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 BackupBlockJob {
  32     BlockJob common;
  33     BlockBackend *target;
  34     /* bitmap for sync=incremental */
  35     BdrvDirtyBitmap *sync_bitmap;
  36     MirrorSyncMode sync_mode;
  37     BlockdevOnError on_source_error;
  38     BlockdevOnError on_target_error;
  39     CoRwlock flush_rwlock;
  40     uint64_t len;
  41     uint64_t bytes_read;
  42     int64_t cluster_size;
  43     bool compress;
  44     NotifierWithReturn before_write;
  45     QLIST_HEAD(, CowRequest) inflight_reqs;
  46
  47     HBitmap *copy_bitmap;
  48     bool use_copy_range;
  49     int64_t copy_range_size;
  50
  51     bool serialize_target_writes;
  52 } BackupBlockJob;
  53
  54 static const BlockJobDriver backup_job_driver;
  55
  56 /* See if in-flight requests overlap and wait for them to complete */
  57 static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
  58                                                        int64_t start,
  59                                                        int64_t end)
  60 {
  61     CowRequest *req;
  62     bool retry;
  63
  64     do {
  65         retry = false;
  66         QLIST_FOREACH(req, &job->inflight_reqs, list) {
  67             if (end > req->start_byte && start < req->end_byte) {
  68                 qemu_co_queue_wait(&req->wait_queue, NULL);
  69                 retry = true;
  70                 break;
  71             }
  72         }
  73     } while (retry);
  74 }
  75
  76 /* Keep track of an in-flight request */
  77 static void cow_request_begin(CowRequest *req, BackupBlockJob *job,
  78                               int64_t start, int64_t end)
  79 {
  80     req->start_byte = start;
  81     req->end_byte = end;
  82     qemu_co_queue_init(&req->wait_queue);
  83     QLIST_INSERT_HEAD(&job->inflight_reqs, req, list);
  84 }
  85
  86 /* Forget about a completed request */
  87 static void cow_request_end(CowRequest *req)
  88 {
  89     QLIST_REMOVE(req, list);
  90     qemu_co_queue_restart_all(&req->wait_queue);
  91 }
  92
  93 /* Copy range to target with a bounce buffer and return the bytes copied. If
  94  * error occurred, return a negative error number */
  95 static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
  96                                                       int64_t start,
  97                                                       int64_t end,
  98                                                       bool is_write_notifier,
  99                                                       bool *error_is_read,
 100                                                       void **bounce_buffer)
 101 {
 102     int ret;
 103     struct iovec iov;
 104     QEMUIOVector qiov;
 105     BlockBackend *blk = job->common.blk;
 106     int nbytes;
 107     int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
 108     int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
 109
 110     hbitmap_reset(job->copy_bitmap, start / job->cluster_size, 1);
 111     nbytes = MIN(job->cluster_size, job->len - start);
 112     if (!*bounce_buffer) {
 113         *bounce_buffer = blk_blockalign(blk, job->cluster_size);
 114     }
 115     iov.iov_base = *bounce_buffer;
 116     iov.iov_len = nbytes;
 117     qemu_iovec_init_external(&qiov, &iov, 1);
 118
 119     ret = blk_co_preadv(blk, start, qiov.size, &qiov, read_flags);
 120     if (ret < 0) {
 121         trace_backup_do_cow_read_fail(job, start, ret);
 122         if (error_is_read) {
 123             *error_is_read = true;
 124         }
 125         goto fail;
 126     }
 127
 128     if (qemu_iovec_is_zero(&qiov)) {
 129         ret = blk_co_pwrite_zeroes(job->target, start,
 130                                    qiov.size, write_flags | BDRV_REQ_MAY_UNMAP);
