qemu

mirror.c (65836B)

---

 /*
  * Image mirroring
  *
  * Copyright Red Hat, Inc. 2012
  *
  * Authors:
  *  Paolo Bonzini  <pbonzini@redhat.com>
  *
  * This work is licensed under the terms of the GNU LGPL, version 2 or later.
  * See the COPYING.LIB file in the top-level directory.
  *
  */
 
 #include "qemu/osdep.h"
 #include "qemu/cutils.h"
 #include "qemu/coroutine.h"
 #include "qemu/range.h"
 #include "trace.h"
 #include "block/blockjob_int.h"
 #include "block/block_int.h"
 #include "sysemu/block-backend.h"
 #include "qapi/error.h"
 #include "qapi/qmp/qerror.h"
 #include "qemu/ratelimit.h"
 #include "qemu/bitmap.h"
 #include "qemu/memalign.h"
 
 #define MAX_IN_FLIGHT 16
 #define MAX_IO_BYTES (1 << 20) /* 1 Mb */
 #define DEFAULT_MIRROR_BUF_SIZE (MAX_IN_FLIGHT * MAX_IO_BYTES)
 
 /* The mirroring buffer is a list of granularity-sized chunks.
  * Free chunks are organized in a list.
  */
 typedef struct MirrorBuffer {
     QSIMPLEQ_ENTRY(MirrorBuffer) next;
 } MirrorBuffer;
 
 typedef struct MirrorOp MirrorOp;
 
 typedef struct MirrorBlockJob {
     BlockJob common;
     BlockBackend *target;
     BlockDriverState *mirror_top_bs;
     BlockDriverState *base;
     BlockDriverState *base_overlay;
 
     /* The name of the graph node to replace */
     char *replaces;
     /* The BDS to replace */
     BlockDriverState *to_replace;
     /* Used to block operations on the drive-mirror-replace target */
     Error *replace_blocker;
     bool is_none_mode;
     BlockMirrorBackingMode backing_mode;
     /* Whether the target image requires explicit zero-initialization */
     bool zero_target;
     MirrorCopyMode copy_mode;
     BlockdevOnError on_source_error, on_target_error;
     /* Set when the target is synced (dirty bitmap is clean, nothing
      * in flight) and the job is running in active mode */
     bool actively_synced;
     bool should_complete;
     int64_t granularity;
     size_t buf_size;
     int64_t bdev_length;
     unsigned long *cow_bitmap;
     BdrvDirtyBitmap *dirty_bitmap;
     BdrvDirtyBitmapIter *dbi;
     uint8_t *buf;
     QSIMPLEQ_HEAD(, MirrorBuffer) buf_free;
     int buf_free_count;
 
     uint64_t last_pause_ns;
     unsigned long *in_flight_bitmap;
     unsigned in_flight;
     int64_t bytes_in_flight;
     QTAILQ_HEAD(, MirrorOp) ops_in_flight;
     int ret;
     bool unmap;
     int target_cluster_size;
     int max_iov;
     bool initial_zeroing_ongoing;
     int in_active_write_counter;
     int64_t active_write_bytes_in_flight;
     bool prepared;
     bool in_drain;
 } MirrorBlockJob;
 
 typedef struct MirrorBDSOpaque {
     MirrorBlockJob *job;
     bool stop;
     bool is_commit;
 } MirrorBDSOpaque;
 
 struct MirrorOp {
     MirrorBlockJob *s;
     QEMUIOVector qiov;
     int64_t offset;
     uint64_t bytes;
 
     /* The pointee is set by mirror_co_read(), mirror_co_zero(), and
      * mirror_co_discard() before yielding for the first time */
     int64_t *bytes_handled;
 
     bool is_pseudo_op;
     bool is_active_write;
     bool is_in_flight;
     CoQueue waiting_requests;
     Coroutine *co;
     MirrorOp *waiting_for_op;
 
     QTAILQ_ENTRY(MirrorOp) next;
 };
 
 typedef enum MirrorMethod {
     MIRROR_METHOD_COPY,
     MIRROR_METHOD_ZERO,
     MIRROR_METHOD_DISCARD,
 } MirrorMethod;
 
 static BlockErrorAction mirror_error_action(MirrorBlockJob *s, bool read,
                                             int error)
 {
     s->actively_synced = false;
     if (read) {
         return block_job_error_action(&s->common, s->on_source_error,
                                       true, error);
     } else {
         return block_job_error_action(&s->common, s->on_target_error,
                                       false, error);
     }
 }
 
 static void coroutine_fn mirror_wait_on_conflicts(MirrorOp *self,
                                                   MirrorBlockJob *s,
                                                   uint64_t offset,
                                                   uint64_t bytes)
 {
     uint64_t self_start_chunk = offset / s->granularity;
     uint64_t self_end_chunk = DIV_ROUND_UP(offset + bytes, s->granularity);
     uint64_t self_nb_chunks = self_end_chunk - self_start_chunk;
 
     while (find_next_bit(s->in_flight_bitmap, self_end_chunk,
                          self_start_chunk) < self_end_chunk &&
            s->ret >= 0)
     {
         MirrorOp *op;
 
         QTAILQ_FOREACH(op, &s->ops_in_flight, next) {
             uint64_t op_start_chunk = op->offset / s->granularity;
             uint64_t op_nb_chunks = DIV_ROUND_UP(op->offset + op->bytes,
                                                  s->granularity) -
                                     op_start_chunk;
 
             if (op == self) {
                 continue;
             }
 
             if (ranges_overlap(self_start_chunk, self_nb_chunks,
                                op_start_chunk, op_nb_chunks))
             {
                 if (self) {
                     /*
                      * If the operation is already (indirectly) waiting for us,
                      * or will wait for us as soon as it wakes up, then just go
                      * on (instead of producing a deadlock in the former case).
                      */
                     if (op->waiting_for_op) {
                         continue;
                     }
 
                     self->waiting_for_op = op;
                 }
 
                 qemu_co_queue_wait(&op->waiting_requests, NULL);
 
                 if (self) {
                     self->waiting_for_op = NULL;
                 }
 
                 break;
             }
         }
     }
 }
 
 static void coroutine_fn mirror_iteration_done(MirrorOp *op, int ret)
 {
     MirrorBlockJob *s = op->s;
     struct iovec *iov;
     int64_t chunk_num;
     int i, nb_chunks;
 
     trace_mirror_iteration_done(s, op->offset, op->bytes, ret);
 
     s->in_flight--;
     s->bytes_in_flight -= op->bytes;
     iov = op->qiov.iov;
     for (i = 0; i < op->qiov.niov; i++) {
         MirrorBuffer *buf = (MirrorBuffer *) iov[i].iov_base;
         QSIMPLEQ_INSERT_TAIL(&s->buf_free, buf, next);
         s->buf_free_count++;
     }
 
     chunk_num = op->offset / s->granularity;
     nb_chunks = DIV_ROUND_UP(op->bytes, s->granularity);
 
     bitmap_clear(s->in_flight_bitmap, chunk_num, nb_chunks);
     QTAILQ_REMOVE(&s->ops_in_flight, op, next);
     if (ret >= 0) {
         if (s->cow_bitmap) {
             bitmap_set(s->cow_bitmap, chunk_num, nb_chunks);
         }
         if (!s->initial_zeroing_ongoing) {
             job_progress_update(&s->common.job, op->bytes);
         }
     }
     qemu_iovec_destroy(&op->qiov);
 
     qemu_co_queue_restart_all(&op->waiting_requests);
     g_free(op);
 }
 
 static void coroutine_fn mirror_write_complete(MirrorOp *op, int ret)
 {
     MirrorBlockJob *s = op->s;
 
     if (ret < 0) {
         BlockErrorAction action;
 
         bdrv_set_dirty_bitmap(s->dirty_bitmap, op->offset, op->bytes);
         action = mirror_error_action(s, false, -ret);
         if (action == BLOCK_ERROR_ACTION_REPORT && s->ret >= 0) {
             s->ret = ret;
         }
     }
 
     mirror_iteration_done(op, ret);
 }
 
 static void coroutine_fn mirror_read_complete(MirrorOp *op, int ret)
 {
     MirrorBlockJob *s = op->s;
 
     if (ret < 0) {
         BlockErrorAction action;
 
         bdrv_set_dirty_bitmap(s->dirty_bitmap, op->offset, op->bytes);
         action = mirror_error_action(s, true, -ret);
         if (action == BLOCK_ERROR_ACTION_REPORT && s->ret >= 0) {
             s->ret = ret;
         }
 
         mirror_iteration_done(op, ret);
         return;
     }
 
     ret = blk_co_pwritev(s->target, op->offset, op->qiov.size, &op->qiov, 0);
     mirror_write_complete(op, ret);
 }
 
