block/backup.c - qemu - Git at Google

 /*
  * QEMU backup
  *
  * Copyright (C) 2013 Proxmox Server Solutions
  *
  * 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 "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"

 #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)
 #define SLICE_TIME 100000000ULL /* ns */

 typedef struct BackupBlockJob {
     BlockJob common;
     BlockBackend *target;
     /* bitmap for sync=incremental */
     BdrvDirtyBitmap *sync_bitmap;
     MirrorSyncMode sync_mode;
     RateLimit limit;
     BlockdevOnError on_source_error;
     BlockdevOnError on_target_error;
     CoRwlock flush_rwlock;
     uint64_t sectors_read;
     unsigned long *done_bitmap;
     int64_t cluster_size;
     bool compress;
     NotifierWithReturn before_write;
     QLIST_HEAD(, CowRequest) inflight_reqs;
 } BackupBlockJob;

 /* Size of a cluster in sectors, instead of bytes. */
 static inline int64_t cluster_size_sectors(BackupBlockJob *job)
 {
   return job->cluster_size / BDRV_SECTOR_SIZE;
 }

 /* See if in-flight requests overlap and wait for them to complete */
 static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
                                                        int64_t start,
                                                        int64_t end)
 {
     CowRequest *req;
     bool retry;

     do {
         retry = false;
         QLIST_FOREACH(req, &job->inflight_reqs, list) {
             if (end > req->start && start < req->end) {
                 qemu_co_queue_wait(&req->wait_queue, NULL);
                 retry = true;
                 break;
             }
         }
     } while (retry);
 }

 /* Keep track of an in-flight request */
 static void cow_request_begin(CowRequest *req, BackupBlockJob *job,
                                      int64_t start, int64_t end)
 {
     req->start = start;
     req->end = end;
     qemu_co_queue_init(&req->wait_queue);
     QLIST_INSERT_HEAD(&job->inflight_reqs, req, list);
 }

 /* Forget about a completed request */
 static void cow_request_end(CowRequest *req)
 {
     QLIST_REMOVE(req, list);
     qemu_co_queue_restart_all(&req->wait_queue);
 }

 static int coroutine_fn backup_do_cow(BackupBlockJob *job,
                                       int64_t sector_num, int nb_sectors,
                                       bool *error_is_read,
                                       bool is_write_notifier)
 {
     BlockBackend *blk = job->common.blk;
     CowRequest cow_request;
     struct iovec iov;
     QEMUIOVector bounce_qiov;
     void *bounce_buffer = NULL;
     int ret = 0;
     int64_t sectors_per_cluster = cluster_size_sectors(job);
     int64_t start, end;
     int n;

     qemu_co_rwlock_rdlock(&job->flush_rwlock);

     start = sector_num / sectors_per_cluster;
     end = DIV_ROUND_UP(sector_num + nb_sectors, sectors_per_cluster);

     trace_backup_do_cow_enter(job, start, sector_num, nb_sectors);

     wait_for_overlapping_requests(job, start, end);
     cow_request_begin(&cow_request, job, start, end);

     for (; start < end; start++) {
         if (test_bit(start, job->done_bitmap)) {
             trace_backup_do_cow_skip(job, start);
             continue; /* already copied */
         }

         trace_backup_do_cow_process(job, start);

         n = MIN(sectors_per_cluster,
                 job->common.len / BDRV_SECTOR_SIZE -
                 start * sectors_per_cluster);

         if (!bounce_buffer) {
             bounce_buffer = blk_blockalign(blk, job->cluster_size);
         }
         iov.iov_base = bounce_buffer;
         iov.iov_len = n * BDRV_SECTOR_SIZE;
         qemu_iovec_init_external(&bounce_qiov, &iov, 1);

         ret = blk_co_preadv(blk, start * job->cluster_size,
                             bounce_qiov.size, &bounce_qiov,
                             is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0);
         if (ret < 0) {
             trace_backup_do_cow_read_fail(job, start, ret);
             if (error_is_read) {
                 *error_is_read = true;
             }
             goto out;
         }

         if (buffer_is_zero(iov.iov_base, iov.iov_len)) {
             ret = blk_co_pwrite_zeroes(job->target, start * job->cluster_size,
                                        bounce_qiov.size, BDRV_REQ_MAY_UNMAP);
         } else {
             ret = blk_co_pwritev(job->target, start * job->cluster_size,
                                  bounce_qiov.size, &bounce_qiov,
                                  job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
         }
         if (ret < 0) {
             trace_backup_do_cow_write_fail(job, start, ret);
             if (error_is_read) {
                 *error_is_read = false;
             }
             goto out;
         }

         set_bit(start, job->done_bitmap);