 131     } else {
 132         ret = blk_co_pwritev(job->target, start,
 133                              qiov.size, &qiov, write_flags |
 134                              (job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0));
 135     }
 136     if (ret < 0) {
 137         trace_backup_do_cow_write_fail(job, start, ret);
 138         if (error_is_read) {
 139             *error_is_read = false;
 140         }
 141         goto fail;
 142     }
 143
 144     return nbytes;
 145 fail:
 146     hbitmap_set(job->copy_bitmap, start / job->cluster_size, 1);
 147     return ret;
 148
 149 }
 150
 151 /* Copy range to target and return the bytes copied. If error occurred, return a
 152  * negative error number. */
 153 static int coroutine_fn backup_cow_with_offload(BackupBlockJob *job,
 154                                                 int64_t start,
 155                                                 int64_t end,
 156                                                 bool is_write_notifier)
 157 {
 158     int ret;
 159     int nr_clusters;
 160     BlockBackend *blk = job->common.blk;
 161     int nbytes;
 162     int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
 163     int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
 164
 165     assert(QEMU_IS_ALIGNED(job->copy_range_size, job->cluster_size));
 166     nbytes = MIN(job->copy_range_size, end - start);
 167     nr_clusters = DIV_ROUND_UP(nbytes, job->cluster_size);
 168     hbitmap_reset(job->copy_bitmap, start / job->cluster_size,
 169                   nr_clusters);
 170     ret = blk_co_copy_range(blk, start, job->target, start, nbytes,
 171                             read_flags, write_flags);
 172     if (ret < 0) {
 173         trace_backup_do_cow_copy_range_fail(job, start, ret);
 174         hbitmap_set(job->copy_bitmap, start / job->cluster_size,
 175                     nr_clusters);
 176         return ret;
 177     }
 178
 179     return nbytes;
 180 }
 181
 182 static int coroutine_fn backup_do_cow(BackupBlockJob *job,
 183                                       int64_t offset, uint64_t bytes,
 184                                       bool *error_is_read,
 185                                       bool is_write_notifier)
 186 {
 187     CowRequest cow_request;
 188     int ret = 0;
 189     int64_t start, end; /* bytes */
 190     void *bounce_buffer = NULL;
 191
 192     qemu_co_rwlock_rdlock(&job->flush_rwlock);
 193
 194     start = QEMU_ALIGN_DOWN(offset, job->cluster_size);
 195     end = QEMU_ALIGN_UP(bytes + offset, job->cluster_size);
 196
 197     trace_backup_do_cow_enter(job, start, offset, bytes);
 198
 199     wait_for_overlapping_requests(job, start, end);
 200     cow_request_begin(&cow_request, job, start, end);
 201
 202     while (start < end) {
 203         if (!hbitmap_get(job->copy_bitmap, start / job->cluster_size)) {
 204             trace_backup_do_cow_skip(job, start);
 205             start += job->cluster_size;
 206             continue; /* already copied */
 207         }
 208
 209         trace_backup_do_cow_process(job, start);
 210
 211         if (job->use_copy_range) {
 212             ret = backup_cow_with_offload(job, start, end, is_write_notifier);
 213             if (ret < 0) {
 214                 job->use_copy_range = false;
 215             }
 216         }
 217         if (!job->use_copy_range) {
 218             ret = backup_cow_with_bounce_buffer(job, start, end, is_write_notifier,
 219                                                 error_is_read, &bounce_buffer);
 220         }
 221         if (ret < 0) {
 222             break;
 223         }
 224
 225         /* Publish progress, guest I/O counts as progress too.  Note that the
 226          * offset field is an opaque progress value, it is not a disk offset.
