block/backup.c - third_party/qemu - Git at Google

 /*
  * QEMU backup
  *
  * Copyright (C) 2013 Proxmox Server Solutions
  * Copyright (c) 2019 Virtuozzo International GmbH.
  *
  * Authors:
  *  Dietmar Maurer (dietmar@proxmox.com)
  *
  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  * See the COPYING file in the top-level directory.
  *
  */

 #include "qemu/osdep.h"

 #include "trace.h"
 #include "block/block.h"
 #include "block/block_int.h"
 #include "block/blockjob_int.h"
 #include "block/block_backup.h"
 #include "block/block-copy.h"
 #include "qapi/error.h"
 #include "qapi/qmp/qerror.h"
 #include "qemu/ratelimit.h"
 #include "qemu/cutils.h"
 #include "sysemu/block-backend.h"
 #include "qemu/bitmap.h"
 #include "qemu/error-report.h"

 #include "block/backup-top.h"

 #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)

 typedef struct BackupBlockJob {
     BlockJob common;
     BlockDriverState *backup_top;
     BlockDriverState *source_bs;

     BdrvDirtyBitmap *sync_bitmap;

     MirrorSyncMode sync_mode;
     BitmapSyncMode bitmap_mode;
     BlockdevOnError on_source_error;
     BlockdevOnError on_target_error;
     uint64_t len;
     uint64_t bytes_read;
     int64_t cluster_size;

     BlockCopyState *bcs;
 } BackupBlockJob;

 static const BlockJobDriver backup_job_driver;

 static void backup_progress_bytes_callback(int64_t bytes, void *opaque)
 {
     BackupBlockJob *s = opaque;

     s->bytes_read += bytes;
 }

 static int coroutine_fn backup_do_cow(BackupBlockJob *job,
                                       int64_t offset, uint64_t bytes,
                                       bool *error_is_read)
 {
     int ret = 0;
     int64_t start, end; /* bytes */

     start = QEMU_ALIGN_DOWN(offset, job->cluster_size);
     end = QEMU_ALIGN_UP(bytes + offset, job->cluster_size);

     trace_backup_do_cow_enter(job, start, offset, bytes);

     ret = block_copy(job->bcs, start, end - start, error_is_read);

     trace_backup_do_cow_return(job, offset, bytes, ret);

     return ret;
 }

 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 {
     BdrvDirtyBitmap *bm;
     bool sync = (((ret == 0) || (job->bitmap_mode == BITMAP_SYNC_MODE_ALWAYS)) \
                  && (job->bitmap_mode != BITMAP_SYNC_MODE_NEVER));

     if (sync) {
         /*
          * We succeeded, or we always intended to sync the bitmap.
          * Delete this bitmap and install the child.
          */
         bm = bdrv_dirty_bitmap_abdicate(job->sync_bitmap, NULL);
     } else {
         /*
          * We failed, or we never intended to sync the bitmap anyway.
          * Merge the successor back into the parent, keeping all data.
          */
         bm = bdrv_reclaim_dirty_bitmap(job->sync_bitmap, NULL);
     }

     assert(bm);

     if (ret < 0 && job->bitmap_mode == BITMAP_SYNC_MODE_ALWAYS) {
         /* If we failed and synced, merge in the bits we didn't copy: */
         bdrv_dirty_bitmap_merge_internal(bm, block_copy_dirty_bitmap(job->bcs),
                                          NULL, true);
     }
 }

 static void backup_commit(Job *job)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
     if (s->sync_bitmap) {
         backup_cleanup_sync_bitmap(s, 0);
     }
 }

 static void backup_abort(Job *job)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
     if (s->sync_bitmap) {
         backup_cleanup_sync_bitmap(s, -1);
     }
 }

 static void backup_clean(Job *job)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
     bdrv_backup_top_drop(s->backup_top);
 }

 void backup_do_checkpoint(BlockJob *job, Error **errp)
 {
     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);

     assert(block_job_driver(job) == &backup_job_driver);

     if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
         error_setg(errp, "The backup job only supports block checkpoint in"
                    " sync=none mode");
         return;
     }

     bdrv_set_dirty_bitmap(block_copy_dirty_bitmap(backup_job->bcs), 0,
                           backup_job->len);
 }

 static BlockErrorAction backup_error_action(BackupBlockJob *job,
                                             bool read, int error)
 {
     if (read) {
         return block_job_error_action(&job->common, job->on_source_error,
                                       true, error);
     } else {
         return block_job_error_action(&job->common, job->on_target_error,
                                       false, error);
     }
 }