         /* Publish progress, guest I/O counts as progress too.  Note that the
          * offset field is an opaque progress value, it is not a disk offset.
          */
         job->sectors_read += n;
         job->common.offset += n * BDRV_SECTOR_SIZE;
     }

 out:
     if (bounce_buffer) {
         qemu_vfree(bounce_buffer);
     }

     cow_request_end(&cow_request);

     trace_backup_do_cow_return(job, sector_num, nb_sectors, ret);

     qemu_co_rwlock_unlock(&job->flush_rwlock);

     return ret;
 }

 static int coroutine_fn backup_before_write_notify(
         NotifierWithReturn *notifier,
         void *opaque)
 {
     BackupBlockJob *job = container_of(notifier, BackupBlockJob, before_write);
     BdrvTrackedRequest *req = opaque;
     int64_t sector_num = req->offset >> BDRV_SECTOR_BITS;
     int nb_sectors = req->bytes >> BDRV_SECTOR_BITS;

     assert(req->bs == blk_bs(job->common.blk));
     assert((req->offset & (BDRV_SECTOR_SIZE - 1)) == 0);
     assert((req->bytes & (BDRV_SECTOR_SIZE - 1)) == 0);

     return backup_do_cow(job, sector_num, nb_sectors, NULL, true);
 }

 static void backup_set_speed(BlockJob *job, int64_t speed, Error **errp)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common);

     if (speed < 0) {
         error_setg(errp, QERR_INVALID_PARAMETER, "speed");
         return;
     }
     ratelimit_set_speed(&s->limit, speed / BDRV_SECTOR_SIZE, SLICE_TIME);
 }

 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 {
     BdrvDirtyBitmap *bm;
     BlockDriverState *bs = blk_bs(job->common.blk);

     if (ret < 0 || block_job_is_cancelled(&job->common)) {
         /* Merge the successor back into the parent, delete nothing. */
         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
         assert(bm);
     } else {
         /* Everything is fine, delete this bitmap and install the backup. */
         bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
         assert(bm);
     }
 }

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

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

 static void backup_clean(BlockJob *job)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
     assert(s->target);
     blk_unref(s->target);
     s->target = NULL;
 }

 static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common);

     blk_set_aio_context(s->target, aio_context);
 }

 void backup_do_checkpoint(BlockJob *job, Error **errp)
 {
     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
     int64_t len;

     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);

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

     len = DIV_ROUND_UP(backup_job->common.len, backup_job->cluster_size);
     bitmap_zero(backup_job->done_bitmap, len);
 }

 void backup_wait_for_overlapping_requests(BlockJob *job, int64_t sector_num,
                                           int nb_sectors)
 {
     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
     int64_t sectors_per_cluster = cluster_size_sectors(backup_job);
     int64_t start, end;

     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);

     start = sector_num / sectors_per_cluster;
     end = DIV_ROUND_UP(sector_num + nb_sectors, sectors_per_cluster);
     wait_for_overlapping_requests(backup_job, start, end);
 }

 void backup_cow_request_begin(CowRequest *req, BlockJob *job,
                               int64_t sector_num,
                               int nb_sectors)
 {
     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
     int64_t sectors_per_cluster = cluster_size_sectors(backup_job);
     int64_t start, end;

     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);

     start = sector_num / sectors_per_cluster;
     end = DIV_ROUND_UP(sector_num + nb_sectors, sectors_per_cluster);
     cow_request_begin(req, backup_job, start, end);
 }

 void backup_cow_request_end(CowRequest *req)
 {
     cow_request_end(req);
 }

 static void backup_drain(BlockJob *job)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common);

     /* Need to keep a reference in case blk_drain triggers execution
      * of backup_complete...
      */
     if (s->target) {
         BlockBackend *target = s->target;
         blk_ref(target);
         blk_drain(target);
         blk_unref(target);
     }
 }

 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);
     }
 }

 typedef struct {
     int ret;
 } BackupCompleteData;

 static void backup_complete(BlockJob *job, void *opaque)
 {
     BackupCompleteData *data = opaque;

     block_job_completed(job, data->ret);
     g_free(data);
 }

 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 {
     if (block_job_is_cancelled(&job->common)) {
         return true;
     }