 /* Clip bytes relative to offset to not exceed end-of-file */
 static inline int64_t mirror_clip_bytes(MirrorBlockJob *s,
                                         int64_t offset,
                                         int64_t bytes)
 {
     return MIN(bytes, s->bdev_length - offset);
 }
 
 /* Round offset and/or bytes to target cluster if COW is needed, and
  * return the offset of the adjusted tail against original. */
 static int mirror_cow_align(MirrorBlockJob *s, int64_t *offset,
                             uint64_t *bytes)
 {
     bool need_cow;
     int ret = 0;
     int64_t align_offset = *offset;
     int64_t align_bytes = *bytes;
     int max_bytes = s->granularity * s->max_iov;
 
     need_cow = !test_bit(*offset / s->granularity, s->cow_bitmap);
     need_cow |= !test_bit((*offset + *bytes - 1) / s->granularity,
                           s->cow_bitmap);
     if (need_cow) {
         bdrv_round_to_clusters(blk_bs(s->target), *offset, *bytes,
                                &align_offset, &align_bytes);
     }
 
     if (align_bytes > max_bytes) {
         align_bytes = max_bytes;
         if (need_cow) {
             align_bytes = QEMU_ALIGN_DOWN(align_bytes, s->target_cluster_size);
         }
     }
     /* Clipping may result in align_bytes unaligned to chunk boundary, but
      * that doesn't matter because it's already the end of source image. */
     align_bytes = mirror_clip_bytes(s, align_offset, align_bytes);
 
     ret = align_offset + align_bytes - (*offset + *bytes);
     *offset = align_offset;
     *bytes = align_bytes;
     assert(ret >= 0);
     return ret;
 }
 
 static inline void coroutine_fn
 mirror_wait_for_free_in_flight_slot(MirrorBlockJob *s)
 {
     MirrorOp *op;
 
     QTAILQ_FOREACH(op, &s->ops_in_flight, next) {
         /*
          * Do not wait on pseudo ops, because it may in turn wait on
          * some other operation to start, which may in fact be the
          * caller of this function.  Since there is only one pseudo op
          * at any given time, we will always find some real operation
          * to wait on.
          * Also, do not wait on active operations, because they do not
          * use up in-flight slots.
          */
         if (!op->is_pseudo_op && op->is_in_flight && !op->is_active_write) {
             qemu_co_queue_wait(&op->waiting_requests, NULL);
             return;
         }
     }
     abort();
 }
 
 /* Perform a mirror copy operation.
  *
  * *op->bytes_handled is set to the number of bytes copied after and
  * including offset, excluding any bytes copied prior to offset due
  * to alignment.  This will be op->bytes if no alignment is necessary,
  * or (new_end - op->offset) if the tail is rounded up or down due to
  * alignment or buffer limit.
  */
 static void coroutine_fn mirror_co_read(void *opaque)
 {
     MirrorOp *op = opaque;
     MirrorBlockJob *s = op->s;
     int nb_chunks;
     uint64_t ret;
     uint64_t max_bytes;
 
     max_bytes = s->granularity * s->max_iov;
 
     /* We can only handle as much as buf_size at a time. */
     op->bytes = MIN(s->buf_size, MIN(max_bytes, op->bytes));
     assert(op->bytes);
     assert(op->bytes < BDRV_REQUEST_MAX_BYTES);
     *op->bytes_handled = op->bytes;
 
     if (s->cow_bitmap) {
         *op->bytes_handled += mirror_cow_align(s, &op->offset, &op->bytes);
     }
     /* Cannot exceed BDRV_REQUEST_MAX_BYTES + INT_MAX */
     assert(*op->bytes_handled <= UINT_MAX);
     assert(op->bytes <= s->buf_size);
     /* The offset is granularity-aligned because:
      * 1) Caller passes in aligned values;
      * 2) mirror_cow_align is used only when target cluster is larger. */
     assert(QEMU_IS_ALIGNED(op->offset, s->granularity));
     /* The range is sector-aligned, since bdrv_getlength() rounds up. */
     assert(QEMU_IS_ALIGNED(op->bytes, BDRV_SECTOR_SIZE));
     nb_chunks = DIV_ROUND_UP(op->bytes, s->granularity);
 
     while (s->buf_free_count < nb_chunks) {
         trace_mirror_yield_in_flight(s, op->offset, s->in_flight);
         mirror_wait_for_free_in_flight_slot(s);
     }
 
     /* Now make a QEMUIOVector taking enough granularity-sized chunks
      * from s->buf_free.
      */
     qemu_iovec_init(&op->qiov, nb_chunks);
     while (nb_chunks-- > 0) {
         MirrorBuffer *buf = QSIMPLEQ_FIRST(&s->buf_free);
         size_t remaining = op->bytes - op->qiov.size;
 
         QSIMPLEQ_REMOVE_HEAD(&s->buf_free, next);
         s->buf_free_count--;
         qemu_iovec_add(&op->qiov, buf, MIN(s->granularity, remaining));
     }
 
     /* Copy the dirty cluster.  */
     s->in_flight++;
     s->bytes_in_flight += op->bytes;
     op->is_in_flight = true;
     trace_mirror_one_iteration(s, op->offset, op->bytes);
 
     ret = bdrv_co_preadv(s->mirror_top_bs->backing, op->offset, op->bytes,
                          &op->qiov, 0);
     mirror_read_complete(op, ret);
 }
 
 static void coroutine_fn mirror_co_zero(void *opaque)
 {
     MirrorOp *op = opaque;
     int ret;
 
     op->s->in_flight++;
     op->s->bytes_in_flight += op->bytes;
     *op->bytes_handled = op->bytes;
     op->is_in_flight = true;
 
     ret = blk_co_pwrite_zeroes(op->s->target, op->offset, op->bytes,
                                op->s->unmap ? BDRV_REQ_MAY_UNMAP : 0);
     mirror_write_complete(op, ret);
 }
 
 static void coroutine_fn mirror_co_discard(void *opaque)
 {
     MirrorOp *op = opaque;
     int ret;
 
     op->s->in_flight++;
     op->s->bytes_in_flight += op->bytes;
     *op->bytes_handled = op->bytes;
     op->is_in_flight = true;
 
     ret = blk_co_pdiscard(op->s->target, op->offset, op->bytes);
     mirror_write_complete(op, ret);
 }
 
 static unsigned mirror_perform(MirrorBlockJob *s, int64_t offset,
                                unsigned bytes, MirrorMethod mirror_method)
 {
     MirrorOp *op;
     Coroutine *co;
     int64_t bytes_handled = -1;
 
     op = g_new(MirrorOp, 1);
     *op = (MirrorOp){
         .s              = s,
         .offset         = offset,
         .bytes          = bytes,
         .bytes_handled  = &bytes_handled,
     };
     qemu_co_queue_init(&op->waiting_requests);
 
     switch (mirror_method) {
     case MIRROR_METHOD_COPY:
         co = qemu_coroutine_create(mirror_co_read, op);
         break;
     case MIRROR_METHOD_ZERO:
         co = qemu_coroutine_create(mirror_co_zero, op);
         break;
     case MIRROR_METHOD_DISCARD:
         co = qemu_coroutine_create(mirror_co_discard, op);
         break;
     default:
         abort();
     }
     op->co = co;
 