 227          */
 228         start += ret;
 229         job->bytes_read += ret;
 230         job_progress_update(&job->common.job, ret);
 231         ret = 0;
 232     }
 233
 234     if (bounce_buffer) {
 235         qemu_vfree(bounce_buffer);
 236     }
 237
 238     cow_request_end(&cow_request);
 239
 240     trace_backup_do_cow_return(job, offset, bytes, ret);
 241
 242     qemu_co_rwlock_unlock(&job->flush_rwlock);
 243
 244     return ret;
 245 }
 246
 247 static int coroutine_fn backup_before_write_notify(
 248         NotifierWithReturn *notifier,
 249         void *opaque)
 250 {
 251     BackupBlockJob *job = container_of(notifier, BackupBlockJob, before_write);
 252     BdrvTrackedRequest *req = opaque;
 253
 254     assert(req->bs == blk_bs(job->common.blk));
 255     assert(QEMU_IS_ALIGNED(req->offset, BDRV_SECTOR_SIZE));
 256     assert(QEMU_IS_ALIGNED(req->bytes, BDRV_SECTOR_SIZE));
 257
 258     return backup_do_cow(job, req->offset, req->bytes, NULL, true);
 259 }
 260
 261 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 262 {
 263     BdrvDirtyBitmap *bm;
 264     BlockDriverState *bs = blk_bs(job->common.blk);
 265
 266     if (ret < 0) {
 267         /* Merge the successor back into the parent, delete nothing. */
 268         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
 269         assert(bm);
 270     } else {
 271         /* Everything is fine, delete this bitmap and install the backup. */
 272         bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
 273         assert(bm);
 274     }
 275 }
 276
 277 static void backup_commit(Job *job)
 278 {
 279     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 280     if (s->sync_bitmap) {
 281         backup_cleanup_sync_bitmap(s, 0);
 282     }
 283 }
 284
 285 static void backup_abort(Job *job)
 286 {
 287     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 288     if (s->sync_bitmap) {
 289         backup_cleanup_sync_bitmap(s, -1);
 290     }
 291 }
 292
 293 static void backup_clean(Job *job)
 294 {
 295     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 296     assert(s->target);
 297     blk_unref(s->target);
 298     s->target = NULL;
 299 }
 300
 301 static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
 302 {
 303     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 304
 305     blk_set_aio_context(s->target, aio_context);
 306 }
 307
 308 void backup_do_checkpoint(BlockJob *job, Error **errp)
 309 {
 310     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 311     int64_t len;
 312
 313     assert(block_job_driver(job) == &backup_job_driver);
 314
 315     if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
 316         error_setg(errp, "The backup job only supports block checkpoint in"
 317                    " sync=none mode");
 318         return;
 319     }
 320
 321     len = DIV_ROUND_UP(backup_job->len, backup_job->cluster_size);
 322     hbitmap_set(backup_job->copy_bitmap, 0, len);
 323 }
 324
 325 void backup_wait_for_overlapping_requests(BlockJob *job, int64_t offset,
 326                                           uint64_t bytes)
 327 {
 328     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 329     int64_t start, end;
 330
 331     assert(block_job_driver(job) == &backup_job_driver);
 332
 333     start = QEMU_ALIGN_DOWN(offset, backup_job->cluster_size);
 334     end = QEMU_ALIGN_UP(offset + bytes, backup_job->cluster_size);
 335     wait_for_overlapping_requests(backup_job, start, end);
 336 }
 337
 338 void backup_cow_request_begin(CowRequest *req, BlockJob *job,
 339                               int64_t offset, uint64_t bytes)
 340 {
 341     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 342     int64_t start, end;
 343
 344     assert(block_job_driver(job) == &backup_job_driver);
 345
 346     start = QEMU_ALIGN_DOWN(offset, backup_job->cluster_size);
 347     end = QEMU_ALIGN_UP(offset + bytes, backup_job->cluster_size);
 348     cow_request_begin(req, backup_job, start, end);
 349 }
 350
 351 void backup_cow_request_end(CowRequest *req)
 352 {
 353     cow_request_end(req);
 354 }
 355
 356 static void backup_drain(BlockJob *job)
 357 {
 358     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 359
 360     /* Need to keep a reference in case blk_drain triggers execution
 361      * of backup_complete...