     /* we need to yield so that bdrv_drain_all() returns.
      * (without, VM does not reboot)
      */
     if (job->common.speed) {
         uint64_t delay_ns = ratelimit_calculate_delay(&job->limit,
                                                       job->sectors_read);
         job->sectors_read = 0;
         block_job_sleep_ns(&job->common, QEMU_CLOCK_REALTIME, delay_ns);
     } else {
         block_job_sleep_ns(&job->common, QEMU_CLOCK_REALTIME, 0);
     }

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

     return false;
 }

 static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
 {
     bool error_is_read;
     int ret = 0;
     int clusters_per_iter;
     uint32_t granularity;
     int64_t sector;
     int64_t cluster;
     int64_t end;
     int64_t last_cluster = -1;
     int64_t sectors_per_cluster = cluster_size_sectors(job);
     BdrvDirtyBitmapIter *dbi;

     granularity = bdrv_dirty_bitmap_granularity(job->sync_bitmap);
     clusters_per_iter = MAX((granularity / job->cluster_size), 1);
     dbi = bdrv_dirty_iter_new(job->sync_bitmap, 0);

     /* Find the next dirty sector(s) */
     while ((sector = bdrv_dirty_iter_next(dbi)) != -1) {
         cluster = sector / sectors_per_cluster;

         /* Fake progress updates for any clusters we skipped */
         if (cluster != last_cluster + 1) {
             job->common.offset += ((cluster - last_cluster - 1) *
                                    job->cluster_size);
         }

         for (end = cluster + clusters_per_iter; cluster < end; cluster++) {
             do {
                 if (yield_and_check(job)) {
                     goto out;
                 }
                 ret = backup_do_cow(job, cluster * sectors_per_cluster,
                                     sectors_per_cluster, &error_is_read,
                                     false);
                 if ((ret < 0) &&
                     backup_error_action(job, error_is_read, -ret) ==
                     BLOCK_ERROR_ACTION_REPORT) {
                     goto out;
                 }
             } while (ret < 0);
         }

         /* If the bitmap granularity is smaller than the backup granularity,
          * we need to advance the iterator pointer to the next cluster. */
         if (granularity < job->cluster_size) {
             bdrv_set_dirty_iter(dbi, cluster * sectors_per_cluster);
         }

         last_cluster = cluster - 1;
     }

     /* Play some final catchup with the progress meter */
     end = DIV_ROUND_UP(job->common.len, job->cluster_size);
     if (last_cluster + 1 < end) {
         job->common.offset += ((end - last_cluster - 1) * job->cluster_size);
     }

 out:
     bdrv_dirty_iter_free(dbi);
     return ret;
 }

 static void coroutine_fn backup_run(void *opaque)
 {
     BackupBlockJob *job = opaque;
     BackupCompleteData *data;
     BlockDriverState *bs = blk_bs(job->common.blk);
     int64_t start, end;
     int64_t sectors_per_cluster = cluster_size_sectors(job);
     int ret = 0;

     QLIST_INIT(&job->inflight_reqs);
     qemu_co_rwlock_init(&job->flush_rwlock);

     start = 0;
     end = DIV_ROUND_UP(job->common.len, job->cluster_size);

     job->done_bitmap = bitmap_new(end);

     job->before_write.notify = backup_before_write_notify;
     bdrv_add_before_write_notifier(bs, &job->before_write);

     if (job->sync_mode == MIRROR_SYNC_MODE_NONE) {
         while (!block_job_is_cancelled(&job->common)) {
             /* Yield until the job is cancelled.  We just let our before_write
              * notify callback service CoW requests. */
             block_job_yield(&job->common);
         }
     } else if (job->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
         ret = backup_run_incremental(job);
     } else {
         /* Both FULL and TOP SYNC_MODE's require copying.. */
         for (; start < end; start++) {
             bool error_is_read;
             if (yield_and_check(job)) {
                 break;
             }

             if (job->sync_mode == MIRROR_SYNC_MODE_TOP) {
                 int i, n;
                 int alloced = 0;

                 /* Check to see if these blocks are already in the
                  * backing file. */

                 for (i = 0; i < sectors_per_cluster;) {
                     /* bdrv_is_allocated() only returns true/false based
                      * on the first set of sectors it comes across that
                      * are are all in the same state.
                      * For that reason we must verify each sector in the
                      * backup cluster length.  We end up copying more than
                      * needed but at some point that is always the case. */
                     alloced =
                         bdrv_is_allocated(bs,
                                 start * sectors_per_cluster + i,
                                 sectors_per_cluster - i, &n);
                     i += n;

                     if (alloced == 1 || n == 0) {
                         break;
                     }
                 }