     QTAILQ_INSERT_TAIL(&s->ops_in_flight, op, next);
     qemu_coroutine_enter(co);
     /* At this point, ownership of op has been moved to the coroutine
      * and the object may already be freed */
 
     /* Assert that this value has been set */
     assert(bytes_handled >= 0);
 
     /* Same assertion as in mirror_co_read() (and for mirror_co_read()
      * and mirror_co_discard(), bytes_handled == op->bytes, which
      * is the @bytes parameter given to this function) */
     assert(bytes_handled <= UINT_MAX);
     return bytes_handled;
 }
 
 static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
 {
     BlockDriverState *source = s->mirror_top_bs->backing->bs;
     MirrorOp *pseudo_op;
     int64_t offset;
     uint64_t delay_ns = 0, ret = 0;
     /* At least the first dirty chunk is mirrored in one iteration. */
     int nb_chunks = 1;
     bool write_zeroes_ok = bdrv_can_write_zeroes_with_unmap(blk_bs(s->target));
     int max_io_bytes = MAX(s->buf_size / MAX_IN_FLIGHT, MAX_IO_BYTES);
 
     bdrv_dirty_bitmap_lock(s->dirty_bitmap);
     offset = bdrv_dirty_iter_next(s->dbi);
     if (offset < 0) {
         bdrv_set_dirty_iter(s->dbi, 0);
         offset = bdrv_dirty_iter_next(s->dbi);
         trace_mirror_restart_iter(s, bdrv_get_dirty_count(s->dirty_bitmap));
         assert(offset >= 0);
     }
     bdrv_dirty_bitmap_unlock(s->dirty_bitmap);
 
     /*
      * Wait for concurrent requests to @offset.  The next loop will limit the
      * copied area based on in_flight_bitmap so we only copy an area that does
      * not overlap with concurrent in-flight requests.  Still, we would like to
      * copy something, so wait until there are at least no more requests to the
      * very beginning of the area.
      */
     mirror_wait_on_conflicts(NULL, s, offset, 1);
 
     job_pause_point(&s->common.job);
 
     /* Find the number of consective dirty chunks following the first dirty
      * one, and wait for in flight requests in them. */
     bdrv_dirty_bitmap_lock(s->dirty_bitmap);
     while (nb_chunks * s->granularity < s->buf_size) {
         int64_t next_dirty;
         int64_t next_offset = offset + nb_chunks * s->granularity;
         int64_t next_chunk = next_offset / s->granularity;
         if (next_offset >= s->bdev_length ||
             !bdrv_dirty_bitmap_get_locked(s->dirty_bitmap, next_offset)) {
             break;
         }
         if (test_bit(next_chunk, s->in_flight_bitmap)) {
             break;
         }
 
         next_dirty = bdrv_dirty_iter_next(s->dbi);
         if (next_dirty > next_offset || next_dirty < 0) {
             /* The bitmap iterator's cache is stale, refresh it */
             bdrv_set_dirty_iter(s->dbi, next_offset);
             next_dirty = bdrv_dirty_iter_next(s->dbi);
         }
         assert(next_dirty == next_offset);
         nb_chunks++;
     }
 
     /* Clear dirty bits before querying the block status, because
      * calling bdrv_block_status_above could yield - if some blocks are
      * marked dirty in this window, we need to know.
      */
     bdrv_reset_dirty_bitmap_locked(s->dirty_bitmap, offset,
                                    nb_chunks * s->granularity);
     bdrv_dirty_bitmap_unlock(s->dirty_bitmap);
 
     /* Before claiming an area in the in-flight bitmap, we have to
      * create a MirrorOp for it so that conflicting requests can wait
      * for it.  mirror_perform() will create the real MirrorOps later,
      * for now we just create a pseudo operation that will wake up all
      * conflicting requests once all real operations have been
      * launched. */
     pseudo_op = g_new(MirrorOp, 1);
     *pseudo_op = (MirrorOp){
         .offset         = offset,
         .bytes          = nb_chunks * s->granularity,
         .is_pseudo_op   = true,
     };
     qemu_co_queue_init(&pseudo_op->waiting_requests);
     QTAILQ_INSERT_TAIL(&s->ops_in_flight, pseudo_op, next);
 
     bitmap_set(s->in_flight_bitmap, offset / s->granularity, nb_chunks);
     while (nb_chunks > 0 && offset < s->bdev_length) {
         int ret;
         int64_t io_bytes;
         int64_t io_bytes_acct;
         MirrorMethod mirror_method = MIRROR_METHOD_COPY;
 
         assert(!(offset % s->granularity));
         ret = bdrv_block_status_above(source, NULL, offset,
                                       nb_chunks * s->granularity,
                                       &io_bytes, NULL, NULL);
         if (ret < 0) {
             io_bytes = MIN(nb_chunks * s->granularity, max_io_bytes);
         } else if (ret & BDRV_BLOCK_DATA) {
             io_bytes = MIN(io_bytes, max_io_bytes);
         }
 
         io_bytes -= io_bytes % s->granularity;
         if (io_bytes < s->granularity) {
             io_bytes = s->granularity;
         } else if (ret >= 0 && !(ret & BDRV_BLOCK_DATA)) {
             int64_t target_offset;
             int64_t target_bytes;
             bdrv_round_to_clusters(blk_bs(s->target), offset, io_bytes,
                                    &target_offset, &target_bytes);
             if (target_offset == offset &&
                 target_bytes == io_bytes) {
                 mirror_method = ret & BDRV_BLOCK_ZERO ?
                                     MIRROR_METHOD_ZERO :
                                     MIRROR_METHOD_DISCARD;
             }
         }
 
         while (s->in_flight >= MAX_IN_FLIGHT) {
             trace_mirror_yield_in_flight(s, offset, s->in_flight);
             mirror_wait_for_free_in_flight_slot(s);
         }
 
         if (s->ret < 0) {
             ret = 0;
             goto fail;
         }
 
         io_bytes = mirror_clip_bytes(s, offset, io_bytes);
         io_bytes = mirror_perform(s, offset, io_bytes, mirror_method);
         if (mirror_method != MIRROR_METHOD_COPY && write_zeroes_ok) {
             io_bytes_acct = 0;
         } else {
             io_bytes_acct = io_bytes;
         }
         assert(io_bytes);
         offset += io_bytes;
         nb_chunks -= DIV_ROUND_UP(io_bytes, s->granularity);
         delay_ns = block_job_ratelimit_get_delay(&s->common, io_bytes_acct);
     }
 
     ret = delay_ns;
 fail:
     QTAILQ_REMOVE(&s->ops_in_flight, pseudo_op, next);
     qemu_co_queue_restart_all(&pseudo_op->waiting_requests);
     g_free(pseudo_op);
 
     return ret;
 }
 
 static void mirror_free_init(MirrorBlockJob *s)
 {
     int granularity = s->granularity;
     size_t buf_size = s->buf_size;
     uint8_t *buf = s->buf;
 
     assert(s->buf_free_count == 0);
     QSIMPLEQ_INIT(&s->buf_free);
     while (buf_size != 0) {
         MirrorBuffer *cur = (MirrorBuffer *)buf;
         QSIMPLEQ_INSERT_TAIL(&s->buf_free, cur, next);
         s->buf_free_count++;
         buf_size -= granularity;
         buf += granularity;
     }
 }
 
 /* This is also used for the .pause callback. There is no matching
  * mirror_resume() because mirror_run() will begin iterating again
  * when the job is resumed.
  */
 static void coroutine_fn mirror_wait_for_all_io(MirrorBlockJob *s)
 {
     while (s->in_flight > 0) {
         mirror_wait_for_free_in_flight_slot(s);
     }
 }
 
