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"
 #include "qemu/error-report.h"

 #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)

 typedef struct CowRequest {
     int64_t start_byte;
     int64_t end_byte;
     QLIST_ENTRY(CowRequest) list;
     CoQueue wait_queue; /* coroutines blocked on this request */
 } CowRequest;

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

     HBitmap *copy_bitmap;
     bool use_copy_range;
     int64_t copy_range_size;

     bool serialize_target_writes;
 } BackupBlockJob;

 static const BlockJobDriver backup_job_driver;

 /* 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_byte && start < req->end_byte) {
                 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_byte = start;
     req->end_byte = 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);
 }

 /* Copy range to target with a bounce buffer and return the bytes copied. If
  * error occurred, return a negative error number */
 static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
                                                       int64_t start,
                                                       int64_t end,
                                                       bool is_write_notifier,
                                                       bool *error_is_read,
                                                       void **bounce_buffer)
 {
     int ret;
     struct iovec iov;
     QEMUIOVector qiov;
     BlockBackend *blk = job->common.blk;
     int nbytes;
     int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
     int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;

     hbitmap_reset(job->copy_bitmap, start / job->cluster_size, 1);
     nbytes = MIN(job->cluster_size, job->len - start);
     if (!*bounce_buffer) {
         *bounce_buffer = blk_blockalign(blk, job->cluster_size);
     }
     iov.iov_base = *bounce_buffer;
     iov.iov_len = nbytes;
     qemu_iovec_init_external(&qiov, &iov, 1);

     ret = blk_co_preadv(blk, start, qiov.size, &qiov, read_flags);
     if (ret < 0) {
         trace_backup_do_cow_read_fail(job, start, ret);
         if (error_is_read) {
             *error_is_read = true;
         }
         goto fail;
     }

     if (qemu_iovec_is_zero(&qiov)) {
         ret = blk_co_pwrite_zeroes(job->target, start,
                                    qiov.size, write_flags | BDRV_REQ_MAY_UNMAP);
     } else {
         ret = blk_co_pwritev(job->target, start,
                              qiov.size, &qiov, write_flags |
                              (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 fail;
     }

     return nbytes;
 fail:
     hbitmap_set(job->copy_bitmap, start / job->cluster_size, 1);
     return ret;

 }

 /* Copy range to target and return the bytes copied. If error occurred, return a
  * negative error number. */
 static int coroutine_fn backup_cow_with_offload(BackupBlockJob *job,
                                                 int64_t start,
                                                 int64_t end,
                                                 bool is_write_notifier)
 {
     int ret;
     int nr_clusters;
     BlockBackend *blk = job->common.blk;
     int nbytes;
     int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
     int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;

     assert(QEMU_IS_ALIGNED(job->copy_range_size, job->cluster_size));
     nbytes = MIN(job->copy_range_size, end - start);
     nr_clusters = DIV_ROUND_UP(nbytes, job->cluster_size);
     hbitmap_reset(job->copy_bitmap, start / job->cluster_size,
                   nr_clusters);
     ret = blk_co_copy_range(blk, start, job->target, start, nbytes,
                             read_flags, write_flags);
     if (ret < 0) {
         trace_backup_do_cow_copy_range_fail(job, start, ret);
         hbitmap_set(job->copy_bitmap, start / job->cluster_size,
                     nr_clusters);
         return ret;
     }

     return nbytes;
 }

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

     qemu_co_rwlock_rdlock(&job->flush_rwlock);

     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);

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

     while (start < end) {
         if (!hbitmap_get(job->copy_bitmap, start / job->cluster_size)) {
             trace_backup_do_cow_skip(job, start);
             start += job->cluster_size;
             continue; /* already copied */
         }

         trace_backup_do_cow_process(job, start);

         if (job->use_copy_range) {
             ret = backup_cow_with_offload(job, start, end, is_write_notifier);
             if (ret < 0) {
                 job->use_copy_range = false;
             }
         }
         if (!job->use_copy_range) {
             ret = backup_cow_with_bounce_buffer(job, start, end, is_write_notifier,
                                                 error_is_read, &bounce_buffer);
         }
         if (ret < 0) {
             break;
         }