                 /* If the above loop never found any sectors that are in
                  * the topmost image, skip this backup. */
                 if (alloced == 0) {
                     continue;
                 }
             }
             /* FULL sync mode we copy the whole drive. */
             ret = backup_do_cow(job, start * sectors_per_cluster,
                                 sectors_per_cluster, &error_is_read, false);
             if (ret < 0) {
                 /* Depending on error action, fail now or retry cluster */
                 BlockErrorAction action =
                     backup_error_action(job, error_is_read, -ret);
                 if (action == BLOCK_ERROR_ACTION_REPORT) {
                     break;
                 } else {
                     start--;
                     continue;
                 }
             }
         }
     }

     notifier_with_return_remove(&job->before_write);

     /* wait until pending backup_do_cow() calls have completed */
     qemu_co_rwlock_wrlock(&job->flush_rwlock);
     qemu_co_rwlock_unlock(&job->flush_rwlock);
     g_free(job->done_bitmap);

     data = g_malloc(sizeof(*data));
     data->ret = ret;
     block_job_defer_to_main_loop(&job->common, backup_complete, data);
 }

 static const BlockJobDriver backup_job_driver = {
     .instance_size          = sizeof(BackupBlockJob),
     .job_type               = BLOCK_JOB_TYPE_BACKUP,
     .start                  = backup_run,
     .set_speed              = backup_set_speed,
     .commit                 = backup_commit,
     .abort                  = backup_abort,
     .clean                  = backup_clean,
     .attached_aio_context   = backup_attached_aio_context,
     .drain                  = backup_drain,
 };

 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *target, int64_t speed,
                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
                   bool compress,
                   BlockdevOnError on_source_error,
                   BlockdevOnError on_target_error,
                   int creation_flags,
                   BlockCompletionFunc *cb, void *opaque,
                   BlockJobTxn *txn, Error **errp)
 {
     int64_t len;
     BlockDriverInfo bdi;
     BackupBlockJob *job = NULL;
     int ret;

     assert(bs);
     assert(target);

     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 && target->drv->bdrv_co_pwritev_compressed == NULL) {
         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_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
         if (!sync_bitmap) {
             error_setg(errp, "must provide a valid bitmap name for "
                              "\"incremental\" sync mode");
             return NULL;
         }

         /* Create a new bitmap, and freeze/disable this one. */
         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
             return NULL;
         }
     } else if (sync_bitmap) {
         error_setg(errp,
                    "a sync_bitmap was provided to backup_run, "
                    "but received an incompatible sync_mode (%s)",
                    MirrorSyncMode_lookup[sync_mode]);
         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;
     }

     /* job->common.len is fixed, so we can't allow resize */
     job = block_job_create(job_id, &backup_job_driver, bs,
                            BLK_PERM_CONSISTENT_READ,
                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
                            BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD,
                            speed, creation_flags, cb, opaque, errp);
     if (!job) {
         goto error;
     }

     /* The target must match the source in size, so no resize here either */
     job->target = blk_new(BLK_PERM_WRITE,
                           BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
                           BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD);
     ret = blk_insert_bs(job->target, target, errp);
     if (ret < 0) {
         goto error;
     }

     job->on_source_error = on_source_error;
     job->on_target_error = on_target_error;
     job->sync_mode = sync_mode;
     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
                        sync_bitmap : NULL;
     job->compress = compress;

     /* 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 < 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");
         goto error;
     } else if (ret < 0 && target->backing) {
         /* Not fatal; just trudge on ahead. */
         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
     } else {
         job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
     }

     /* Required permissions are already taken with target's blk_new() */
     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
                        &error_abort);
     job->common.len = len;
     block_job_txn_add_job(txn, &job->common);

     return &job->common;

  error:
     if (sync_bitmap) {
         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
     }
     if (job) {
         backup_clean(&job->common);
         block_job_unref(&job->common);
     }

     return NULL;
 }
	/*
	* QEMU backup
	*
	* Copyright (C) 2013 Proxmox Server Solutions
	*
	* 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 "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"

