_range_size, job->cluster_size));
 170     nbytes = MIN(job->copy_range_size, end - start);
 171     nr_clusters = DIV_ROUND_UP(nbytes, job->cluster_size);
 172     hbitmap_reset(job->copy_bitmap, start / job->cluster_size,
 173                   nr_clusters);
 174     ret = blk_co_copy_range(blk, start, job->target, start, nbytes,
 175                             read_flags, write_flags);
 176     if (ret < 0) {
 177         trace_backup_do_cow_copy_range_fail(job, start, ret);
 178         hbitmap_set(job->copy_bitmap, start / job->cluster_size,
 179                     nr_clusters);
 180         return ret;
 181     }
 182
 183     return nbytes;
 184 }
 185
 186 static int coroutine_fn backup_do_cow(BackupBlockJob *job,
 187                                       int64_t offset, uint64_t bytes,
 188                                       bool *error_is_read,
 189                                       bool is_write_notifier)
 190 {
 191     CowRequest cow_request;
 192     int ret = 0;
 193     int64_t start, end; /* bytes */
 194     void *bounce_buffer = NULL;
 195
 196     qemu_co_rwlock_rdlock(&job->flush_rwlock);
 197
 198     start = QEMU_ALIGN_DOWN(offset, job->cluster_size);
 199     end = QEMU_ALIGN_UP(bytes + offset, job->cluster_size);
 200
 201     trace_backup_do_cow_enter(job, start, offset, bytes);
 202
 203     wait_for_overlapping_requests(job, start, end);
 204     cow_request_begin(&cow_request, job, start, end);
 205
 206     while (start < end) {
 207         if (!hbitmap_get(job->copy_bitmap, start / job->cluster_size)) {
 208             trace_backup_do_cow_skip(job, start);
 209             start += job->cluster_size;
 210             continue; /* already copied */
 211         }
 212
 213         trace_backup_do_cow_process(job, start);
 214
 215         if (job->use_copy_range) {
 216             ret = backup_cow_with_offload(job, start, end, is_write_notifier);
 217             if (ret < 0) {
 218                 job->use_copy_range = false;
 219             }
 220         }
 221         if (!job->use_copy_range) {
 222             ret = backup_cow_with_bounce_buffer(job, start, end, is_write_notifier,
 223                                                 error_is_read, &bounce_buffer);
 224         }
 225         if (ret < 0) {
 226             break;
 227         }
 228
 229         /* Publish progress, guest I/O counts as progress too.  Note that the
 230          * offset field is an opaque progress value, it is not a disk offset.
 231          */
 232         start += ret;
 233         job->bytes_read += ret;
 234         job_progress_update(&job->common.job, ret);
 235         ret = 0;
 236     }
 237
 238     if (bounce_buffer) {
 239         qemu_vfree(bounce_buffer);
 240     }
 241
 242     cow_request_end(&cow_request);
 243
 244     trace_backup_do_cow_return(job, offset, bytes, ret);
 245
 246     qemu_co_rwlock_unlock(&job->flush_rwlock);
 247
 248     return ret;
 249 }
 250
 251 static int coroutine_fn backup_before_write_notify(
 252         NotifierWithReturn *notifier,
 253         void *opaque)
 254 {
 255     BackupBlockJob *job = container_of(notifier, BackupBlockJob, before_write);
 256     BdrvTrackedRequest *req = opaque;
 257
 258     assert(req->bs == blk_bs(job->common.blk));
 259     assert(QEMU_IS_ALIGNED(req->offset, BDRV_SECTOR_SIZE));
 260     assert(QEMU_IS_ALIGNED(req->bytes, BDRV_SECTOR_SIZE));
 261
 262     return backup_do_cow(job, req->offset, req->bytes, NULL, true);
 263 }
 264
 265 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 266 {
 267     BdrvDirtyBitmap *bm;
 268     BlockDriverState *bs = blk_bs(job->common.blk);
 269
 270     if (ret < 0) {
 271         /* Merge the successor back into the parent, delete nothing. */
 272         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
 273         assert(bm);
 274     } else {
 275         /* Everything is fine, delete this bitmap and install the backup. */
 276         bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
 277         assert(bm);
 278     }
 279 }
 280
 281 static void backup_commit(Job *job)
 282 {
 283     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 284     if (s->sync_bitmap) {
 285         backup_cleanup_sync_bitmap(s, 0);
 286     }
 287 }
 288
 289 static void backup_abort(Job *job)
 290 {
 291     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 292     if (s->sync_bitmap) {
 293         backup_cleanup_sync_bitmap(s, -1);
 294     }
 295 }
 296
 297 static void backup_clean(Job *job)
 298 {
 299     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 300     assert(s->target);
 301     blk_unref(s->target);
 302     s->target = NULL;
 303 }
 304
 305 static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
 306 {
 307     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 308
 309     blk_set_aio_context(s->target, aio_context);
 310 }
 311
 312 void backup_do_checkpoint(BlockJob *job, Error **errp)
 313 {
 314     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 315     int64_t len;
 316
 317     assert(block_job_driver(job) == &backup_job_driver);
 318
 319     if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
 320         error_setg(errp, "The backup job only supports block checkpoint in"
 321                    " sync=none mode");
 322         return;
 323     }
 324
 325     len = DIV_ROUND_UP(backup_job->len, backup_job->cluster_size);
 326     hbitmap_set(backup_job->copy_bitmap, 0, len);
 327 }
 328
 329 static void backup_drain(BlockJob *job)
 330 {
 331     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 332
 333     /* Need to keep a reference in case blk_drain triggers execution
 334      * of backup_complete...