 /**
  * mirror_exit_common: handle both abort() and prepare() cases.
  * for .prepare, returns 0 on success and -errno on failure.
  * for .abort cases, denoted by abort = true, MUST return 0.
  */
 static int mirror_exit_common(Job *job)
 {
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
     BlockJob *bjob = &s->common;
     MirrorBDSOpaque *bs_opaque;
     AioContext *replace_aio_context = NULL;
     BlockDriverState *src;
     BlockDriverState *target_bs;
     BlockDriverState *mirror_top_bs;
     Error *local_err = NULL;
     bool abort = job->ret < 0;
     int ret = 0;
 
     if (s->prepared) {
         return 0;
     }
     s->prepared = true;
 
     mirror_top_bs = s->mirror_top_bs;
     bs_opaque = mirror_top_bs->opaque;
     src = mirror_top_bs->backing->bs;
     target_bs = blk_bs(s->target);
 
     if (bdrv_chain_contains(src, target_bs)) {
         bdrv_unfreeze_backing_chain(mirror_top_bs, target_bs);
     }
 
     bdrv_release_dirty_bitmap(s->dirty_bitmap);
 
     /* Make sure that the source BDS doesn't go away during bdrv_replace_node,
      * before we can call bdrv_drained_end */
     bdrv_ref(src);
     bdrv_ref(mirror_top_bs);
     bdrv_ref(target_bs);
 
     /*
      * Remove target parent that still uses BLK_PERM_WRITE/RESIZE before
      * inserting target_bs at s->to_replace, where we might not be able to get
      * these permissions.
      */
     blk_unref(s->target);
     s->target = NULL;
 
     /* We don't access the source any more. Dropping any WRITE/RESIZE is
      * required before it could become a backing file of target_bs. Not having
      * these permissions any more means that we can't allow any new requests on
      * mirror_top_bs from now on, so keep it drained. */
     bdrv_drained_begin(mirror_top_bs);
     bs_opaque->stop = true;
     bdrv_child_refresh_perms(mirror_top_bs, mirror_top_bs->backing,
                              &error_abort);
     if (!abort && s->backing_mode == MIRROR_SOURCE_BACKING_CHAIN) {
         BlockDriverState *backing = s->is_none_mode ? src : s->base;
         BlockDriverState *unfiltered_target = bdrv_skip_filters(target_bs);
 
         if (bdrv_cow_bs(unfiltered_target) != backing) {
             bdrv_set_backing_hd(unfiltered_target, backing, &local_err);
             if (local_err) {
                 error_report_err(local_err);
                 local_err = NULL;
                 ret = -EPERM;
             }
         }
     } else if (!abort && s->backing_mode == MIRROR_OPEN_BACKING_CHAIN) {
         assert(!bdrv_backing_chain_next(target_bs));
         ret = bdrv_open_backing_file(bdrv_skip_filters(target_bs), NULL,
                                      "backing", &local_err);
         if (ret < 0) {
             error_report_err(local_err);
             local_err = NULL;
         }
     }
 
     if (s->to_replace) {
         replace_aio_context = bdrv_get_aio_context(s->to_replace);
         aio_context_acquire(replace_aio_context);
     }
 
     if (s->should_complete && !abort) {
         BlockDriverState *to_replace = s->to_replace ?: src;
         bool ro = bdrv_is_read_only(to_replace);
 
         if (ro != bdrv_is_read_only(target_bs)) {
             bdrv_reopen_set_read_only(target_bs, ro, NULL);
         }
 
         /* The mirror job has no requests in flight any more, but we need to
          * drain potential other users of the BDS before changing the graph. */
         assert(s->in_drain);
         bdrv_drained_begin(target_bs);
         /*
          * Cannot use check_to_replace_node() here, because that would
          * check for an op blocker on @to_replace, and we have our own
          * there.
          */
         if (bdrv_recurse_can_replace(src, to_replace)) {
             bdrv_replace_node(to_replace, target_bs, &local_err);
         } else {
             error_setg(&local_err, "Can no longer replace '%s' by '%s', "
                        "because it can no longer be guaranteed that doing so "
                        "would not lead to an abrupt change of visible data",
                        to_replace->node_name, target_bs->node_name);
         }
         bdrv_drained_end(target_bs);
         if (local_err) {
             error_report_err(local_err);
             ret = -EPERM;
         }
     }
     if (s->to_replace) {
         bdrv_op_unblock_all(s->to_replace, s->replace_blocker);
         error_free(s->replace_blocker);
         bdrv_unref(s->to_replace);
     }
     if (replace_aio_context) {
         aio_context_release(replace_aio_context);
     }
     g_free(s->replaces);
     bdrv_unref(target_bs);
 
     /*
      * Remove the mirror filter driver from the graph. Before this, get rid of
      * the blockers on the intermediate nodes so that the resulting state is
      * valid.
      */
     block_job_remove_all_bdrv(bjob);
     bdrv_replace_node(mirror_top_bs, mirror_top_bs->backing->bs, &error_abort);
 
     bs_opaque->job = NULL;
 
     bdrv_drained_end(src);
     bdrv_drained_end(mirror_top_bs);
     s->in_drain = false;
     bdrv_unref(mirror_top_bs);
     bdrv_unref(src);
 
     return ret;
 }
 
 static int mirror_prepare(Job *job)
 {
     return mirror_exit_common(job);
 }
 
 static void mirror_abort(Job *job)
 {
     int ret = mirror_exit_common(job);
     assert(ret == 0);
 }
 
 static void coroutine_fn mirror_throttle(MirrorBlockJob *s)
 {
     int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
 
     if (now - s->last_pause_ns > BLOCK_JOB_SLICE_TIME) {
         s->last_pause_ns = now;
         job_sleep_ns(&s->common.job, 0);
     } else {
         job_pause_point(&s->common.job);
     }
 }
 
 static int coroutine_fn mirror_dirty_init(MirrorBlockJob *s)
 {
     int64_t offset;
     BlockDriverState *bs = s->mirror_top_bs->backing->bs;
     BlockDriverState *target_bs = blk_bs(s->target);
     int ret;
     int64_t count;
 
     if (s->zero_target) {
         if (!bdrv_can_write_zeroes_with_unmap(target_bs)) {
             bdrv_set_dirty_bitmap(s->dirty_bitmap, 0, s->bdev_length);
             return 0;
         }
 
         s->initial_zeroing_ongoing = true;
         for (offset = 0; offset < s->bdev_length; ) {
             int bytes = MIN(s->bdev_length - offset,
                             QEMU_ALIGN_DOWN(INT_MAX, s->granularity));
 
             mirror_throttle(s);
 
             if (job_is_cancelled(&s->common.job)) {
                 s->initial_zeroing_ongoing = false;
                 return 0;
             }
 
             if (s->in_flight >= MAX_IN_FLIGHT) {
                 trace_mirror_yield(s, UINT64_MAX, s->buf_free_count,
                                    s->in_flight);
                 mirror_wait_for_free_in_flight_slot(s);
                 continue;
             }
 
             mirror_perform(s, offset, bytes, MIRROR_METHOD_ZERO);
             offset += bytes;
         }
 
         mirror_wait_for_all_io(s);
         s->initial_zeroing_ongoing = false;
     }
 
     /* First part, loop on the sectors and initialize the dirty bitmap.  */
     for (offset = 0; offset < s->bdev_length; ) {
         /* Just to make sure we are not exceeding int limit. */
         int bytes = MIN(s->bdev_length - offset,
                         QEMU_ALIGN_DOWN(INT_MAX, s->granularity));
 
         mirror_throttle(s);
 
         if (job_is_cancelled(&s->common.job)) {
             return 0;
         }
 
         ret = bdrv_is_allocated_above(bs, s->base_overlay, true, offset, bytes,
                                       &count);
         if (ret < 0) {
             return ret;
         }
 
         assert(count);
         if (ret > 0) {
             bdrv_set_dirty_bitmap(s->dirty_bitmap, offset, count);
         }
         offset += count;
     }
     return 0;
 }
 