	#define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)
	#define SLICE_TIME 100000000ULL /* ns */

	typedef struct BackupBlockJob {
	BlockJob common;
	BlockBackend *target;
	/* bitmap for sync=incremental */
	BdrvDirtyBitmap *sync_bitmap;
	MirrorSyncMode sync_mode;
	RateLimit limit;
	BlockdevOnError on_source_error;
	BlockdevOnError on_target_error;
	CoRwlock flush_rwlock;
	uint64_t sectors_read;
	unsigned long *done_bitmap;
	int64_t cluster_size;
	bool compress;
	NotifierWithReturn before_write;
	QLIST_HEAD(, CowRequest) inflight_reqs;
	} BackupBlockJob;

	/* Size of a cluster in sectors, instead of bytes. */
	static inline int64_t cluster_size_sectors(BackupBlockJob *job)
	{
	return job->cluster_size / BDRV_SECTOR_SIZE;
	}

	/* See if in-flight requests overlap and wait for them to complete */
	static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
	int64_t start,
	int64_t end)
	{
	CowRequest *req;
	bool retry;

	do {
	retry = false;
	QLIST_FOREACH(req, &job->inflight_reqs, list) {
	if (end > req->start && start < req->end) {
	qemu_co_queue_wait(&req->wait_queue, NULL);
	retry = true;
	break;
	}
	}
	} while (retry);
	}

	/* Keep track of an in-flight request */
	static void cow_request_begin(CowRequest req, BackupBlockJob job,
	int64_t start, int64_t end)
	{
	req->start = start;
	req->end = end;
	qemu_co_queue_init(&req->wait_queue);
	QLIST_INSERT_HEAD(&job->inflight_reqs, req, list);
	}

	/* Forget about a completed request */
	static void cow_request_end(CowRequest *req)
	{
	QLIST_REMOVE(req, list);
	qemu_co_queue_restart_all(&req->wait_queue);
	}

	static int coroutine_fn backup_do_cow(BackupBlockJob *job,
	int64_t sector_num, int nb_sectors,
	bool *error_is_read,
	bool is_write_notifier)
	{
	BlockBackend *blk = job->common.blk;
	CowRequest cow_request;
	struct iovec iov;
	QEMUIOVector bounce_qiov;
	void *bounce_buffer = NULL;
	int ret = 0;
	int64_t sectors_per_cluster = cluster_size_sectors(job);
	int64_t start, end;
	int n;

	qemu_co_rwlock_rdlock(&job->flush_rwlock);

	start = sector_num / sectors_per_cluster;
	end = DIV_ROUND_UP(sector_num + nb_sectors, sectors_per_cluster);

	trace_backup_do_cow_enter(job, start, sector_num, nb_sectors);

	wait_for_overlapping_requests(job, start, end);
	cow_request_begin(&cow_request, job, start, end);

	for (; start < end; start++) {
	if (test_bit(start, job->done_bitmap)) {
	trace_backup_do_cow_skip(job, start);
	continue; /* already copied */
	}

	trace_backup_do_cow_process(job, start);

	n = MIN(sectors_per_cluster,
	job->common.len / BDRV_SECTOR_SIZE -
	start * sectors_per_cluster);

	if (!bounce_buffer) {
	bounce_buffer = blk_blockalign(blk, job->cluster_size);
	}
	iov.iov_base = bounce_buffer;
	iov.iov_len = n * BDRV_SECTOR_SIZE;
	qemu_iovec_init_external(&bounce_qiov, &iov, 1);

	ret = blk_co_preadv(blk, start * job->cluster_size,
	bounce_qiov.size, &bounce_qiov,
	is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0);
	if (ret < 0) {
	trace_backup_do_cow_read_fail(job, start, ret);
	if (error_is_read) {
	*error_is_read = true;
	}
	goto out;
	}

	if (buffer_is_zero(iov.iov_base, iov.iov_len)) {
	ret = blk_co_pwrite_zeroes(job->target, start * job->cluster_size,
	bounce_qiov.size, BDRV_REQ_MAY_UNMAP);
	} else {
	ret = blk_co_pwritev(job->target, start * job->cluster_size,
	bounce_qiov.size, &bounce_qiov,
	job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
	}
	if (ret < 0) {
	trace_backup_do_cow_write_fail(job, start, ret);
	if (error_is_read) {
	*error_is_read = false;
	}
	goto out;
	}

	set_bit(start, job->done_bitmap);