 335      */
 336     if (s->target) {
 337         BlockBackend *target = s->target;
 338         blk_ref(target);
 339         blk_drain(target);
 340         blk_unref(target);
 341     }
 342 }
 343
 344 static BlockErrorAction backup_error_action(BackupBlockJob *job,
 345                                             bool read, int error)
 346 {
 347     if (read) {
 348         return block_job_error_action(&job->common, job->on_source_error,
 349                                       true, error);
 350     } else {
 351         return block_job_error_action(&job->common, job->on_target_error,
 352                                       false, error);
 353     }
 354 }
 355
 356 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 357 {
 358     uint64_t delay_ns;
 359
 360     if (job_is_cancelled(&job->common.job)) {
 361         return true;
 362     }
 363
 364     /* We need to yield even for delay_ns = 0 so that bdrv_drain_all() can
 365      * return. Without a yield, the VM would not reboot. */
 366     delay_ns = block_job_ratelimit_get_delay(&job->common, job->bytes_read);
 367     job->bytes_read = 0;
 368     job_sleep_ns(&job->common.job, delay_ns);
 369
 370     if (job_is_cancelled(&job->common.job)) {
 371         return true;
 372     }
 373
 374     return false;
 375 }
 376
 377 static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
 378 {
 379     int ret;
 380     bool error_is_read;
 381     int64_t cluster;
 382     HBitmapIter hbi;
 383
 384     hbitmap_iter_init(&hbi, job->copy_bitmap, 0);
 385     while ((cluster = hbitmap_iter_next(&hbi)) != -1) {
 386         do {
 387             if (yield_and_check(job)) {
 388                 return 0;
 389             }
 390             ret = backup_do_cow(job, cluster * job->cluster_size,
 391                                 job->cluster_size, &error_is_read, false);
 392             if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
 393                            BLOCK_ERROR_ACTION_REPORT)
 394             {
 395                 return ret;
 396             }
 397         } while (ret < 0);
 398     }
 399
 400     return 0;
 401 }
 402
 403 /* init copy_bitmap from sync_bitmap */
 404 static void backup_incremental_init_copy_bitmap(BackupBlockJob *job)
 405 {
 406     BdrvDirtyBitmapIter *dbi;
 407     int64_t offset;
 408     int64_t end = DIV_ROUND_UP(bdrv_dirty_bitmap_size(job->sync_bitmap),
 409                                job->cluster_size);
 410
 411     dbi = bdrv_dirty_iter_new(job->sync_bitmap);
 412     while ((offset = bdrv_dirty_iter_next(dbi)) != -1) {
 413         int64_t cluster = offset / job->cluster_size;
 414         int64_t next_cluster;
 415
 416         offset += bdrv_dirty_bitmap_granularity(job->sync_bitmap);
 417         if (offset >= bdrv_dirty_bitmap_size(job->sync_bitmap)) {
 418             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 419             break;
 420         }
 421
 422         offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset,
 423                                              UINT64_MAX);
 424         if (offset == -1) {
 425             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 426             break;
 427         }
 428
 429         next_cluster = DIV_ROUND_UP(offset, job->cluster_size);
 430         hbitmap_set(job->copy_bitmap, cluster, next_cluster - cluster);
 431         if (next_cluster >= end) {
 432             break;
 433         }
 434
 435         bdrv_set_dirty_iter(dbi, next_cluster * job->cluster_size);
 436     }
 437
 438     /* TODO job_progress_set_remaining() would make more sense */
 439     job_progress_update(&job->common.job,
 440         job->len - hbitmap_count(job->copy_bitmap) * job->cluster_size);
 441
 442     bdrv_dirty_iter_free(dbi);
 443 }
 444
 445 static int coroutine_fn backup_run(Job *job, Error **errp)
 446 {
 447     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 448     BlockDriverState *bs = blk_bs(s->common.blk);
 449     int64_t offset, nb_clusters;
 450     int ret = 0;
 451
 452     QLIST_INIT(&s->inflight_reqs);
 453     qemu_co_rwlock_init(&s->flush_rwlock);
 454
 455     nb_clusters = DIV_ROUND_UP(s->len, s->cluster_size);
 456     job_progress_set_remaining(job, s->len);
 457
 458     s->copy_bitmap = hbitmap_alloc(nb_clusters, 0);
 459     if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 460         backup_incremental_init_copy_bitmap(s);
 461     } else {
 462         hbitmap_set(s->copy_bitmap, 0, nb_clusters);
 463     }
 464
 465
 466     s->before_write.notify = backup_before_write_notify;
 467     bdrv_add_before_write_notifier(bs, &s->before_write);
 468
 469     if (s->sync_mode == MIRROR_SYNC_MODE_NONE) {
 470         /* All bits are set in copy_bitmap to allow any cluster to be copied.