 /* Called when going out of the streaming phase to flush the bulk of the
  * data to the medium, or just before completing.
  */
 static int mirror_flush(MirrorBlockJob *s)
 {
     int ret = blk_flush(s->target);
     if (ret < 0) {
         if (mirror_error_action(s, false, -ret) == BLOCK_ERROR_ACTION_REPORT) {
             s->ret = ret;
         }
     }
     return ret;
 }
 
 static int coroutine_fn mirror_run(Job *job, Error **errp)
 {
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
     BlockDriverState *bs = s->mirror_top_bs->backing->bs;
     BlockDriverState *target_bs = blk_bs(s->target);
     bool need_drain = true;
     BlockDeviceIoStatus iostatus;
     int64_t length;
     int64_t target_length;
     BlockDriverInfo bdi;
     char backing_filename[2]; /* we only need 2 characters because we are only
                                  checking for a NULL string */
     int ret = 0;
 
     if (job_is_cancelled(&s->common.job)) {
         goto immediate_exit;
     }
 
     s->bdev_length = bdrv_getlength(bs);
     if (s->bdev_length < 0) {
         ret = s->bdev_length;
         goto immediate_exit;
     }
 
     target_length = blk_getlength(s->target);
     if (target_length < 0) {
         ret = target_length;
         goto immediate_exit;
     }
 
     /* Active commit must resize the base image if its size differs from the
      * active layer. */
     if (s->base == blk_bs(s->target)) {
         if (s->bdev_length > target_length) {
             ret = blk_co_truncate(s->target, s->bdev_length, false,
                                   PREALLOC_MODE_OFF, 0, NULL);
             if (ret < 0) {
                 goto immediate_exit;
             }
         }
     } else if (s->bdev_length != target_length) {
         error_setg(errp, "Source and target image have different sizes");
         ret = -EINVAL;
         goto immediate_exit;
     }
 
     if (s->bdev_length == 0) {
         /* Transition to the READY state and wait for complete. */
         job_transition_to_ready(&s->common.job);
         s->actively_synced = true;
         while (!job_cancel_requested(&s->common.job) && !s->should_complete) {
             job_yield(&s->common.job);
         }
         goto immediate_exit;
     }
 
     length = DIV_ROUND_UP(s->bdev_length, s->granularity);
     s->in_flight_bitmap = bitmap_new(length);
 
     /* If we have no backing file yet in the destination, we cannot let
      * the destination do COW.  Instead, we copy sectors around the
      * dirty data if needed.  We need a bitmap to do that.
      */
     bdrv_get_backing_filename(target_bs, backing_filename,
                               sizeof(backing_filename));
     if (!bdrv_get_info(target_bs, &bdi) && bdi.cluster_size) {
         s->target_cluster_size = bdi.cluster_size;
     } else {
         s->target_cluster_size = BDRV_SECTOR_SIZE;
     }
     if (backing_filename[0] && !bdrv_backing_chain_next(target_bs) &&
         s->granularity < s->target_cluster_size) {
         s->buf_size = MAX(s->buf_size, s->target_cluster_size);
         s->cow_bitmap = bitmap_new(length);
     }
     s->max_iov = MIN(bs->bl.max_iov, target_bs->bl.max_iov);
 
     s->buf = qemu_try_blockalign(bs, s->buf_size);
     if (s->buf == NULL) {
         ret = -ENOMEM;
         goto immediate_exit;
     }
 
     mirror_free_init(s);
 
     s->last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     if (!s->is_none_mode) {
         ret = mirror_dirty_init(s);
         if (ret < 0 || job_is_cancelled(&s->common.job)) {
             goto immediate_exit;
         }
     }
 
     assert(!s->dbi);
     s->dbi = bdrv_dirty_iter_new(s->dirty_bitmap);
     for (;;) {
         uint64_t delay_ns = 0;
         int64_t cnt, delta;
         bool should_complete;
 
         if (s->ret < 0) {
             ret = s->ret;
             goto immediate_exit;
         }
 
         job_pause_point(&s->common.job);
 
         if (job_is_cancelled(&s->common.job)) {
             ret = 0;
             goto immediate_exit;
         }
 
         cnt = bdrv_get_dirty_count(s->dirty_bitmap);
         /* cnt is the number of dirty bytes remaining and s->bytes_in_flight is
          * the number of bytes currently being processed; together those are
          * the current remaining operation length */
         job_progress_set_remaining(&s->common.job,
                                    s->bytes_in_flight + cnt +
                                    s->active_write_bytes_in_flight);
 
         /* Note that even when no rate limit is applied we need to yield
          * periodically with no pending I/O so that bdrv_drain_all() returns.
          * We do so every BLKOCK_JOB_SLICE_TIME nanoseconds, or when there is
          * an error, or when the source is clean, whichever comes first. */
         delta = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - s->last_pause_ns;
         WITH_JOB_LOCK_GUARD() {
             iostatus = s->common.iostatus;
         }
         if (delta < BLOCK_JOB_SLICE_TIME &&
             iostatus == BLOCK_DEVICE_IO_STATUS_OK) {
             if (s->in_flight >= MAX_IN_FLIGHT || s->buf_free_count == 0 ||
                 (cnt == 0 && s->in_flight > 0)) {
                 trace_mirror_yield(s, cnt, s->buf_free_count, s->in_flight);
                 mirror_wait_for_free_in_flight_slot(s);
                 continue;
             } else if (cnt != 0) {
                 delay_ns = mirror_iteration(s);
             }
         }
 
         should_complete = false;
         if (s->in_flight == 0 && cnt == 0) {
             trace_mirror_before_flush(s);
             if (!job_is_ready(&s->common.job)) {
                 if (mirror_flush(s) < 0) {
                     /* Go check s->ret.  */
                     continue;
                 }
                 /* We're out of the streaming phase.  From now on, if the job
                  * is cancelled we will actually complete all pending I/O and
                  * report completion.  This way, block-job-cancel will leave
                  * the target in a consistent state.
                  */
                 job_transition_to_ready(&s->common.job);
                 if (s->copy_mode != MIRROR_COPY_MODE_BACKGROUND) {
                     s->actively_synced = true;
                 }
             }
 
             should_complete = s->should_complete ||
                 job_cancel_requested(&s->common.job);
             cnt = bdrv_get_dirty_count(s->dirty_bitmap);
         }
 
         if (cnt == 0 && should_complete) {
             /* The dirty bitmap is not updated while operations are pending.
              * If we're about to exit, wait for pending operations before
              * calling bdrv_get_dirty_count(bs), or we may exit while the
              * source has dirty data to copy!
              *
              * Note that I/O can be submitted by the guest while
              * mirror_populate runs, so pause it now.  Before deciding
              * whether to switch to target check one last time if I/O has
              * come in the meanwhile, and if not flush the data to disk.
              */
             trace_mirror_before_drain(s, cnt);
 
             s->in_drain = true;
             bdrv_drained_begin(bs);
 
             /* Must be zero because we are drained */
             assert(s->in_active_write_counter == 0);
 
             cnt = bdrv_get_dirty_count(s->dirty_bitmap);
             if (cnt > 0 || mirror_flush(s) < 0) {
                 bdrv_drained_end(bs);
                 s->in_drain = false;
                 continue;
             }
 