 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 {
     uint64_t delay_ns;

     if (job_is_cancelled(&job->common.job)) {
         return true;
     }

     /*
      * We need to yield even for delay_ns = 0 so that bdrv_drain_all() can
      * return. Without a yield, the VM would not reboot.
      */
     delay_ns = block_job_ratelimit_get_delay(&job->common, job->bytes_read);
     job->bytes_read = 0;
     job_sleep_ns(&job->common.job, delay_ns);

     if (job_is_cancelled(&job->common.job)) {
         return true;
     }

     return false;
 }

 static int coroutine_fn backup_loop(BackupBlockJob *job)
 {
     bool error_is_read;
     int64_t offset;
     BdrvDirtyBitmapIter *bdbi;
     int ret = 0;

     bdbi = bdrv_dirty_iter_new(block_copy_dirty_bitmap(job->bcs));
     while ((offset = bdrv_dirty_iter_next(bdbi)) != -1) {
         do {
             if (yield_and_check(job)) {
                 goto out;
             }
             ret = backup_do_cow(job, offset, job->cluster_size, &error_is_read);
             if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
                            BLOCK_ERROR_ACTION_REPORT)
             {
                 goto out;
             }
         } while (ret < 0);
     }

  out:
     bdrv_dirty_iter_free(bdbi);
     return ret;
 }

 static void backup_init_bcs_bitmap(BackupBlockJob *job)
 {
     bool ret;
     uint64_t estimate;
     BdrvDirtyBitmap *bcs_bitmap = block_copy_dirty_bitmap(job->bcs);

     if (job->sync_mode == MIRROR_SYNC_MODE_BITMAP) {
         ret = bdrv_dirty_bitmap_merge_internal(bcs_bitmap, job->sync_bitmap,
                                                NULL, true);
         assert(ret);
     } else {
         if (job->sync_mode == MIRROR_SYNC_MODE_TOP) {
             /*
              * We can't hog the coroutine to initialize this thoroughly.
              * Set a flag and resume work when we are able to yield safely.
              */
             block_copy_set_skip_unallocated(job->bcs, true);
         }
         bdrv_set_dirty_bitmap(bcs_bitmap, 0, job->len);
     }

     estimate = bdrv_get_dirty_count(bcs_bitmap);
     job_progress_set_remaining(&job->common.job, estimate);
 }

 static int coroutine_fn backup_run(Job *job, Error **errp)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
     int ret = 0;

     backup_init_bcs_bitmap(s);

     if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
         int64_t offset = 0;
         int64_t count;

         for (offset = 0; offset < s->len; ) {
             if (yield_and_check(s)) {
                 ret = -ECANCELED;
                 goto out;
             }

             ret = block_copy_reset_unallocated(s->bcs, offset, &count);
             if (ret < 0) {
                 goto out;
             }

             offset += count;
         }
         block_copy_set_skip_unallocated(s->bcs, false);
     }

     if (s->sync_mode == MIRROR_SYNC_MODE_NONE) {
         /*
          * All bits are set in bcs bitmap to allow any cluster to be copied.
          * This does not actually require them to be copied.
          */
         while (!job_is_cancelled(job)) {
             /*
              * Yield until the job is cancelled.  We just let our before_write
              * notify callback service CoW requests.
              */
             job_yield(job);
         }
     } else {
         ret = backup_loop(s);
     }

  out:
     return ret;
 }

 static const BlockJobDriver backup_job_driver = {
     .job_driver = {
         .instance_size          = sizeof(BackupBlockJob),
         .job_type               = JOB_TYPE_BACKUP,
         .free                   = block_job_free,
         .user_resume            = block_job_user_resume,
         .run                    = backup_run,
         .commit                 = backup_commit,
         .abort                  = backup_abort,
         .clean                  = backup_clean,
     }
 };

 static int64_t backup_calculate_cluster_size(BlockDriverState *target,
                                              Error **errp)
 {
     int ret;
     BlockDriverInfo bdi;