 471          * This does not actually require them to be copied. */
 472         while (!job_is_cancelled(job)) {
 473             /* Yield until the job is cancelled.  We just let our before_write
 474              * notify callback service CoW requests. */
 475             job_yield(job);
 476         }
 477     } else if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 478         ret = backup_run_incremental(s);
 479     } else {
 480         /* Both FULL and TOP SYNC_MODE's require copying.. */
 481         for (offset = 0; offset < s->len;
 482              offset += s->cluster_size) {
 483             bool error_is_read;
 484             int alloced = 0;
 485
 486             if (yield_and_check(s)) {
 487                 break;
 488             }
 489
 490             if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
 491                 int i;
 492                 int64_t n;
 493
 494                 /* Check to see if these blocks are already in the
 495                  * backing file. */
 496
 497                 for (i = 0; i < s->cluster_size;) {
 498                     /* bdrv_is_allocated() only returns true/false based
 499                      * on the first set of sectors it comes across that
 500                      * are are all in the same state.
 501                      * For that reason we must verify each sector in the
 502                      * backup cluster length.  We end up copying more than
 503                      * needed but at some point that is always the case. */
 504                     alloced =
 505                         bdrv_is_allocated(bs, offset + i,
 506                                           s->cluster_size - i, &n);
 507                     i += n;
 508
 509                     if (alloced || n == 0) {
 510                         break;
 511                     }
 512                 }
 513
 514                 /* If the above loop never found any sectors that are in
 515                  * the topmost image, skip this backup. */
 516                 if (alloced == 0) {
 517                     continue;
 518                 }
 519             }
 520             /* FULL sync mode we copy the whole drive. */
 521             if (alloced < 0) {
 522                 ret = alloced;
 523             } else {
 524                 ret = backup_do_cow(s, offset, s->cluster_size,
 525                                     &error_is_read, false);
 526             }
 527             if (ret < 0) {
 528                 /* Depending on error action, fail now or retry cluster */
 529                 BlockErrorAction action =
 530                     backup_error_action(s, error_is_read, -ret);
 531                 if (action == BLOCK_ERROR_ACTION_REPORT) {
 532                     break;
 533                 } else {
 534                     offset -= s->cluster_size;
 535                     continue;
 536                 }
 537             }
 538         }
 539     }
 540
 541     notifier_with_return_remove(&s->before_write);
 542
 543     /* wait until pending backup_do_cow() calls have completed */
 544     qemu_co_rwlock_wrlock(&s->flush_rwlock);
 545     qemu_co_rwlock_unlock(&s->flush_rwlock);
 546     hbitmap_free(s->copy_bitmap);
 547
 548     return ret;
 549 }
 550
 551 static const BlockJobDriver backup_job_driver = {
 552     .job_driver = {
 553         .instance_size          = sizeof(BackupBlockJob),
 554         .job_type               = JOB_TYPE_BACKUP,
 555         .free                   = block_job_free,
 556         .user_resume            = block_job_user_resume,
 557         .drain                  = block_job_drain,
 558         .run                    = backup_run,
 559         .commit                 = backup_commit,
 560         .abort                  = backup_abort,
 561         .clean                  = backup_clean,
 562     },
 563     .attached_aio_context   = backup_attached_aio_context,
 564     .drain                  = backup_drain,
 565 };
 566
 567 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
 568                   BlockDriverState *target, int64_t speed,
 569                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
 570                   bool compress,
 571                   BlockdevOnError on_source_error,
 572                   BlockdevOnError on_target_error,
 573                   int creation_flags,
 574                   BlockCompletionFunc *cb, void *opaque,
 575                   JobTxn *txn, Error **errp)
 576 {
 577     int64_t len;
 578     BlockDriverInfo bdi;
 579     BackupBlockJob *job = NULL;
 580     int ret;
 581
 582     assert(bs);
 583     assert(target);
 584
 585     if (bs == target) {
 586         error_setg(errp, "Source and target cannot be the same");
 587         return NULL;
 588     }
 589
 590     if (!bdrv_is_inserted(bs)) {
 591         error_setg(errp, "Device is not inserted: %s",
 592                    bdrv_get_device_name(bs));
 593         return NULL;
 594     }
 595
 596     if (!bdrv_is_inserted(target)) {
 597         error_setg(errp, "Device is not inserted: %s",
 598                    bdrv_get_device_name(target));
 599         return NULL;
 600     }
 601