 362      */
 363     if (s->target) {
 364         BlockBackend *target = s->target;
 365         blk_ref(target);
 366         blk_drain(target);
 367         blk_unref(target);
 368     }
 369 }
 370
 371 static BlockErrorAction backup_error_action(BackupBlockJob *job,
 372                                             bool read, int error)
 373 {
 374     if (read) {
 375         return block_job_error_action(&job->common, job->on_source_error,
 376                                       true, error);
 377     } else {
 378         return block_job_error_action(&job->common, job->on_target_error,
 379                                       false, error);
 380     }
 381 }
 382
 383 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 384 {
 385     uint64_t delay_ns;
 386
 387     if (job_is_cancelled(&job->common.job)) {
 388         return true;
 389     }
 390
 391     /* We need to yield even for delay_ns = 0 so that bdrv_drain_all() can
 392      * return. Without a yield, the VM would not reboot. */
 393     delay_ns = block_job_ratelimit_get_delay(&job->common, job->bytes_read);
 394     job->bytes_read = 0;
 395     job_sleep_ns(&job->common.job, delay_ns);
 396
 397     if (job_is_cancelled(&job->common.job)) {
 398         return true;
 399     }
 400
 401     return false;
 402 }
 403
 404 static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
 405 {
 406     int ret;
 407     bool error_is_read;
 408     int64_t cluster;
 409     HBitmapIter hbi;
 410
 411     hbitmap_iter_init(&hbi, job->copy_bitmap, 0);
 412     while ((cluster = hbitmap_iter_next(&hbi, true)) != -1) {
 413         do {
 414             if (yield_and_check(job)) {
 415                 return 0;
 416             }
 417             ret = backup_do_cow(job, cluster * job->cluster_size,
 418                                 job->cluster_size, &error_is_read, false);
 419             if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
 420                            BLOCK_ERROR_ACTION_REPORT)
 421             {
 422                 return ret;
 423             }
 424         } while (ret < 0);
 425     }
 426
 427     return 0;
 428 }
 429
 430 /* init copy_bitmap from sync_bitmap */
 431 static void backup_incremental_init_copy_bitmap(BackupBlockJob *job)
 432 {
 433     BdrvDirtyBitmapIter *dbi;
 434     int64_t offset;
 435     int64_t end = DIV_ROUND_UP(bdrv_dirty_bitmap_size(job->sync_bitmap),
 436                                job->cluster_size);
 437
 438     dbi = bdrv_dirty_iter_new(job->sync_bitmap);
 439     while ((offset = bdrv_dirty_iter_next(dbi)) != -1) {
 440         int64_t cluster = offset / job->cluster_size;
 441         int64_t next_cluster;
 442
 443         offset += bdrv_dirty_bitmap_granularity(job->sync_bitmap);
 444         if (offset >= bdrv_dirty_bitmap_size(job->sync_bitmap)) {
 445             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 446             break;
 447         }
 448
 449         offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset);
 450         if (offset == -1) {
 451             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 452             break;
 453         }
 454
 455         next_cluster = DIV_ROUND_UP(offset, job->cluster_size);
 456         hbitmap_set(job->copy_bitmap, cluster, next_cluster - cluster);
 457         if (next_cluster >= end) {
 458             break;
 459         }
 460
 461         bdrv_set_dirty_iter(dbi, next_cluster * job->cluster_size);
 462     }
 463
 464     /* TODO job_progress_set_remaining() would make more sense */
 465     job_progress_update(&job->common.job,
 466         job->len - hbitmap_count(job->copy_bitmap) * job->cluster_size);
 467
 468     bdrv_dirty_iter_free(dbi);
 469 }
 470
 471 static int coroutine_fn backup_run(Job *job, Error **errp)
 472 {
 473     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 474     BlockDriverState *bs = blk_bs(s->common.blk);
 475     int64_t offset, nb_clusters;
 476     int ret = 0;
 477
 478     QLIST_INIT(&s->inflight_reqs);
 479     qemu_co_rwlock_init(&s->flush_rwlock);
 480
 481     nb_clusters = DIV_ROUND_UP(s->len, s->cluster_size);
 482     job_progress_set_remaining(job, s->len);
 483
 484     s->copy_bitmap = hbitmap_alloc(nb_clusters, 0);
 485     if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 486         backup_incremental_init_copy_bitmap(s);
 487     } else {
 488         hbitmap_set(s->copy_bitmap, 0, nb_clusters);
 489     }
 490
 491
 492     s->before_write.notify = backup_before_write_notify;
 493     bdrv_add_before_write_notifier(bs, &s->before_write);
 494
 495     if (s->sync_mode == MIRROR_SYNC_MODE_NONE) {
 496         /* All bits are set in copy_bitmap to allow any cluster to be copied.