         /* 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.
          */
         start += ret;
         job->bytes_read += ret;
         job_progress_update(&job->common.job, ret);
         ret = 0;
     }

     if (bounce_buffer) {
         qemu_vfree(bounce_buffer);
     }

     cow_request_end(&cow_request);

     trace_backup_do_cow_return(job, offset, bytes, 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;

     assert(req->bs == blk_bs(job->common.blk));
     assert(QEMU_IS_ALIGNED(req->offset, BDRV_SECTOR_SIZE));
     assert(QEMU_IS_ALIGNED(req->bytes, BDRV_SECTOR_SIZE));

     return backup_do_cow(job, req->offset, req->bytes, NULL, true);
 }

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

     if (ret < 0) {
         /* 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(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);
     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(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;
     }

     len = DIV_ROUND_UP(backup_job->len, backup_job->cluster_size);
     hbitmap_set(backup_job->copy_bitmap, 0, len);
 }

 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);
     }
 }

 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_run_incremental(BackupBlockJob *job)
 {
     int ret;
     bool error_is_read;
     int64_t cluster;
     HBitmapIter hbi;

     hbitmap_iter_init(&hbi, job->copy_bitmap, 0);
     while ((cluster = hbitmap_iter_next(&hbi, true)) != -1) {
         do {
             if (yield_and_check(job)) {
                 return 0;
             }
             ret = backup_do_cow(job, cluster * job->cluster_size,
                                 job->cluster_size, &error_is_read, false);
             if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
                            BLOCK_ERROR_ACTION_REPORT)
             {
                 return ret;
             }
         } while (ret < 0);
     }

     return 0;
 }

 /* init copy_bitmap from sync_bitmap */
 static void backup_incremental_init_copy_bitmap(BackupBlockJob *job)
 {
     BdrvDirtyBitmapIter *dbi;
     int64_t offset;
     int64_t end = DIV_ROUND_UP(bdrv_dirty_bitmap_size(job->sync_bitmap),
                                job->cluster_size);

     dbi = bdrv_dirty_iter_new(job->sync_bitmap);
     while ((offset = bdrv_dirty_iter_next(dbi)) != -1) {
         int64_t cluster = offset / job->cluster_size;
         int64_t next_cluster;

         offset += bdrv_dirty_bitmap_granularity(job->sync_bitmap);
         if (offset >= bdrv_dirty_bitmap_size(job->sync_bitmap)) {
             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
             break;
         }

         offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset);
         if (offset == -1) {
             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
             break;
         }

         next_cluster = DIV_ROUND_UP(offset, job->cluster_size);
         hbitmap_set(job->copy_bitmap, cluster, next_cluster - cluster);
         if (next_cluster >= end) {
             break;
         }

         bdrv_set_dirty_iter(dbi, next_cluster * job->cluster_size);
     }

     /* TODO job_progress_set_remaining() would make more sense */
     job_progress_update(&job->common.job,
         job->len - hbitmap_count(job->copy_bitmap) * job->cluster_size);

     bdrv_dirty_iter_free(dbi);
 }

 static int coroutine_fn backup_run(Job *job, Error **errp)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
     BlockDriverState *bs = blk_bs(s->common.blk);
     int64_t offset, nb_clusters;
     int ret = 0;

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

     nb_clusters = DIV_ROUND_UP(s->len, s->cluster_size);
     job_progress_set_remaining(job, s->len);

     s->copy_bitmap = hbitmap_alloc(nb_clusters, 0);
     if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
         backup_incremental_init_copy_bitmap(s);
     } else {
         hbitmap_set(s->copy_bitmap, 0, nb_clusters);
     }


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

     if (s->sync_mode == MIRROR_SYNC_MODE_NONE) {
         /* All bits are set in copy_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 if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
         ret = backup_run_incremental(s);
     } else {
         /* Both FULL and TOP SYNC_MODE's require copying.. */
         for (offset = 0; offset < s->len;
              offset += s->cluster_size) {
             bool error_is_read;
             int alloced = 0;

             if (yield_and_check(s)) {
                 break;
             }

             if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
                 int i;
                 int64_t n;