 602     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
 603         error_setg(errp, "Compression is not supported for this drive %s",
 604                    bdrv_get_device_name(target));
 605         return NULL;
 606     }
 607
 608     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
 609         return NULL;
 610     }
 611
 612     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
 613         return NULL;
 614     }
 615
 616     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 617         if (!sync_bitmap) {
 618             error_setg(errp, "must provide a valid bitmap name for "
 619                              "\"incremental\" sync mode");
 620             return NULL;
 621         }
 622
 623         /* Create a new bitmap, and freeze/disable this one. */
 624         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
 625             return NULL;
 626         }
 627     } else if (sync_bitmap) {
 628         error_setg(errp,
 629                    "a sync_bitmap was provided to backup_run, "
 630                    "but received an incompatible sync_mode (%s)",
 631                    MirrorSyncMode_str(sync_mode));
 632         return NULL;
 633     }
 634
 635     len = bdrv_getlength(bs);
 636     if (len < 0) {
 637         error_setg_errno(errp, -len, "unable to get length for '%s'",
 638                          bdrv_get_device_name(bs));
 639         goto error;
 640     }
 641
 642     /* job->len is fixed, so we can't allow resize */
 643     job = block_job_create(job_id, &backup_job_driver, txn, bs,
 644                            BLK_PERM_CONSISTENT_READ,
 645                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 646                            BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD,
 647                            speed, creation_flags, cb, opaque, errp);
 648     if (!job) {
 649         goto error;
 650     }
 651
 652     /* The target must match the source in size, so no resize here either */
 653     job->target = blk_new(BLK_PERM_WRITE,
 654                           BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 655                           BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD);
 656     ret = blk_insert_bs(job->target, target, errp);
 657     if (ret < 0) {
 658         goto error;
 659     }
 660
 661     job->on_source_error = on_source_error;
 662     job->on_target_error = on_target_error;
 663     job->sync_mode = sync_mode;
 664     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
 665                        sync_bitmap : NULL;
 666     job->compress = compress;
 667
 668     /* Detect image-fleecing (and similar) schemes */
 669     job->serialize_target_writes = bdrv_chain_contains(target, bs);
 670
 671     /* If there is no backing file on the target, we cannot rely on COW if our
 672      * backup cluster size is smaller than the target cluster size. Even for
 673      * targets with a backing file, try to avoid COW if possible. */
 674     ret = bdrv_get_info(target, &bdi);
 675     if (ret == -ENOTSUP && !target->backing) {
 676         /* Cluster size is not defined */
 677         warn_report("The target block device doesn't provide "
 678                     "information about the block size and it doesn't have a "
 679                     "backing file. The default block size of %u bytes is "
 680                     "used. If the actual block size of the target exceeds "
 681                     "this default, the backup may be unusable",
 682                     BACKUP_CLUSTER_SIZE_DEFAULT);
 683         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 684     } else if (ret < 0 && !target->backing) {
 685         error_setg_errno(errp, -ret,
 686             "Couldn't determine the cluster size of the target image, "
 687             "which has no backing file");
 688         error_append_hint(errp,
 689             "Aborting, since this may create an unusable destination image\n");
 690         goto error;
 691     } else if (ret < 0 && target->backing) {
 692         /* Not fatal; just trudge on ahead. */
 693         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 694     } else {
 695         job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
 696     }
 697     job->use_copy_range = true;
 698     job->copy_range_size = MIN_NON_ZERO(blk_get_max_transfer(job->common.blk),
 699                                         blk_get_max_transfer(job->target));
 700     job->copy_range_size = MAX(job->cluster_size,
 701                                QEMU_ALIGN_UP(job->copy_range_size,
 702                                              job->cluster_size));
 703
 704     /* Required permissions are already taken with target's blk_new() */
 705     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
 706                        &error_abort);
 707     job->len = len;
 708
 709     return &job->common;
 710
 711  error:
 712     if (sync_bitmap) {
 713         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
 714     }
 715     if (job) {
 716         backup_clean(&job->common.job);
 717         job_early_fail(&job->common.job);
 718     }
 719
 720     return NULL;
 721 }