             /* The two disks are in sync.  Exit and report successful
              * completion.
              */
             assert(QLIST_EMPTY(&bs->tracked_requests));
             need_drain = false;
             break;
         }
 
         if (job_is_ready(&s->common.job) && !should_complete) {
             delay_ns = (s->in_flight == 0 &&
                         cnt == 0 ? BLOCK_JOB_SLICE_TIME : 0);
         }
         trace_mirror_before_sleep(s, cnt, job_is_ready(&s->common.job),
                                   delay_ns);
         job_sleep_ns(&s->common.job, delay_ns);
         s->last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     }
 
 immediate_exit:
     if (s->in_flight > 0) {
         /* We get here only if something went wrong.  Either the job failed,
          * or it was cancelled prematurely so that we do not guarantee that
          * the target is a copy of the source.
          */
         assert(ret < 0 || job_is_cancelled(&s->common.job));
         assert(need_drain);
         mirror_wait_for_all_io(s);
     }
 
     assert(s->in_flight == 0);
     qemu_vfree(s->buf);
     g_free(s->cow_bitmap);
     g_free(s->in_flight_bitmap);
     bdrv_dirty_iter_free(s->dbi);
 
     if (need_drain) {
         s->in_drain = true;
         bdrv_drained_begin(bs);
     }
 
     return ret;
 }
 
 static void mirror_complete(Job *job, Error **errp)
 {
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
 
     if (!job_is_ready(job)) {
         error_setg(errp, "The active block job '%s' cannot be completed",
                    job->id);
         return;
     }
 
     /* block all operations on to_replace bs */
     if (s->replaces) {
         AioContext *replace_aio_context;
 
         s->to_replace = bdrv_find_node(s->replaces);
         if (!s->to_replace) {
             error_setg(errp, "Node name '%s' not found", s->replaces);
             return;
         }
 
         replace_aio_context = bdrv_get_aio_context(s->to_replace);
         aio_context_acquire(replace_aio_context);
 
         /* TODO Translate this into child freeze system. */
         error_setg(&s->replace_blocker,
                    "block device is in use by block-job-complete");
         bdrv_op_block_all(s->to_replace, s->replace_blocker);
         bdrv_ref(s->to_replace);
 
         aio_context_release(replace_aio_context);
     }
 
     s->should_complete = true;
 
     /* If the job is paused, it will be re-entered when it is resumed */
     WITH_JOB_LOCK_GUARD() {
         if (!job->paused) {
             job_enter_cond_locked(job, NULL);
         }
     }
 }
 
 static void coroutine_fn mirror_pause(Job *job)
 {
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
 
     mirror_wait_for_all_io(s);
 }
 
 static bool mirror_drained_poll(BlockJob *job)
 {
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common);
 
     /* If the job isn't paused nor cancelled, we can't be sure that it won't
      * issue more requests. We make an exception if we've reached this point
      * from one of our own drain sections, to avoid a deadlock waiting for
      * ourselves.
      */
     WITH_JOB_LOCK_GUARD() {
         if (!s->common.job.paused && !job_is_cancelled_locked(&job->job)
             && !s->in_drain) {
             return true;
         }
     }
 
     return !!s->in_flight;
 }
 
 static bool mirror_cancel(Job *job, bool force)
 {
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
     BlockDriverState *target = blk_bs(s->target);
 
     /*
      * Before the job is READY, we treat any cancellation like a
      * force-cancellation.
      */
     force = force || !job_is_ready(job);
 
     if (force) {
         bdrv_cancel_in_flight(target);
     }
     return force;
 }
 
 static bool commit_active_cancel(Job *job, bool force)
 {
     /* Same as above in mirror_cancel() */
     return force || !job_is_ready(job);
 }
 
 static const BlockJobDriver mirror_job_driver = {
     .job_driver = {
         .instance_size          = sizeof(MirrorBlockJob),
         .job_type               = JOB_TYPE_MIRROR,
         .free                   = block_job_free,
         .user_resume            = block_job_user_resume,
         .run                    = mirror_run,
         .prepare                = mirror_prepare,
         .abort                  = mirror_abort,
         .pause                  = mirror_pause,
         .complete               = mirror_complete,
         .cancel                 = mirror_cancel,
     },
     .drained_poll           = mirror_drained_poll,
 };
 
 static const BlockJobDriver commit_active_job_driver = {
     .job_driver = {
         .instance_size          = sizeof(MirrorBlockJob),
         .job_type               = JOB_TYPE_COMMIT,
         .free                   = block_job_free,
         .user_resume            = block_job_user_resume,
         .run                    = mirror_run,
         .prepare                = mirror_prepare,
         .abort                  = mirror_abort,
         .pause                  = mirror_pause,
         .complete               = mirror_complete,
         .cancel                 = commit_active_cancel,
     },
     .drained_poll           = mirror_drained_poll,
 };
 
 static void coroutine_fn
 do_sync_target_write(MirrorBlockJob *job, MirrorMethod method,
                      uint64_t offset, uint64_t bytes,
                      QEMUIOVector *qiov, int flags)
 {
     int ret;
     size_t qiov_offset = 0;
     int64_t bitmap_offset, bitmap_end;
 
     if (!QEMU_IS_ALIGNED(offset, job->granularity) &&
         bdrv_dirty_bitmap_get(job->dirty_bitmap, offset))
     {
             /*
              * Dirty unaligned padding: ignore it.
              *
              * Reasoning:
              * 1. If we copy it, we can't reset corresponding bit in
              *    dirty_bitmap as there may be some "dirty" bytes still not
              *    copied.
              * 2. It's already dirty, so skipping it we don't diverge mirror
              *    progress.
              *
              * Note, that because of this, guest write may have no contribution
              * into mirror converge, but that's not bad, as we have background
              * process of mirroring. If under some bad circumstances (high guest
              * IO load) background process starve, we will not converge anyway,
              * even if each write will contribute, as guest is not guaranteed to
              * rewrite the whole disk.
              */
             qiov_offset = QEMU_ALIGN_UP(offset, job->granularity) - offset;
             if (bytes <= qiov_offset) {
                 /* nothing to do after shrink */
                 return;
             }
             offset += qiov_offset;
             bytes -= qiov_offset;
     }
 
     if (!QEMU_IS_ALIGNED(offset + bytes, job->granularity) &&
         bdrv_dirty_bitmap_get(job->dirty_bitmap, offset + bytes - 1))
     {
         uint64_t tail = (offset + bytes) % job->granularity;
 
         if (bytes <= tail) {
             /* nothing to do after shrink */
             return;
         }
         bytes -= tail;
     }
 
     /*
      * Tails are either clean or shrunk, so for bitmap resetting
      * we safely align the range down.
      */
     bitmap_offset = QEMU_ALIGN_UP(offset, job->granularity);
     bitmap_end = QEMU_ALIGN_DOWN(offset + bytes, job->granularity);
     if (bitmap_offset < bitmap_end) {
         bdrv_reset_dirty_bitmap(job->dirty_bitmap, bitmap_offset,
                                 bitmap_end - bitmap_offset);
     }
 
     job_progress_increase_remaining(&job->common.job, bytes);
     job->active_write_bytes_in_flight += bytes;
 
     switch (method) {
     case MIRROR_METHOD_COPY:
         ret = blk_co_pwritev_part(job->target, offset, bytes,
                                   qiov, qiov_offset, flags);
         break;
 
     case MIRROR_METHOD_ZERO:
         assert(!qiov);
         ret = blk_co_pwrite_zeroes(job->target, offset, bytes, flags);
         break;
 
     case MIRROR_METHOD_DISCARD:
         assert(!qiov);
         ret = blk_co_pdiscard(job->target, offset, bytes);
         break;
 
     default:
         abort();
     }
 
     job->active_write_bytes_in_flight -= bytes;
     if (ret >= 0) {
         job_progress_update(&job->common.job, bytes);
     } else {
         BlockErrorAction action;
 