     /*
      * If there is no backing file on the target, we cannot rely on COW if our
      * backup cluster size is smaller than the target cluster size. Even for
      * targets with a backing file, try to avoid COW if possible.
      */
     ret = bdrv_get_info(target, &bdi);
     if (ret == -ENOTSUP && !target->backing) {
         /* Cluster size is not defined */
         warn_report("The target block device doesn't provide "
                     "information about the block size and it doesn't have a "
                     "backing file. The default block size of %u bytes is "
                     "used. If the actual block size of the target exceeds "
                     "this default, the backup may be unusable",
                     BACKUP_CLUSTER_SIZE_DEFAULT);
         return BACKUP_CLUSTER_SIZE_DEFAULT;
     } else if (ret < 0 && !target->backing) {
         error_setg_errno(errp, -ret,
             "Couldn't determine the cluster size of the target image, "
             "which has no backing file");
         error_append_hint(errp,
             "Aborting, since this may create an unusable destination image\n");
         return ret;
     } else if (ret < 0 && target->backing) {
         /* Not fatal; just trudge on ahead. */
         return BACKUP_CLUSTER_SIZE_DEFAULT;
     }

     return MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
 }

 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *target, int64_t speed,
                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
                   BitmapSyncMode bitmap_mode,
                   bool compress,
                   const char *filter_node_name,
                   BlockdevOnError on_source_error,
                   BlockdevOnError on_target_error,
                   int creation_flags,
                   BlockCompletionFunc *cb, void *opaque,
                   JobTxn *txn, Error **errp)
 {
     int64_t len;
     BackupBlockJob *job = NULL;
     int64_t cluster_size;
     BdrvRequestFlags write_flags;
     BlockDriverState *backup_top = NULL;
     BlockCopyState *bcs = NULL;

     assert(bs);
     assert(target);

     /* QMP interface protects us from these cases */
     assert(sync_mode != MIRROR_SYNC_MODE_INCREMENTAL);
     assert(sync_bitmap || sync_mode != MIRROR_SYNC_MODE_BITMAP);

     if (bs == target) {
         error_setg(errp, "Source and target cannot be the same");
         return NULL;
     }

     if (!bdrv_is_inserted(bs)) {
         error_setg(errp, "Device is not inserted: %s",
                    bdrv_get_device_name(bs));
         return NULL;
     }

     if (!bdrv_is_inserted(target)) {
         error_setg(errp, "Device is not inserted: %s",
                    bdrv_get_device_name(target));
         return NULL;
     }

     if (compress && !block_driver_can_compress(target->drv)) {
         error_setg(errp, "Compression is not supported for this drive %s",
                    bdrv_get_device_name(target));
         return NULL;
     }

     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
         return NULL;
     }

     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
         return NULL;
     }

     if (sync_bitmap) {
         /* If we need to write to this bitmap, check that we can: */
         if (bitmap_mode != BITMAP_SYNC_MODE_NEVER &&
             bdrv_dirty_bitmap_check(sync_bitmap, BDRV_BITMAP_DEFAULT, errp)) {
             return NULL;
         }

         /* Create a new bitmap, and freeze/disable this one. */
         if (bdrv_dirty_bitmap_create_successor(sync_bitmap, errp) < 0) {
             return NULL;
         }
     }

     len = bdrv_getlength(bs);
     if (len < 0) {
         error_setg_errno(errp, -len, "unable to get length for '%s'",
                          bdrv_get_device_name(bs));
         goto error;
     }

     cluster_size = backup_calculate_cluster_size(target, errp);
     if (cluster_size < 0) {
         goto error;
     }