	/* Publish progress, guest I/O counts as progress too. Note that the
	* offset field is an opaque progress value, it is not a disk offset.
	*/
	job->sectors_read += n;
	job->common.offset += n * BDRV_SECTOR_SIZE;
	}

	out:
	if (bounce_buffer) {
	qemu_vfree(bounce_buffer);
	}

	cow_request_end(&cow_request);

	trace_backup_do_cow_return(job, sector_num, nb_sectors, ret);

	qemu_co_rwlock_unlock(&job->flush_rwlock);

	return ret;
	}

	static int coroutine_fn backup_before_write_notify(
	NotifierWithReturn *notifier,
	void *opaque)
	{
	BackupBlockJob *job = container_of(notifier, BackupBlockJob, before_write);
	BdrvTrackedRequest *req = opaque;
	int64_t sector_num = req->offset >> BDRV_SECTOR_BITS;
	int nb_sectors = req->bytes >> BDRV_SECTOR_BITS;

	assert(req->bs == blk_bs(job->common.blk));
	assert((req->offset & (BDRV_SECTOR_SIZE - 1)) == 0);
	assert((req->bytes & (BDRV_SECTOR_SIZE - 1)) == 0);

	return backup_do_cow(job, sector_num, nb_sectors, NULL, true);
	}

	static void backup_set_speed(BlockJob job, int64_t speed, Error *errp)
	{
	BackupBlockJob *s = container_of(job, BackupBlockJob, common);

	if (speed < 0) {
	error_setg(errp, QERR_INVALID_PARAMETER, "speed");
	return;
	}
	ratelimit_set_speed(&s->limit, speed / BDRV_SECTOR_SIZE, SLICE_TIME);
	}

	static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
	{
	BdrvDirtyBitmap *bm;
	BlockDriverState *bs = blk_bs(job->common.blk);

	if (ret < 0 \|\| block_job_is_cancelled(&job->common)) {
	/* Merge the successor back into the parent, delete nothing. */
	bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
	assert(bm);
	} else {
	/* Everything is fine, delete this bitmap and install the backup. */
	bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
	assert(bm);
	}
	}

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

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

	static void backup_clean(BlockJob *job)
	{
	BackupBlockJob *s = container_of(job, BackupBlockJob, common);
	assert(s->target);
	blk_unref(s->target);
	s->target = NULL;
	}

	static void backup_attached_aio_context(BlockJob job, AioContext aio_context)
	{
	BackupBlockJob *s = container_of(job, BackupBlockJob, common);

	blk_set_aio_context(s->target, aio_context);
	}

	void backup_do_checkpoint(BlockJob job, Error *errp)
	{
	BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
	int64_t len;

	assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);

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

	len = DIV_ROUND_UP(backup_job->common.len, backup_job->cluster_size);
	bitmap_zero(backup_job->done_bitmap, len);
	}

	void backup_wait_for_overlapping_requests(BlockJob *job, int64_t sector_num,
	int nb_sectors)
	{
	BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
	int64_t sectors_per_cluster = cluster_size_sectors(backup_job);
	int64_t start, end;

	assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);

	start = sector_num / sectors_per_cluster;
	end = DIV_ROUND_UP(sector_num + nb_sectors, sectors_per_cluster);
	wait_for_overlapping_requests(backup_job, start, end);
	}

	void backup_cow_request_begin(CowRequest req, BlockJob job,
	int64_t sector_num,
	int nb_sectors)
	{
	BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
	int64_t sectors_per_cluster = cluster_size_sectors(backup_job);
	int64_t start, end;

	assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);

	start = sector_num / sectors_per_cluster;
	end = DIV_ROUND_UP(sector_num + nb_sectors, sectors_per_cluster);
	cow_request_begin(req, backup_job, start, end);
	}

	void backup_cow_request_end(CowRequest *req)
	{
	cow_request_end(req);
	}

	static void backup_drain(BlockJob *job)
	{
	BackupBlockJob *s = container_of(job, BackupBlockJob, common);

	/* Need to keep a reference in case blk_drain triggers execution
	* of backup_complete...
	*/
	if (s->target) {
	BlockBackend *target = s->target;
	blk_ref(target);
	blk_drain(target);
	blk_unref(target);
	}
	}

	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);
	}
	}

	typedef struct {
	int ret;
	} BackupCompleteData;

	static void backup_complete(BlockJob job, void opaque)
	{
	BackupCompleteData *data = opaque;

	block_job_completed(job, data->ret);
	g_free(data);
	}

	static bool coroutine_fn yield_and_check(BackupBlockJob *job)
	{
	if (block_job_is_cancelled(&job->common)) {
	return true;
	}