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

                 for (i = 0; i < s->cluster_size;) {
                     /* 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, offset + i,
                                           s->cluster_size - i, &n);
                     i += n;

                     if (alloced || 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. */
             if (alloced < 0) {
                 ret = alloced;
             } else {
                 ret = backup_do_cow(s, offset, s->cluster_size,
                                     &error_is_read, false);
             }
             if (ret < 0) {
                 /* Depending on error action, fail now or retry cluster */
                 BlockErrorAction action =
                     backup_error_action(s, error_is_read, -ret);
                 if (action == BLOCK_ERROR_ACTION_REPORT) {
                     break;
                 } else {
                     offset -= s->cluster_size;
                     continue;
                 }
             }
         }
     }

     notifier_with_return_remove(&s->before_write);

     /* wait until pending backup_do_cow() calls have completed */
     qemu_co_rwlock_wrlock(&s->flush_rwlock);
     qemu_co_rwlock_unlock(&s->flush_rwlock);
     hbitmap_free(s->copy_bitmap);

     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,
         .drain                  = block_job_drain,
         .run                    = backup_run,
         .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,
                   JobTxn *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_str(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->len is fixed, so we can't allow resize */
     job = block_job_create(job_id, &backup_job_driver, txn, 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;

     /* Detect image-fleecing (and similar) schemes */
     job->serialize_target_writes = bdrv_chain_contains(target, bs);

     /* 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);
         job->cluster_size = 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");
         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);
     }
     job->use_copy_range = true;
     job->copy_range_size = MIN_NON_ZERO(blk_get_max_transfer(job->common.blk),
                                         blk_get_max_transfer(job->target));
     job->copy_range_size = MAX(job->cluster_size,
                                QEMU_ALIGN_UP(job->copy_range_size,
                                              job->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->len = len;

     return &job->common;

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

     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"
	#include "qemu/error-report.h"

	#define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)

	typedef struct CowRequest {
	int64_t start_byte;
	int64_t end_byte;
	QLIST_ENTRY(CowRequest) list;
	CoQueue wait_queue; /* coroutines blocked on this request */
	} CowRequest;

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

	HBitmap *copy_bitmap;
	bool use_copy_range;
	int64_t copy_range_size;

	bool serialize_target_writes;
	} BackupBlockJob;

	static const BlockJobDriver backup_job_driver;

	/* 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_byte && start < req->end_byte) {
	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_byte = start;
	req->end_byte = 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);
	}

	/* Copy range to target with a bounce buffer and return the bytes copied. If
	* error occurred, return a negative error number */
	static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
	int64_t start,
	int64_t end,
	bool is_write_notifier,
	bool *error_is_read,
	void **bounce_buffer)
	{
	int ret;
	struct iovec iov;
	QEMUIOVector qiov;
	BlockBackend *blk = job->common.blk;
	int nbytes;
	int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
	int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;

	hbitmap_reset(job->copy_bitmap, start / job->cluster_size, 1);
	nbytes = MIN(job->cluster_size, job->len - start);
	if (!*bounce_buffer) {
	*bounce_buffer = blk_blockalign(blk, job->cluster_size);
	}
	iov.iov_base = *bounce_buffer;
	iov.iov_len = nbytes;
	qemu_iovec_init_external(&qiov, &iov, 1);

	ret = blk_co_preadv(blk, start, qiov.size, &qiov, read_flags);
	if (ret < 0) {
	trace_backup_do_cow_read_fail(job, start, ret);
	if (error_is_read) {
	*error_is_read = true;
	}
	goto fail;
	}

	if (qemu_iovec_is_zero(&qiov)) {
	ret = blk_co_pwrite_zeroes(job->target, start,
	qiov.size, write_flags \| BDRV_REQ_MAY_UNMAP);
	} else {
	ret = blk_co_pwritev(job->target, start,
	qiov.size, &qiov, write_flags \|
	(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 fail;
	}

	return nbytes;
	fail:
	hbitmap_set(job->copy_bitmap, start / job->cluster_size, 1);
	return ret;