     /*
      * If source is in backing chain of target assume that target is going to be
      * used for "image fleecing", i.e. it should represent a kind of snapshot of
      * source at backup-start point in time. And target is going to be read by
      * somebody (for example, used as NBD export) during backup job.
      *
      * In this case, we need to add BDRV_REQ_SERIALISING write flag to avoid
      * intersection of backup writes and third party reads from target,
      * otherwise reading from target we may occasionally read already updated by
      * guest data.
      *
      * For more information see commit f8d59dfb40bb and test
      * tests/qemu-iotests/222
      */
     write_flags = (bdrv_chain_contains(target, bs) ? BDRV_REQ_SERIALISING : 0) |
                   (compress ? BDRV_REQ_WRITE_COMPRESSED : 0),

     backup_top = bdrv_backup_top_append(bs, target, filter_node_name,
                                         cluster_size, write_flags, &bcs, errp);
     if (!backup_top) {
         goto error;
     }

     /* job->len is fixed, so we can't allow resize */
     job = block_job_create(job_id, &backup_job_driver, txn, backup_top,
                            0, BLK_PERM_ALL,
                            speed, creation_flags, cb, opaque, errp);
     if (!job) {
         goto error;
     }

     job->backup_top = backup_top;
     job->source_bs = bs;
     job->on_source_error = on_source_error;
     job->on_target_error = on_target_error;
     job->sync_mode = sync_mode;
     job->sync_bitmap = sync_bitmap;
     job->bitmap_mode = bitmap_mode;
     job->bcs = bcs;
     job->cluster_size = cluster_size;
     job->len = len;

     block_copy_set_progress_callback(bcs, backup_progress_bytes_callback, job);
     block_copy_set_progress_meter(bcs, &job->common.job.progress);

     /* Required permissions are already taken by backup-top target */
     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
                        &error_abort);

     return &job->common;

  error:
     if (sync_bitmap) {
         bdrv_reclaim_dirty_bitmap(sync_bitmap, NULL);
     }
     if (backup_top) {
         bdrv_backup_top_drop(backup_top);
     }

     return NULL;
 }
	/*
	* QEMU backup
	*
	* Copyright (C) 2013 Proxmox Server Solutions
	* Copyright (c) 2019 Virtuozzo International GmbH.
	*
	* Authors:
	* Dietmar Maurer (dietmar@proxmox.com)
	*
	* This work is licensed under the terms of the GNU GPL, version 2 or later.
	* See the COPYING file in the top-level directory.
	*
	*/

	#include "qemu/osdep.h"

	#include "trace.h"
	#include "block/block.h"
	#include "block/block_int.h"
	#include "block/blockjob_int.h"
	#include "block/block_backup.h"
	#include "block/block-copy.h"
	#include "qapi/error.h"
	#include "qapi/qmp/qerror.h"
	#include "qemu/ratelimit.h"
	#include "qemu/cutils.h"
	#include "sysemu/block-backend.h"
	#include "qemu/bitmap.h"
	#include "qemu/error-report.h"

	#include "block/backup-top.h"

	#define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)

	typedef struct BackupBlockJob {
	BlockJob common;
	BlockDriverState *backup_top;
	BlockDriverState *source_bs;

	BdrvDirtyBitmap *sync_bitmap;

	MirrorSyncMode sync_mode;
	BitmapSyncMode bitmap_mode;
	BlockdevOnError on_source_error;
	BlockdevOnError on_target_error;
	uint64_t len;
	uint64_t bytes_read;
	int64_t cluster_size;

	BlockCopyState *bcs;
	} BackupBlockJob;

	static const BlockJobDriver backup_job_driver;

	static void backup_progress_bytes_callback(int64_t bytes, void *opaque)
	{
	BackupBlockJob *s = opaque;

	s->bytes_read += bytes;
	}

	static int coroutine_fn backup_do_cow(BackupBlockJob *job,
	int64_t offset, uint64_t bytes,
	bool *error_is_read)
	{
	int ret = 0;
	int64_t start, end; /* bytes */

	start = QEMU_ALIGN_DOWN(offset, job->cluster_size);
	end = QEMU_ALIGN_UP(bytes + offset, job->cluster_size);