 497          * This does not actually require them to be copied. */
 498         while (!job_is_cancelled(job)) {
 499             /* Yield until the job is cancelled.  We just let our before_write
 500              * notify callback service CoW requests. */
 501             job_yield(job);
 502         }
 503     } else if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 504         ret = backup_run_incremental(s);
 505     } else {
 506         /* Both FULL and TOP SYNC_MODE's require copying.. */
 507         for (offset = 0; offset < s->len;
 508              offset += s->cluster_size) {
 509             bool error_is_read;
 510             int alloced = 0;
 511
 512             if (yield_and_check(s)) {
 513                 break;
 514             }
 515
 516             if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
 517                 int i;
 518                 int64_t n;
 519
 520                 /* Check to see if these blocks are already in the
 521                  * backing file. */
 522
 523                 for (i = 0; i < s->cluster_size;) {
 524                     /* bdrv_is_allocated() only returns true/false based
 525                      * on the first set of sectors it comes across that
 526                      * are are all in the same state.
 527                      * For that reason we must verify each sector in the
 528                      * backup cluster length.  We end up copying more than
 529                      * needed but at some point that is always the case. */
 530                     alloced =
 531                         bdrv_is_allocated(bs, offset + i,
 532                                           s->cluster_size - i, &n);
 533                     i += n;
 534
 535                     if (alloced || n == 0) {
 536                         break;
 537                     }
 538                 }
 539
 540                 /* If the above loop never found any sectors that are in
 541                  * the topmost image, skip this backup. */
 542                 if (alloced == 0) {
 543                     continue;
 544                 }
 545             }
 546             /* FULL sync mode we copy the whole drive. */
 547             if (alloced < 0) {
 548                 ret = alloced;
 549             } else {
 550                 ret = backup_do_cow(s, offset, s->cluster_size,
 551                                     &error_is_read, false);
 552             }
 553             if (ret < 0) {
 554                 /* Depending on error action, fail now or retry cluster */
 555                 BlockErrorAction action =
 556                     backup_error_action(s, error_is_read, -ret);
 557                 if (action == BLOCK_ERROR_ACTION_REPORT) {
 558                     break;
 559                 } else {
 560                     offset -= s->cluster_size;
 561                     continue;
 562                 }
 563             }
 564         }
 565     }
 566
 567     notifier_with_return_remove(&s->before_write);
 568
 569     /* wait until pending backup_do_cow() calls have completed */
 570     qemu_co_rwlock_wrlock(&s->flush_rwlock);
 571     qemu_co_rwlock_unlock(&s->flush_rwlock);
 572     hbitmap_free(s->copy_bitmap);
 573
 574     return ret;
 575 }
 576
 577 static const BlockJobDriver backup_job_driver = {
 578     .job_driver = {
 579         .instance_size          = sizeof(BackupBlockJob),
 580         .job_type               = JOB_TYPE_BACKUP,
 581         .free                   = block_job_free,
 582         .user_resume            = block_job_user_resume,
 583         .drain                  = block_job_drain,
 584         .run                    = backup_run,
 585         .commit                 = backup_commit,
 586         .abort                  = backup_abort,
 587         .clean                  = backup_clean,
 588     },
 589     .attached_aio_context   = backup_attached_aio_context,
 590     .drain                  = backup_drain,
 591 };
 592
 593 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
 594                   BlockDriverState *target, int64_t speed,
 595                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
 596                   bool compress,
 597                   BlockdevOnError on_source_error,
 598                   BlockdevOnError on_target_error,
 599                   int creation_flags,
 600                   BlockCompletionFunc *cb, void *opaque,
 601                   JobTxn *txn, Error **errp)
 602 {
 603     int64_t len;
 604     BlockDriverInfo bdi;
 605     BackupBlockJob *job = NULL;
 606     int ret;
 607
 608     assert(bs);
 609     assert(target);
 610
 611     if (bs == target) {
 612         error_setg(errp, "Source and target cannot be the same");
 613         return NULL;
 614     }
 615
 616     if (!bdrv_is_inserted(bs)) {
 617         error_setg(errp, "Device is not inserted: %s",
 618                    bdrv_get_device_name(bs));
 619         return NULL;
 620     }
 621
 622     if (!bdrv_is_inserted(target)) {
 623         error_setg(errp, "Device is not inserted: %s",
 624                    bdrv_get_device_name(target));
 625         return NULL;
 626     }
 627