	}

	/* Copy range to target and return the bytes copied. If error occurred, return a
	* negative error number. */
	static int coroutine_fn backup_cow_with_offload(BackupBlockJob *job,
	int64_t start,
	int64_t end,
	bool is_write_notifier)
	{
	int ret;
	int nr_clusters;
	BlockBackend *blk = job->common.blk;
	int nbytes;
	int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
	int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;

	assert(QEMU_IS_ALIGNED(job->copy_range_size, job->cluster_size));
	nbytes = MIN(job->copy_range_size, end - start);
	nr_clusters = DIV_ROUND_UP(nbytes, job->cluster_size);
	hbitmap_reset(job->copy_bitmap, start / job->cluster_size,
	nr_clusters);
	ret = blk_co_copy_range(blk, start, job->target, start, nbytes,
	read_flags, write_flags);
	if (ret < 0) {
	trace_backup_do_cow_copy_range_fail(job, start, ret);
	hbitmap_set(job->copy_bitmap, start / job->cluster_size,
	nr_clusters);
	return ret;
	}

	return nbytes;
	}

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

	qemu_co_rwlock_rdlock(&job->flush_rwlock);

	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);

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

	while (start < end) {
	if (!hbitmap_get(job->copy_bitmap, start / job->cluster_size)) {
	trace_backup_do_cow_skip(job, start);
	start += job->cluster_size;
	continue; /* already copied */
	}

	trace_backup_do_cow_process(job, start);

	if (job->use_copy_range) {
	ret = backup_cow_with_offload(job, start, end, is_write_notifier);
	if (ret < 0) {
	job->use_copy_range = false;
	}
	}
	if (!job->use_copy_range) {
	ret = backup_cow_with_bounce_buffer(job, start, end, is_write_notifier,
	error_is_read, &bounce_buffer);
	}
	if (ret < 0) {
	break;
	}

	/* 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.
	*/
	start += ret;
	job->bytes_read += ret;
	job_progress_update(&job->common.job, ret);
	ret = 0;
	}

	if (bounce_buffer) {
	qemu_vfree(bounce_buffer);
	}

	cow_request_end(&cow_request);

	trace_backup_do_cow_return(job, offset, bytes, 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;

	assert(req->bs == blk_bs(job->common.blk));
	assert(QEMU_IS_ALIGNED(req->offset, BDRV_SECTOR_SIZE));
	assert(QEMU_IS_ALIGNED(req->bytes, BDRV_SECTOR_SIZE));

	return backup_do_cow(job, req->offset, req->bytes, NULL, true);
	}

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

	if (ret < 0) {
	/* 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(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);
	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(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;
	}

	len = DIV_ROUND_UP(backup_job->len, backup_job->cluster_size);
	hbitmap_set(backup_job->copy_bitmap, 0, len);
	}

	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);
	}
	}

	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_run_incremental(BackupBlockJob *job)
	{
	int ret;
	bool error_is_read;
	int64_t cluster;
	HBitmapIter hbi;

	hbitmap_iter_init(&hbi, job->copy_bitmap, 0);
	while ((cluster = hbitmap_iter_next(&hbi, true)) != -1) {
	do {
	if (yield_and_check(job)) {
	return 0;
	}
	ret = backup_do_cow(job, cluster * job->cluster_size,
	job->cluster_size, &error_is_read, false);
	if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
	BLOCK_ERROR_ACTION_REPORT)
	{
	return ret;
	}
	} while (ret < 0);
	}

	return 0;
	}

	/* init copy_bitmap from sync_bitmap */
	static void backup_incremental_init_copy_bitmap(BackupBlockJob *job)
	{
	BdrvDirtyBitmapIter *dbi;
	int64_t offset;
	int64_t end = DIV_ROUND_UP(bdrv_dirty_bitmap_size(job->sync_bitmap),
	job->cluster_size);