	trace_backup_do_cow_enter(job, start, offset, bytes);

	ret = block_copy(job->bcs, start, end - start, error_is_read);

	trace_backup_do_cow_return(job, offset, bytes, ret);

	return ret;
	}

	static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
	{
	BdrvDirtyBitmap *bm;
	bool sync = (((ret == 0) \|\| (job->bitmap_mode == BITMAP_SYNC_MODE_ALWAYS)) \
	&& (job->bitmap_mode != BITMAP_SYNC_MODE_NEVER));

	if (sync) {
	/*
	* We succeeded, or we always intended to sync the bitmap.
	* Delete this bitmap and install the child.
	*/
	bm = bdrv_dirty_bitmap_abdicate(job->sync_bitmap, NULL);
	} else {
	/*
	* We failed, or we never intended to sync the bitmap anyway.
	* Merge the successor back into the parent, keeping all data.
	*/
	bm = bdrv_reclaim_dirty_bitmap(job->sync_bitmap, NULL);
	}

	assert(bm);

	if (ret < 0 && job->bitmap_mode == BITMAP_SYNC_MODE_ALWAYS) {
	/* If we failed and synced, merge in the bits we didn't copy: */
	bdrv_dirty_bitmap_merge_internal(bm, block_copy_dirty_bitmap(job->bcs),
	NULL, true);
	}
	}

	static void backup_commit(Job *job)
	{
	BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
	if (s->sync_bitmap) {
	backup_cleanup_sync_bitmap(s, 0);
	}
	}

	static void backup_abort(Job *job)
	{
	BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
	if (s->sync_bitmap) {
	backup_cleanup_sync_bitmap(s, -1);
	}
	}

	static void backup_clean(Job *job)
	{
	BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
	bdrv_backup_top_drop(s->backup_top);
	}

	void backup_do_checkpoint(BlockJob job, Error *errp)
	{
	BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);

	assert(block_job_driver(job) == &backup_job_driver);

	if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
	error_setg(errp, "The backup job only supports block checkpoint in"
	" sync=none mode");
	return;
	}

	bdrv_set_dirty_bitmap(block_copy_dirty_bitmap(backup_job->bcs), 0,
	backup_job->len);
	}

	static BlockErrorAction backup_error_action(BackupBlockJob *job,
	bool read, int error)
	{
	if (read) {
	return block_job_error_action(&job->common, job->on_source_error,
	true, error);
	} else {
	return block_job_error_action(&job->common, job->on_target_error,
	false, error);
	}
	}

	static bool coroutine_fn yield_and_check(BackupBlockJob *job)
	{
	uint64_t delay_ns;

	if (job_is_cancelled(&job->common.job)) {
	return true;
	}

	/*
	* We need to yield even for delay_ns = 0 so that bdrv_drain_all() can
	* return. Without a yield, the VM would not reboot.
	*/
	delay_ns = block_job_ratelimit_get_delay(&job->common, job->bytes_read);
	job->bytes_read = 0;
	job_sleep_ns(&job->common.job, delay_ns);

	if (job_is_cancelled(&job->common.job)) {
	return true;
	}

	return false;
	}

	static int coroutine_fn backup_loop(BackupBlockJob *job)
	{
	bool error_is_read;
	int64_t offset;
	BdrvDirtyBitmapIter *bdbi;
	int ret = 0;

	bdbi = bdrv_dirty_iter_new(block_copy_dirty_bitmap(job->bcs));
	while ((offset = bdrv_dirty_iter_next(bdbi)) != -1) {
	do {
	if (yield_and_check(job)) {
	goto out;
	}
	ret = backup_do_cow(job, offset, job->cluster_size, &error_is_read);
	if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
	BLOCK_ERROR_ACTION_REPORT)
	{
	goto out;
	}
	} while (ret < 0);
	}

	out:
	bdrv_dirty_iter_free(bdbi);
	return ret;
	}

	static void backup_init_bcs_bitmap(BackupBlockJob *job)
	{
	bool ret;
	uint64_t estimate;
	BdrvDirtyBitmap *bcs_bitmap = block_copy_dirty_bitmap(job->bcs);