	/* we need to yield so that bdrv_drain_all() returns.
	* (without, VM does not reboot)
	*/
	if (job->common.speed) {
	uint64_t delay_ns = ratelimit_calculate_delay(&job->limit,
	job->sectors_read);
	job->sectors_read = 0;
	block_job_sleep_ns(&job->common, QEMU_CLOCK_REALTIME, delay_ns);
	} else {
	block_job_sleep_ns(&job->common, QEMU_CLOCK_REALTIME, 0);
	}

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

	return false;
	}

	static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
	{
	bool error_is_read;
	int ret = 0;
	int clusters_per_iter;
	uint32_t granularity;
	int64_t sector;
	int64_t cluster;
	int64_t end;
	int64_t last_cluster = -1;
	int64_t sectors_per_cluster = cluster_size_sectors(job);
	BdrvDirtyBitmapIter *dbi;

	granularity = bdrv_dirty_bitmap_granularity(job->sync_bitmap);
	clusters_per_iter = MAX((granularity / job->cluster_size), 1);
	dbi = bdrv_dirty_iter_new(job->sync_bitmap, 0);

	/* Find the next dirty sector(s) */
	while ((sector = bdrv_dirty_iter_next(dbi)) != -1) {
	cluster = sector / sectors_per_cluster;

	/* Fake progress updates for any clusters we skipped */
	if (cluster != last_cluster + 1) {
	job->common.offset += ((cluster - last_cluster - 1) *
	job->cluster_size);
	}

	for (end = cluster + clusters_per_iter; cluster < end; cluster++) {
	do {
	if (yield_and_check(job)) {
	goto out;
	}
	ret = backup_do_cow(job, cluster * sectors_per_cluster,
	sectors_per_cluster, &error_is_read,
	false);
	if ((ret < 0) &&
	backup_error_action(job, error_is_read, -ret) ==
	BLOCK_ERROR_ACTION_REPORT) {
	goto out;
	}
	} while (ret < 0);
	}

	/* If the bitmap granularity is smaller than the backup granularity,
	* we need to advance the iterator pointer to the next cluster. */
	if (granularity < job->cluster_size) {
	bdrv_set_dirty_iter(dbi, cluster * sectors_per_cluster);
	}

	last_cluster = cluster - 1;
	}

	/* Play some final catchup with the progress meter */
	end = DIV_ROUND_UP(job->common.len, job->cluster_size);
	if (last_cluster + 1 < end) {
	job->common.offset += ((end - last_cluster - 1) * job->cluster_size);
	}

	out:
	bdrv_dirty_iter_free(dbi);
	return ret;
	}

	static void coroutine_fn backup_run(void *opaque)
	{
	BackupBlockJob *job = opaque;
	BackupCompleteData *data;
	BlockDriverState *bs = blk_bs(job->common.blk);
	int64_t start, end;
	int64_t sectors_per_cluster = cluster_size_sectors(job);
	int ret = 0;

	QLIST_INIT(&job->inflight_reqs);
	qemu_co_rwlock_init(&job->flush_rwlock);

	start = 0;
	end = DIV_ROUND_UP(job->common.len, job->cluster_size);

	job->done_bitmap = bitmap_new(end);

	job->before_write.notify = backup_before_write_notify;
	bdrv_add_before_write_notifier(bs, &job->before_write);

	if (job->sync_mode == MIRROR_SYNC_MODE_NONE) {
	while (!block_job_is_cancelled(&job->common)) {
	/* Yield until the job is cancelled. We just let our before_write
	* notify callback service CoW requests. */
	block_job_yield(&job->common);
	}
	} else if (job->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
	ret = backup_run_incremental(job);
	} else {
	/* Both FULL and TOP SYNC_MODE's require copying.. */
	for (; start < end; start++) {
	bool error_is_read;
	if (yield_and_check(job)) {
	break;
	}

	if (job->sync_mode == MIRROR_SYNC_MODE_TOP) {
	int i, n;
	int alloced = 0;