 628     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
 629         error_setg(errp, "Compression is not supported for this drive %s",
 630                    bdrv_get_device_name(target));
 631         return NULL;
 632     }
 633
 634     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
 635         return NULL;
 636     }
 637
 638     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
 639         return NULL;
 640     }
 641
 642     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 643         if (!sync_bitmap) {
 644             error_setg(errp, "must provide a valid bitmap name for "
 645                              "\"incremental\" sync mode");
 646             return NULL;
 647         }
 648
 649         /* Create a new bitmap, and freeze/disable this one. */
 650         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
 651             return NULL;
 652         }
 653     } else if (sync_bitmap) {
 654         error_setg(errp,
 655                    "a sync_bitmap was provided to backup_run, "
 656                    "but received an incompatible sync_mode (%s)",
 657                    MirrorSyncMode_str(sync_mode));
 658         return NULL;
 659     }
 660
 661     len = bdrv_getlength(bs);
 662     if (len < 0) {
 663         error_setg_errno(errp, -len, "unable to get length for '%s'",
 664                          bdrv_get_device_name(bs));
 665         goto error;
 666     }
 667
 668     /* job->len is fixed, so we can't allow resize */
 669     job = block_job_create(job_id, &backup_job_driver, txn, bs,
 670                            BLK_PERM_CONSISTENT_READ,
 671                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 672                            BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD,
 673                            speed, creation_flags, cb, opaque, errp);
 674     if (!job) {
 675         goto error;
 676     }
 677
 678     /* The target must match the source in size, so no resize here either */
 679     job->target = blk_new(BLK_PERM_WRITE,
 680                           BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 681                           BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD);
 682     ret = blk_insert_bs(job->target, target, errp);
 683     if (ret < 0) {
 684         goto error;
 685     }
 686
 687     job->on_source_error = on_source_error;
 688     job->on_target_error = on_target_error;
 689     job->sync_mode = sync_mode;
 690     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
 691                        sync_bitmap : NULL;
 692     job->compress = compress;
 693
 694     /* Detect image-fleecing (and similar) schemes */
 695     job->serialize_target_writes = bdrv_chain_contains(target, bs);
 696
 697     /* If there is no backing file on the target, we cannot rely on COW if our
 698      * backup cluster size is smaller than the target cluster size. Even for
 699      * targets with a backing file, try to avoid COW if possible. */
 700     ret = bdrv_get_info(target, &bdi);
 701     if (ret == -ENOTSUP && !target->backing) {
 702         /* Cluster size is not defined */
 703         warn_report("The target block device doesn't provide "
 704                     "information about the block size and it doesn't have a "
 705                     "backing file. The default block size of %u bytes is "
 706                     "used. If the actual block size of the target exceeds "
 707                     "this default, the backup may be unusable",
 708                     BACKUP_CLUSTER_SIZE_DEFAULT);
 709         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 710     } else if (ret < 0 && !target->backing) {
 711         error_setg_errno(errp, -ret,
 712             "Couldn't determine the cluster size of the target image, "
 713             "which has no backing file");
 714         error_append_hint(errp,
 715             "Aborting, since this may create an unusable destination image\n");
 716         goto error;
 717     } else if (ret < 0 && target->backing) {
 718         /* Not fatal; just trudge on ahead. */
 719         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 720     } else {
 721         job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
 722     }
 723     job->use_copy_range = true;
 724     job->copy_range_size = MIN_NON_ZERO(blk_get_max_transfer(job->common.blk),
 725                                         blk_get_max_transfer(job->target));
 726     job->copy_range_size = MAX(job->cluster_size,
 727                                QEMU_ALIGN_UP(job->copy_range_size,
 728                                              job->cluster_size));
 729
 730     /* Required permissions are already taken with target's blk_new() */
 731     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
 732                        &error_abort);
 733     job->len = len;
 734
 735     return &job->common;
 736
 737  error:
 738     if (sync_bitmap) {
 739         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
 740     }
 741     if (job) {
 742         backup_clean(&job->common.job);
 743         job_early_fail(&job->common.job);
 744     }
 745
 746     return NULL;
 747 }