         /*
          * We failed, so we should mark dirty the whole area, aligned up.
          * Note that we don't care about shrunk tails if any: they were dirty
          * at function start, and they must be still dirty, as we've locked
          * the region for in-flight op.
          */
         bitmap_offset = QEMU_ALIGN_DOWN(offset, job->granularity);
         bitmap_end = QEMU_ALIGN_UP(offset + bytes, job->granularity);
         bdrv_set_dirty_bitmap(job->dirty_bitmap, bitmap_offset,
                               bitmap_end - bitmap_offset);
         job->actively_synced = false;
 
         action = mirror_error_action(job, false, -ret);
         if (action == BLOCK_ERROR_ACTION_REPORT) {
             if (!job->ret) {
                 job->ret = ret;
             }
         }
     }
 }
 
 static MirrorOp *coroutine_fn active_write_prepare(MirrorBlockJob *s,
                                                    uint64_t offset,
                                                    uint64_t bytes)
 {
     MirrorOp *op;
     uint64_t start_chunk = offset / s->granularity;
     uint64_t end_chunk = DIV_ROUND_UP(offset + bytes, s->granularity);
 
     op = g_new(MirrorOp, 1);
     *op = (MirrorOp){
         .s                  = s,
         .offset             = offset,
         .bytes              = bytes,
         .is_active_write    = true,
         .is_in_flight       = true,
         .co                 = qemu_coroutine_self(),
     };
     qemu_co_queue_init(&op->waiting_requests);
     QTAILQ_INSERT_TAIL(&s->ops_in_flight, op, next);
 
     s->in_active_write_counter++;
 
     /*
      * Wait for concurrent requests affecting the area.  If there are already
      * running requests that are copying off now-to-be stale data in the area,
      * we must wait for them to finish before we begin writing fresh data to the
      * target so that the write operations appear in the correct order.
      * Note that background requests (see mirror_iteration()) in contrast only
      * wait for conflicting requests at the start of the dirty area, and then
      * (based on the in_flight_bitmap) truncate the area to copy so it will not
      * conflict with any requests beyond that.  For active writes, however, we
      * cannot truncate that area.  The request from our parent must be blocked
      * until the area is copied in full.  Therefore, we must wait for the whole
      * area to become free of concurrent requests.
      */
     mirror_wait_on_conflicts(op, s, offset, bytes);
 
     bitmap_set(s->in_flight_bitmap, start_chunk, end_chunk - start_chunk);
 
     return op;
 }
 
 static void coroutine_fn active_write_settle(MirrorOp *op)
 {
     uint64_t start_chunk = op->offset / op->s->granularity;
     uint64_t end_chunk = DIV_ROUND_UP(op->offset + op->bytes,
                                       op->s->granularity);
 
     if (!--op->s->in_active_write_counter && op->s->actively_synced) {
         BdrvChild *source = op->s->mirror_top_bs->backing;
 
         if (QLIST_FIRST(&source->bs->parents) == source &&
             QLIST_NEXT(source, next_parent) == NULL)
         {
             /* Assert that we are back in sync once all active write
              * operations are settled.
              * Note that we can only assert this if the mirror node
              * is the source node's only parent. */
             assert(!bdrv_get_dirty_count(op->s->dirty_bitmap));
         }
     }
     bitmap_clear(op->s->in_flight_bitmap, start_chunk, end_chunk - start_chunk);
     QTAILQ_REMOVE(&op->s->ops_in_flight, op, next);
     qemu_co_queue_restart_all(&op->waiting_requests);
     g_free(op);
 }
 
 static int coroutine_fn bdrv_mirror_top_preadv(BlockDriverState *bs,
     int64_t offset, int64_t bytes, QEMUIOVector *qiov, BdrvRequestFlags flags)
 {
     return bdrv_co_preadv(bs->backing, offset, bytes, qiov, flags);
 }
 
 static int coroutine_fn bdrv_mirror_top_do_write(BlockDriverState *bs,
     MirrorMethod method, uint64_t offset, uint64_t bytes, QEMUIOVector *qiov,
     int flags)
 {
     MirrorOp *op = NULL;
     MirrorBDSOpaque *s = bs->opaque;
     int ret = 0;
     bool copy_to_target = false;
 
     if (s->job) {
         copy_to_target = s->job->ret >= 0 &&
                          !job_is_cancelled(&s->job->common.job) &&
                          s->job->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING;
     }
 
     if (copy_to_target) {
         op = active_write_prepare(s->job, offset, bytes);
     }
 
     switch (method) {
     case MIRROR_METHOD_COPY:
         ret = bdrv_co_pwritev(bs->backing, offset, bytes, qiov, flags);
         break;
 
     case MIRROR_METHOD_ZERO:
         ret = bdrv_co_pwrite_zeroes(bs->backing, offset, bytes, flags);
         break;
 
     case MIRROR_METHOD_DISCARD:
         ret = bdrv_co_pdiscard(bs->backing, offset, bytes);
         break;
 
     default:
         abort();
     }
 
     if (ret < 0) {
         goto out;
     }
 
     if (copy_to_target) {
         do_sync_target_write(s->job, method, offset, bytes, qiov, flags);
     }
 
 out:
     if (copy_to_target) {
         active_write_settle(op);
     }
     return ret;
 }
 
 static int coroutine_fn bdrv_mirror_top_pwritev(BlockDriverState *bs,
     int64_t offset, int64_t bytes, QEMUIOVector *qiov, BdrvRequestFlags flags)
 {
     MirrorBDSOpaque *s = bs->opaque;
     QEMUIOVector bounce_qiov;
     void *bounce_buf;
     int ret = 0;
     bool copy_to_target = false;
 
     if (s->job) {
         copy_to_target = s->job->ret >= 0 &&
                          !job_is_cancelled(&s->job->common.job) &&
                          s->job->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING;
     }
 
     if (copy_to_target) {
         /* The guest might concurrently modify the data to write; but
          * the data on source and destination must match, so we have
          * to use a bounce buffer if we are going to write to the
          * target now. */
         bounce_buf = qemu_blockalign(bs, bytes);
         iov_to_buf_full(qiov->iov, qiov->niov, 0, bounce_buf, bytes);
 
         qemu_iovec_init(&bounce_qiov, 1);
         qemu_iovec_add(&bounce_qiov, bounce_buf, bytes);
         qiov = &bounce_qiov;
 
         flags &= ~BDRV_REQ_REGISTERED_BUF;
     }
 
     ret = bdrv_mirror_top_do_write(bs, MIRROR_METHOD_COPY, offset, bytes, qiov,
                                    flags);
 
     if (copy_to_target) {
         qemu_iovec_destroy(&bounce_qiov);
         qemu_vfree(bounce_buf);
     }
 
     return ret;
 }
 
 static int coroutine_fn bdrv_mirror_top_flush(BlockDriverState *bs)
 {
     if (bs->backing == NULL) {
         /* we can be here after failed bdrv_append in mirror_start_job */
         return 0;
     }
     return bdrv_co_flush(bs->backing->bs);
 }
 
 static int coroutine_fn bdrv_mirror_top_pwrite_zeroes(BlockDriverState *bs,
     int64_t offset, int64_t bytes, BdrvRequestFlags flags)
 {
     return bdrv_mirror_top_do_write(bs, MIRROR_METHOD_ZERO, offset, bytes, NULL,
                                     flags);
 }
 
 static int coroutine_fn bdrv_mirror_top_pdiscard(BlockDriverState *bs,
     int64_t offset, int64_t bytes)
 {
     return bdrv_mirror_top_do_write(bs, MIRROR_METHOD_DISCARD, offset, bytes,
                                     NULL, 0);
 }
 
 static void bdrv_mirror_top_refresh_filename(BlockDriverState *bs)
 {
     if (bs->backing == NULL) {
         /* we can be here after failed bdrv_attach_child in
          * bdrv_set_backing_hd */
         return;
     }
     pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
             bs->backing->bs->filename);
 }
 
 static void bdrv_mirror_top_child_perm(BlockDriverState *bs, BdrvChild *c,
                                        BdrvChildRole role,
                                        BlockReopenQueue *reopen_queue,
                                        uint64_t perm, uint64_t shared,
                                        uint64_t *nperm, uint64_t *nshared)
 {
     MirrorBDSOpaque *s = bs->opaque;
 
     if (s->stop) {
         /*
          * If the job is to be stopped, we do not need to forward
          * anything to the real image.
          */
         *nperm = 0;
         *nshared = BLK_PERM_ALL;
         return;
     }
 
     bdrv_default_perms(bs, c, role, reopen_queue,
                        perm, shared, nperm, nshared);
 
     if (s->is_commit) {
         /*
          * For commit jobs, we cannot take CONSISTENT_READ, because
          * that permission is unshared for everything above the base
          * node (except for filters on the base node).
          * We also have to force-share the WRITE permission, or
          * otherwise we would block ourselves at the base node (if
          * writes are blocked for a node, they are also blocked for
          * its backing file).
          * (We could also share RESIZE, because it may be needed for
          * the target if its size is less than the top node's; but
          * bdrv_default_perms_for_cow() automatically shares RESIZE
          * for backing nodes if WRITE is shared, so there is no need
          * to do it here.)
          */
         *nperm &= ~BLK_PERM_CONSISTENT_READ;
         *nshared |= BLK_PERM_WRITE;
     }
 }
 