	if (job->sync_mode == MIRROR_SYNC_MODE_BITMAP) {
	ret = bdrv_dirty_bitmap_merge_internal(bcs_bitmap, job->sync_bitmap,
	NULL, true);
	assert(ret);
	} else {
	if (job->sync_mode == MIRROR_SYNC_MODE_TOP) {
	/*
	* We can't hog the coroutine to initialize this thoroughly.
	* Set a flag and resume work when we are able to yield safely.
	*/
	block_copy_set_skip_unallocated(job->bcs, true);
	}
	bdrv_set_dirty_bitmap(bcs_bitmap, 0, job->len);
	}

	estimate = bdrv_get_dirty_count(bcs_bitmap);
	job_progress_set_remaining(&job->common.job, estimate);
	}

	static int coroutine_fn backup_run(Job job, Error *errp)
	{
	BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
	int ret = 0;

	backup_init_bcs_bitmap(s);

	if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
	int64_t offset = 0;
	int64_t count;

	for (offset = 0; offset < s->len; ) {
	if (yield_and_check(s)) {
	ret = -ECANCELED;
	goto out;
	}

	ret = block_copy_reset_unallocated(s->bcs, offset, &count);
	if (ret < 0) {
	goto out;
	}

	offset += count;
	}
	block_copy_set_skip_unallocated(s->bcs, false);
	}

	if (s->sync_mode == MIRROR_SYNC_MODE_NONE) {
	/*
	* All bits are set in bcs bitmap to allow any cluster to be copied.
	* This does not actually require them to be copied.
	*/
	while (!job_is_cancelled(job)) {
	/*
	* Yield until the job is cancelled. We just let our before_write
	* notify callback service CoW requests.
	*/
	job_yield(job);
	}
	} else {
	ret = backup_loop(s);
	}

	out:
	return ret;
	}

	static const BlockJobDriver backup_job_driver = {
	.job_driver = {
	.instance_size = sizeof(BackupBlockJob),
	.job_type = JOB_TYPE_BACKUP,
	.free = block_job_free,
	.user_resume = block_job_user_resume,
	.run = backup_run,
	.commit = backup_commit,
	.abort = backup_abort,
	.clean = backup_clean,
	}
	};

	static int64_t backup_calculate_cluster_size(BlockDriverState *target,
	Error **errp)
	{
	int ret;
	BlockDriverInfo bdi;

	/*
	* If there is no backing file on the target, we cannot rely on COW if our
	* backup cluster size is smaller than the target cluster size. Even for
	* targets with a backing file, try to avoid COW if possible.
	*/
	ret = bdrv_get_info(target, &bdi);
	if (ret == -ENOTSUP && !target->backing) {
	/* Cluster size is not defined */
	warn_report("The target block device doesn't provide "
	"information about the block size and it doesn't have a "
	"backing file. The default block size of %u bytes is "
	"used. If the actual block size of the target exceeds "
	"this default, the backup may be unusable",
	BACKUP_CLUSTER_SIZE_DEFAULT);
	return BACKUP_CLUSTER_SIZE_DEFAULT;
	} else if (ret < 0 && !target->backing) {
	error_setg_errno(errp, -ret,
	"Couldn't determine the cluster size of the target image, "
	"which has no backing file");
	error_append_hint(errp,
	"Aborting, since this may create an unusable destination image\n");
	return ret;
	} else if (ret < 0 && target->backing) {
	/* Not fatal; just trudge on ahead. */
	return BACKUP_CLUSTER_SIZE_DEFAULT;
	}

	return MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
	}

	BlockJob backup_job_create(const char job_id, BlockDriverState *bs,
	BlockDriverState *target, int64_t speed,
	MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
	BitmapSyncMode bitmap_mode,
	bool compress,
	const char *filter_node_name,
	BlockdevOnError on_source_error,
	BlockdevOnError on_target_error,
	int creation_flags,
	BlockCompletionFunc cb, void opaque,
	JobTxn txn, Error *errp)
	{
	int64_t len;
	BackupBlockJob *job = NULL;
	int64_t cluster_size;
	BdrvRequestFlags write_flags;
	BlockDriverState *backup_top = NULL;
	BlockCopyState *bcs = NULL;

	assert(bs);
	assert(target);

