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     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 void backup_do_checkpoint(BlockJob *job, Error **errp)
 304 {
 305     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 306     int64_t len;
 307
 308     assert(block_job_driver(job) == &backup_job_driver);
 309
 310     if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
 311         error_setg(errp, "The backup job only supports block checkpoint in"
 312                    " sync=none mode");
 313         return;
 314     }
 315
 316     len = DIV_ROUND_UP(backup_job->len, backup_job->cluster_size);
 317     hbitmap_set(backup_job->copy_bitmap, 0, len);
 318 }
 319
 320 static void backup_drain(BlockJob *job)
 321 {
 322     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 323
 324     /* Need to keep a reference in case blk_drain triggers execution
 325      * of backup_complete...
 326      */
 327     if (s->target) {
 328         BlockBackend *target = s->target;
 329         blk_ref(target);
 330         blk_drain(target);
 331         blk_unref(target);
 332     }
 333 }
 334
 335 static BlockErrorAction backup_error_action(BackupBlockJob *job,
 336                                             bool read, int error)
 337 {
 338     if (read) {
 339         return block_job_error_action(&job->common, job->on_source_error,
 340                                       true, error);
 341     } else {
 342         return block_job_error_action(&job->common, job->on_target_error,
 343                                       false, error);
 344     }
 345 }
 346
 347 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 348 {
 349     uint64_t delay_ns;
 350
 351     if (job_is_cancelled(&job->common.job)) {
 352         return true;
 353     }
 354
 355     /* We need to yield even for delay_ns = 0 so that bdrv_drain_all() can
 356      * return. Without a yield, the VM would not reboot. */
 357     delay_ns = block_job_ratelimit_get_delay(&job->common, job->bytes_read);
 358     job->bytes_read = 0;
 359     job_sleep_ns(&job->common.job, delay_ns);
 360
 361     if (job_is_cancelled(&job->common.job)) {
 362         return true;
 363     }
 364
 365     return false;
 366 }
 367
 368 static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
 369 {
 370     int ret;
 371     bool error_is_read;
 372     int64_t cluster;
 373     HBitmapIter hbi;
 374
 375     hbitmap_iter_init(&hbi, job->copy_bitmap, 0);
 376     while ((cluster = hbitmap_iter_next(&hbi)) != -1) {
 377         do {
 378             if (yield_and_check(job)) {
 379                 return 0;
 380             }
 381             ret = backup_do_cow(job, cluster * job->cluster_size,
 382                                 job->cluster_size, &error_is_read, false);
 383             if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
 384                            BLOCK_ERROR_ACTION_REPORT)
 385             {
 386                 return ret;
 387             }
 388         } while (ret < 0);
 389     }
 390
 391     return 0;
 392 }
 393
 394 /* init copy_bitmap from sync_bitmap */
 395 static void backup_incremental_init_copy_bitmap(BackupBlockJob *job)
 396 {
 397     BdrvDirtyBitmapIter *dbi;
 398     int64_t offset;
 399     int64_t end = DIV_ROUND_UP(bdrv_dirty_bitmap_size(job->sync_bitmap),
 400                                job->cluster_size);
 401
 402     dbi = bdrv_dirty_iter_new(job->sync_bitmap);
 403     while ((offset = bdrv_dirty_iter_next(dbi)) != -1) {
 404         int64_t cluster = offset / job->cluster_size;
 405         int64_t next_cluster;
 406
 407         offset += bdrv_dirty_bitmap_granularity(job->sync_bitmap);
 408         if (offset >= bdrv_dirty_bitmap_size(job->sync_bitmap)) {
 409             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 410             break;
 411         }
 412
 413         offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset,
 414                                              UINT64_MAX);
 415         if (offset == -1) {
 416             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 417             break;
 418         }
 419
 420         next_cluster = DIV_ROUND_UP(offset, job->cluster_size);
 421         hbitmap_set(job->copy_bitmap, cluster, next_cluster - cluster);
 422         if (next_cluster >= end) {
 423             break;
 424         }
 425
 426         bdrv_set_dirty_iter(dbi, next_cluster * job->cluster_size);
 427     }
 428
 429     /* TODO job_progress_set_remaining() would make more sense */
 430     job_progress_update(&job->common.job,
 431         job->len - hbitmap_count(job->copy_bitmap) * job->cluster_size);
 432
 433     bdrv_dirty_iter_free(dbi);
 434 }
 435
 436 static int coroutine_fn backup_run(Job *job, Error **errp)
 437 {
 438     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 439     BlockDriverState *bs = blk_bs(s->common.blk);
 440     int64_t offset, nb_clusters;
 441     int ret = 0;
 442
 443     QLIST_INIT(&s->inflight_reqs);
 444     qemu_co_rwlock_init(&s->flush_rwlock);
 445
 446     nb_clusters = DIV_ROUND_UP(s->len, s->cluster_size);
 447     job_progress_set_remaining(job, s->len);
 448
 449     s->copy_bitmap = hbitmap_alloc(nb_clusters, 0);
 450     if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 451         backup_incremental_init_copy_bitmap(s);
 452     } else {
 453         hbitmap_set(s->copy_bitmap, 0, nb_clusters);
 454     }
 455
 456
 457     s->before_write.notify = backup_before_write_notify;
 458     bdrv_add_before_write_notifier(bs, &s->before_write);
 459
 460     if (s->sync_mode == MIRROR_SYNC_MODE_NONE) {
 461         /* All bits are set in copy_bitmap to allow any cluster to be copied.