	/* Check to see if these blocks are already in the
	* backing file. */

	for (i = 0; i < sectors_per_cluster;) {
	/* bdrv_is_allocated() only returns true/false based
	* on the first set of sectors it comes across that
	* are are all in the same state.
	* For that reason we must verify each sector in the
	* backup cluster length. We end up copying more than
	* needed but at some point that is always the case. */
	alloced =
	bdrv_is_allocated(bs,
	start * sectors_per_cluster + i,
	sectors_per_cluster - i, &n);
	i += n;

	if (alloced == 1 \|\| n == 0) {
	break;
	}
	}

	/* If the above loop never found any sectors that are in
	* the topmost image, skip this backup. */
	if (alloced == 0) {
	continue;
	}
	}
	/* FULL sync mode we copy the whole drive. */
	ret = backup_do_cow(job, start * sectors_per_cluster,
	sectors_per_cluster, &error_is_read, false);
	if (ret < 0) {
	/* Depending on error action, fail now or retry cluster */
	BlockErrorAction action =
	backup_error_action(job, error_is_read, -ret);
	if (action == BLOCK_ERROR_ACTION_REPORT) {
	break;
	} else {
	start--;
	continue;
	}
	}
	}
	}

	notifier_with_return_remove(&job->before_write);

	/* wait until pending backup_do_cow() calls have completed */
	qemu_co_rwlock_wrlock(&job->flush_rwlock);
	qemu_co_rwlock_unlock(&job->flush_rwlock);
	g_free(job->done_bitmap);

	data = g_malloc(sizeof(*data));
	data->ret = ret;
	block_job_defer_to_main_loop(&job->common, backup_complete, data);
	}

	static const BlockJobDriver backup_job_driver = {
	.instance_size = sizeof(BackupBlockJob),
	.job_type = BLOCK_JOB_TYPE_BACKUP,
	.start = backup_run,
	.set_speed = backup_set_speed,
	.commit = backup_commit,
	.abort = backup_abort,
	.clean = backup_clean,
	.attached_aio_context = backup_attached_aio_context,
	.drain = backup_drain,
	};

	BlockJob backup_job_create(const char job_id, BlockDriverState *bs,
	BlockDriverState *target, int64_t speed,
	MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
	bool compress,
	BlockdevOnError on_source_error,
	BlockdevOnError on_target_error,
	int creation_flags,
	BlockCompletionFunc cb, void opaque,
	BlockJobTxn txn, Error *errp)
	{
	int64_t len;
	BlockDriverInfo bdi;
	BackupBlockJob *job = NULL;
	int ret;

	assert(bs);
	assert(target);

	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 && target->drv->bdrv_co_pwritev_compressed == NULL) {
	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_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
	if (!sync_bitmap) {
	error_setg(errp, "must provide a valid bitmap name for "
	"\"incremental\" sync mode");
	return NULL;
	}

	/* Create a new bitmap, and freeze/disable this one. */
	if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
	return NULL;
	}
	} else if (sync_bitmap) {
	error_setg(errp,
	"a sync_bitmap was provided to backup_run, "
	"but received an incompatible sync_mode (%s)",
	MirrorSyncMode_lookup[sync_mode]);
	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;
	}

	/* job->common.len is fixed, so we can't allow resize */
	job = block_job_create(job_id, &backup_job_driver, bs,
	BLK_PERM_CONSISTENT_READ,
	BLK_PERM_CONSISTENT_READ \| BLK_PERM_WRITE \|
	BLK_PERM_WRITE_UNCHANGED \| BLK_PERM_GRAPH_MOD,
	speed, creation_flags, cb, opaque, errp);
	if (!job) {
	goto error;
	}

	/* The target must match the source in size, so no resize here either */
	job->target = blk_new(BLK_PERM_WRITE,
	BLK_PERM_CONSISTENT_READ \| BLK_PERM_WRITE \|
	BLK_PERM_WRITE_UNCHANGED \| BLK_PERM_GRAPH_MOD);
	ret = blk_insert_bs(job->target, target, errp);
	if (ret < 0) {
	goto error;
	}

	job->on_source_error = on_source_error;
	job->on_target_error = on_target_error;
	job->sync_mode = sync_mode;
	job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
	sync_bitmap : NULL;
	job->compress = compress;

	/* 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 < 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");
	goto error;
	} else if (ret < 0 && target->backing) {
	/* Not fatal; just trudge on ahead. */
	job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
	} else {
	job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
	}

	/* Required permissions are already taken with target's blk_new() */
	block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
	&error_abort);
	job->common.len = len;
	block_job_txn_add_job(txn, &job->common);

	return &job->common;

	error:
	if (sync_bitmap) {
	bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
	}
	if (job) {
	backup_clean(&job->common);
	block_job_unref(&job->common);
	}

	return NULL;
	}