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