 462          * This does not actually require them to be copied. */
 463         while (!job_is_cancelled(job)) {
 464             /* Yield until the job is cancelled.  We just let our before_write
 465              * notify callback service CoW requests. */
 466             job_yield(job);
 467         }
 468     } else if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 469         ret = backup_run_incremental(s);
 470     } else {
 471         /* Both FULL and TOP SYNC_MODE's require copying.. */
 472         for (offset = 0; offset < s->len;
 473              offset += s->cluster_size) {
 474             bool error_is_read;
 475             int alloced = 0;
 476
 477             if (yield_and_check(s)) {
 478                 break;
 479             }
 480
 481             if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
 482                 int i;
 483                 int64_t n;
 484
 485                 /* Check to see if these blocks are already in the
 486                  * backing file. */
 487
 488                 for (i = 0; i < s->cluster_size;) {
 489                     /* bdrv_is_allocated() only returns true/false based
 490                      * on the first set of sectors it comes across that
 491                      * are are all in the same state.
 492                      * For that reason we must verify each sector in the
 493                      * backup cluster length.  We end up copying more than
 494                      * needed but at some point that is always the case. */
 495                     alloced =
 496                         bdrv_is_allocated(bs, offset + i,
 497                                           s->cluster_size - i, &n);
 498                     i += n;
 499
 500                     if (alloced || n == 0) {
 501                         break;
 502                     }
 503                 }
 504
 505                 /* If the above loop never found any sectors that are in
 506                  * the topmost image, skip this backup. */
 507                 if (alloced == 0) {
 508                     continue;
 509                 }
 510             }
 511             /* FULL sync mode we copy the whole drive. */
 512             if (alloced < 0) {
 513                 ret = alloced;
 514             } else {
 515                 ret = backup_do_cow(s, offset, s->cluster_size,
 516                                     &error_is_read, false);
 517             }
 518             if (ret < 0) {
 519                 /* Depending on error action, fail now or retry cluster */
 520                 BlockErrorAction action =
 521                     backup_error_action(s, error_is_read, -ret);
 522                 if (action == BLOCK_ERROR_ACTION_REPORT) {
 523                     break;
 524                 } else {
 525                     offset -= s->cluster_size;
 526                     continue;
 527                 }
 528             }
 529         }
 530     }
 531
 532     notifier_with_return_remove(&s->before_write);
 533
 534     /* wait until pending backup_do_cow() calls have completed */
 535     qemu_co_rwlock_wrlock(&s->flush_rwlock);
 536     qemu_co_rwlock_unlock(&s->flush_rwlock);
 537     hbitmap_free(s->copy_bitmap);
 538
 539     return ret;
 540 }
 541
 542 static const BlockJobDriver backup_job_driver = {
 543     .job_driver = {
 544         .instance_size          = sizeof(BackupBlockJob),
 545         .job_type               = JOB_TYPE_BACKUP,
 546         .free                   = block_job_free,
 547         .user_resume            = block_job_user_resume,
 548         .drain                  = block_job_drain,
 549         .run                    = backup_run,
 550         .commit                 = backup_commit,
 551         .abort                  = backup_abort,
 552         .clean                  = backup_clean,
 553     },
 554     .drain                  = backup_drain,
 555 };
 556
 557 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
 558                   BlockDriverState *target, int64_t speed,
 559                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
 560                   bool compress,
 561                   BlockdevOnError on_source_error,
 562                   BlockdevOnError on_target_error,
 563                   int creation_flags,
 564                   BlockCompletionFunc *cb, void *opaque,
 565                   JobTxn *txn, Error **errp)
 566 {
 567     int64_t len;
 568     BlockDriverInfo bdi;
 569     BackupBlockJob *job = NULL;
 570     int ret;
 571
 572     assert(bs);
 573     assert(target);
 574
 575     if (bs == target) {
 576         error_setg(errp, "Source and target cannot be the same");
 577         return NULL;
 578     }
 579
 580     if (!bdrv_is_inserted(bs)) {
 581         error_setg(errp, "Device is not inserted: %s",
 582                    bdrv_get_device_name(bs));
 583         return NULL;
 584     }
 585
 586     if (!bdrv_is_inserted(target)) {
 587         error_setg(errp, "Device is not inserted: %s",
 588                    bdrv_get_device_name(target));
 589         return NULL;
 590     }
 591
 592     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