	/* QMP interface protects us from these cases */
	assert(sync_mode != MIRROR_SYNC_MODE_INCREMENTAL);
	assert(sync_bitmap \|\| sync_mode != MIRROR_SYNC_MODE_BITMAP);

	if (bs == target) {
	error_setg(errp, "Source and target cannot be the same");
	return NULL;
	}

	if (!bdrv_is_inserted(bs)) {
	error_setg(errp, "Device is not inserted: %s",
	bdrv_get_device_name(bs));
	return NULL;
	}

	if (!bdrv_is_inserted(target)) {
	error_setg(errp, "Device is not inserted: %s",
	bdrv_get_device_name(target));
	return NULL;
	}

	if (compress && !block_driver_can_compress(target->drv)) {
	error_setg(errp, "Compression is not supported for this drive %s",
	bdrv_get_device_name(target));
	return NULL;
	}

	if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
	return NULL;
	}

	if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
	return NULL;
	}

	if (sync_bitmap) {
	/* If we need to write to this bitmap, check that we can: */
	if (bitmap_mode != BITMAP_SYNC_MODE_NEVER &&
	bdrv_dirty_bitmap_check(sync_bitmap, BDRV_BITMAP_DEFAULT, errp)) {
	return NULL;
	}

	/* Create a new bitmap, and freeze/disable this one. */
	if (bdrv_dirty_bitmap_create_successor(sync_bitmap, errp) < 0) {
	return NULL;
	}
	}

	len = bdrv_getlength(bs);
	if (len < 0) {
	error_setg_errno(errp, -len, "unable to get length for '%s'",
	bdrv_get_device_name(bs));
	goto error;
	}

	cluster_size = backup_calculate_cluster_size(target, errp);
	if (cluster_size < 0) {
	goto error;
	}

	/*
	* If source is in backing chain of target assume that target is going to be
	* used for "image fleecing", i.e. it should represent a kind of snapshot of
	* source at backup-start point in time. And target is going to be read by
	* somebody (for example, used as NBD export) during backup job.
	*
	* In this case, we need to add BDRV_REQ_SERIALISING write flag to avoid
	* intersection of backup writes and third party reads from target,
	* otherwise reading from target we may occasionally read already updated by
	* guest data.
	*
	* For more information see commit f8d59dfb40bb and test
	* tests/qemu-iotests/222
	*/
	write_flags = (bdrv_chain_contains(target, bs) ? BDRV_REQ_SERIALISING : 0) \|
	(compress ? BDRV_REQ_WRITE_COMPRESSED : 0),

	backup_top = bdrv_backup_top_append(bs, target, filter_node_name,
	cluster_size, write_flags, &bcs, errp);
	if (!backup_top) {
	goto error;
	}

	/* job->len is fixed, so we can't allow resize */
	job = block_job_create(job_id, &backup_job_driver, txn, backup_top,
	0, BLK_PERM_ALL,
	speed, creation_flags, cb, opaque, errp);
	if (!job) {
	goto error;
	}

	job->backup_top = backup_top;
	job->source_bs = bs;
	job->on_source_error = on_source_error;
	job->on_target_error = on_target_error;
	job->sync_mode = sync_mode;
	job->sync_bitmap = sync_bitmap;
	job->bitmap_mode = bitmap_mode;
	job->bcs = bcs;
	job->cluster_size = cluster_size;
	job->len = len;

	block_copy_set_progress_callback(bcs, backup_progress_bytes_callback, job);
	block_copy_set_progress_meter(bcs, &job->common.job.progress);

	/* Required permissions are already taken by backup-top target */
	block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
	&error_abort);

	return &job->common;

	error:
	if (sync_bitmap) {
	bdrv_reclaim_dirty_bitmap(sync_bitmap, NULL);
	}
	if (backup_top) {
	bdrv_backup_top_drop(backup_top);
	}

	return NULL;
	}