	dbi = bdrv_dirty_iter_new(job->sync_bitmap);
	while ((offset = bdrv_dirty_iter_next(dbi)) != -1) {
	int64_t cluster = offset / job->cluster_size;
	int64_t next_cluster;

	offset += bdrv_dirty_bitmap_granularity(job->sync_bitmap);
	if (offset >= bdrv_dirty_bitmap_size(job->sync_bitmap)) {
	hbitmap_set(job->copy_bitmap, cluster, end - cluster);
	break;
	}

	offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset);
	if (offset == -1) {
	hbitmap_set(job->copy_bitmap, cluster, end - cluster);
	break;
	}

	next_cluster = DIV_ROUND_UP(offset, job->cluster_size);
	hbitmap_set(job->copy_bitmap, cluster, next_cluster - cluster);
	if (next_cluster >= end) {
	break;
	}

	bdrv_set_dirty_iter(dbi, next_cluster * job->cluster_size);
	}

	/* TODO job_progress_set_remaining() would make more sense */
	job_progress_update(&job->common.job,
	job->len - hbitmap_count(job->copy_bitmap) * job->cluster_size);

	bdrv_dirty_iter_free(dbi);
	}

	static int coroutine_fn backup_run(Job job, Error *errp)
	{
	BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
	BlockDriverState *bs = blk_bs(s->common.blk);
	int64_t offset, nb_clusters;
	int ret = 0;

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

	nb_clusters = DIV_ROUND_UP(s->len, s->cluster_size);
	job_progress_set_remaining(job, s->len);

	s->copy_bitmap = hbitmap_alloc(nb_clusters, 0);
	if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
	backup_incremental_init_copy_bitmap(s);
	} else {
	hbitmap_set(s->copy_bitmap, 0, nb_clusters);
	}


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

	if (s->sync_mode == MIRROR_SYNC_MODE_NONE) {
	/* All bits are set in copy_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 if (s->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
	ret = backup_run_incremental(s);
	} else {
	/* Both FULL and TOP SYNC_MODE's require copying.. */
	for (offset = 0; offset < s->len;
	offset += s->cluster_size) {
	bool error_is_read;
	int alloced = 0;

	if (yield_and_check(s)) {
	break;
	}

	if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
	int i;
	int64_t n;

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

	for (i = 0; i < s->cluster_size;) {
	/* 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, offset + i,
	s->cluster_size - i, &n);
	i += n;

	if (alloced \|\| 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. */
	if (alloced < 0) {
	ret = alloced;
	} else {
	ret = backup_do_cow(s, offset, s->cluster_size,
	&error_is_read, false);
	}
	if (ret < 0) {
	/* Depending on error action, fail now or retry cluster */
	BlockErrorAction action =
	backup_error_action(s, error_is_read, -ret);
	if (action == BLOCK_ERROR_ACTION_REPORT) {
	break;
	} else {
	offset -= s->cluster_size;
	continue;
	}
	}
	}
	}

	notifier_with_return_remove(&s->before_write);

	/* wait until pending backup_do_cow() calls have completed */
	qemu_co_rwlock_wrlock(&s->flush_rwlock);
	qemu_co_rwlock_unlock(&s->flush_rwlock);
	hbitmap_free(s->copy_bitmap);

	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,
	.drain = block_job_drain,
	.run = backup_run,
	.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,
	JobTxn 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_str(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->len is fixed, so we can't allow resize */
	job = block_job_create(job_id, &backup_job_driver, txn, 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;

	/* Detect image-fleecing (and similar) schemes */
	job->serialize_target_writes = bdrv_chain_contains(target, bs);

	/* 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);
	job->cluster_size = 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");
	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);
	}
	job->use_copy_range = true;
	job->copy_range_size = MIN_NON_ZERO(blk_get_max_transfer(job->common.blk),
	blk_get_max_transfer(job->target));
	job->copy_range_size = MAX(job->cluster_size,
	QEMU_ALIGN_UP(job->copy_range_size,
	job->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->len = len;

	return &job->common;

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

	return NULL;
	}