 593         error_setg(errp, "Compression is not supported for this drive %s",
 594                    bdrv_get_device_name(target));
 595         return NULL;
 596     }
 597
 598     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
 599         return NULL;
 600     }
 601
 602     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
 603         return NULL;
 604     }
 605
 606     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 607         if (!sync_bitmap) {
 608             error_setg(errp, "must provide a valid bitmap name for "
 609                              "\"incremental\" sync mode");
 610             return NULL;
 611         }
 612
 613         /* Create a new bitmap, and freeze/disable this one. */
 614         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
 615             return NULL;
 616         }
 617     } else if (sync_bitmap) {
 618         error_setg(errp,
 619                    "a sync_bitmap was provided to backup_run, "
 620                    "but received an incompatible sync_mode (%s)",
 621                    MirrorSyncMode_str(sync_mode));
 622         return NULL;
 623     }
 624
 625     len = bdrv_getlength(bs);
 626     if (len < 0) {
 627         error_setg_errno(errp, -len, "unable to get length for '%s'",
 628                          bdrv_get_device_name(bs));
 629         goto error;
 630     }
 631
 632     /* job->len is fixed, so we can't allow resize */
 633     job = block_job_create(job_id, &backup_job_driver, txn, bs,
 634                            BLK_PERM_CONSISTENT_READ,
 635                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 636                            BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD,
 637                            speed, creation_flags, cb, opaque, errp);
 638     if (!job) {
 639         goto error;
 640     }
 641
 642     /* The target must match the source in size, so no resize here either */
 643     job->target = blk_new(BLK_PERM_WRITE,
 644                           BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 645                           BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD);
 646     ret = blk_insert_bs(job->target, target, errp);
 647     if (ret < 0) {
 648         goto error;
 649     }
 650
 651     job->on_source_error = on_source_error;
 652     job->on_target_error = on_target_error;
 653     job->sync_mode = sync_mode;
 654     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
 655                        sync_bitmap : NULL;
 656     job->compress = compress;
 657
 658     /* Detect image-fleecing (and similar) schemes */
 659     job->serialize_target_writes = bdrv_chain_contains(target, bs);
 660
 661     /* If there is no backing file on the target, we cannot rely on COW if our
 662      * backup cluster size is smaller than the target cluster size. Even for
 663      * targets with a backing file, try to avoid COW if possible. */
 664     ret = bdrv_get_info(target, &bdi);
 665     if (ret == -ENOTSUP && !target->backing) {
 666         /* Cluster size is not defined */
 667         warn_report("The target block device doesn't provide "
 668                     "information about the block size and it doesn't have a "
 669                     "backing file. The default block size of %u bytes is "
 670                     "used. If the actual block size of the target exceeds "
 671                     "this default, the backup may be unusable",
 672                     BACKUP_CLUSTER_SIZE_DEFAULT);
 673         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 674     } else if (ret < 0 && !target->backing) {
 675         error_setg_errno(errp, -ret,
 676             "Couldn't determine the cluster size of the target image, "
 677             "which has no backing file");
 678         error_append_hint(errp,
 679             "Aborting, since this may create an unusable destination image\n");
 680         goto error;
 681     } else if (ret < 0 && target->backing) {
 682         /* Not fatal; just trudge on ahead. */
 683         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 684     } else {
 685         job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
 686     }
 687     job->use_copy_range = true;
 688     job->copy_range_size = MIN_NON_ZERO(blk_get_max_transfer(job->common.blk),
 689                                         blk_get_max_transfer(job->target));
 690     job->copy_range_size = MAX(job->cluster_size,
 691                                QEMU_ALIGN_UP(job->copy_range_size,
 692                                              job->cluster_size));
 693
 694     /* Required permissions are already taken with target's blk_new() */
 695     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
 696                        &error_abort);
 697     job->len = len;
 698
 699     return &job->common;
 700
 701  error:
 702     if (sync_bitmap) {
 703         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
 704     }
 705     if (job) {
 706         backup_clean(&job->common.job);
 707         job_early_fail(&job->common.job);
 708     }
 709
 710     return NULL;
 711 }