 /* Dummy node that provides consistent read to its users without requiring it
  * from its backing file and that allows writes on the backing file chain. */
 static BlockDriver bdrv_mirror_top = {
     .format_name                = "mirror_top",
     .bdrv_co_preadv             = bdrv_mirror_top_preadv,
     .bdrv_co_pwritev            = bdrv_mirror_top_pwritev,
     .bdrv_co_pwrite_zeroes      = bdrv_mirror_top_pwrite_zeroes,
     .bdrv_co_pdiscard           = bdrv_mirror_top_pdiscard,
     .bdrv_co_flush              = bdrv_mirror_top_flush,
     .bdrv_refresh_filename      = bdrv_mirror_top_refresh_filename,
     .bdrv_child_perm            = bdrv_mirror_top_child_perm,
 
     .is_filter                  = true,
     .filtered_child_is_backing  = true,
 };
 
 static BlockJob *mirror_start_job(
                              const char *job_id, BlockDriverState *bs,
                              int creation_flags, BlockDriverState *target,
                              const char *replaces, int64_t speed,
                              uint32_t granularity, int64_t buf_size,
                              BlockMirrorBackingMode backing_mode,
                              bool zero_target,
                              BlockdevOnError on_source_error,
                              BlockdevOnError on_target_error,
                              bool unmap,
                              BlockCompletionFunc *cb,
                              void *opaque,
                              const BlockJobDriver *driver,
                              bool is_none_mode, BlockDriverState *base,
                              bool auto_complete, const char *filter_node_name,
                              bool is_mirror, MirrorCopyMode copy_mode,
                              Error **errp)
 {
     MirrorBlockJob *s;
     MirrorBDSOpaque *bs_opaque;
     BlockDriverState *mirror_top_bs;
     bool target_is_backing;
     uint64_t target_perms, target_shared_perms;
     int ret;
 
     if (granularity == 0) {
         granularity = bdrv_get_default_bitmap_granularity(target);
     }
 
     assert(is_power_of_2(granularity));
 
     if (buf_size < 0) {
         error_setg(errp, "Invalid parameter 'buf-size'");
         return NULL;
     }
 
     if (buf_size == 0) {
         buf_size = DEFAULT_MIRROR_BUF_SIZE;
     }
 
     if (bdrv_skip_filters(bs) == bdrv_skip_filters(target)) {
         error_setg(errp, "Can't mirror node into itself");
         return NULL;
     }
 
     target_is_backing = bdrv_chain_contains(bs, target);
 
     /* In the case of active commit, add dummy driver to provide consistent
      * reads on the top, while disabling it in the intermediate nodes, and make
      * the backing chain writable. */
     mirror_top_bs = bdrv_new_open_driver(&bdrv_mirror_top, filter_node_name,
                                          BDRV_O_RDWR, errp);
     if (mirror_top_bs == NULL) {
         return NULL;
     }
     if (!filter_node_name) {
         mirror_top_bs->implicit = true;
     }
 
     /* So that we can always drop this node */
     mirror_top_bs->never_freeze = true;
 
     mirror_top_bs->total_sectors = bs->total_sectors;
     mirror_top_bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED;
     mirror_top_bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
                                           BDRV_REQ_NO_FALLBACK;
     bs_opaque = g_new0(MirrorBDSOpaque, 1);
     mirror_top_bs->opaque = bs_opaque;
 
     bs_opaque->is_commit = target_is_backing;
 
     bdrv_drained_begin(bs);
     ret = bdrv_append(mirror_top_bs, bs, errp);
     bdrv_drained_end(bs);
 
     if (ret < 0) {
         bdrv_unref(mirror_top_bs);
         return NULL;
     }
 
     /* Make sure that the source is not resized while the job is running */
     s = block_job_create(job_id, driver, NULL, mirror_top_bs,
                          BLK_PERM_CONSISTENT_READ,
                          BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED |
                          BLK_PERM_WRITE, speed,
                          creation_flags, cb, opaque, errp);
     if (!s) {
         goto fail;
     }
     bs_opaque->job = s;
 
     /* The block job now has a reference to this node */
     bdrv_unref(mirror_top_bs);
 
     s->mirror_top_bs = mirror_top_bs;
 
     /* No resize for the target either; while the mirror is still running, a
      * consistent read isn't necessarily possible. We could possibly allow
      * writes and graph modifications, though it would likely defeat the
      * purpose of a mirror, so leave them blocked for now.
      *
      * In the case of active commit, things look a bit different, though,
      * because the target is an already populated backing file in active use.
      * We can allow anything except resize there.*/
 
     target_perms = BLK_PERM_WRITE;
     target_shared_perms = BLK_PERM_WRITE_UNCHANGED;
 
     if (target_is_backing) {
         int64_t bs_size, target_size;
         bs_size = bdrv_getlength(bs);
         if (bs_size < 0) {
             error_setg_errno(errp, -bs_size,
                              "Could not inquire top image size");
             goto fail;
         }
 
         target_size = bdrv_getlength(target);
         if (target_size < 0) {
             error_setg_errno(errp, -target_size,
                              "Could not inquire base image size");
             goto fail;
         }
 
         if (target_size < bs_size) {
             target_perms |= BLK_PERM_RESIZE;
         }
 
         target_shared_perms |= BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE;
     } else if (bdrv_chain_contains(bs, bdrv_skip_filters(target))) {
         /*
          * We may want to allow this in the future, but it would
          * require taking some extra care.
          */
         error_setg(errp, "Cannot mirror to a filter on top of a node in the "
                    "source's backing chain");
         goto fail;
     }
 
     s->target = blk_new(s->common.job.aio_context,
                         target_perms, target_shared_perms);
     ret = blk_insert_bs(s->target, target, errp);
     if (ret < 0) {
         goto fail;
     }
     if (is_mirror) {
         /* XXX: Mirror target could be a NBD server of target QEMU in the case
          * of non-shared block migration. To allow migration completion, we
          * have to allow "inactivate" of the target BB.  When that happens, we
          * know the job is drained, and the vcpus are stopped, so no write
          * operation will be performed. Block layer already has assertions to
          * ensure that. */
         blk_set_force_allow_inactivate(s->target);
     }
     blk_set_allow_aio_context_change(s->target, true);
     blk_set_disable_request_queuing(s->target, true);
 
     s->replaces = g_strdup(replaces);
     s->on_source_error = on_source_error;
     s->on_target_error = on_target_error;
     s->is_none_mode = is_none_mode;
     s->backing_mode = backing_mode;
     s->zero_target = zero_target;
     s->copy_mode = copy_mode;
     s->base = base;
     s->base_overlay = bdrv_find_overlay(bs, base);
     s->granularity = granularity;
     s->buf_size = ROUND_UP(buf_size, granularity);
     s->unmap = unmap;
     if (auto_complete) {
         s->should_complete = true;
     }
 
     s->dirty_bitmap = bdrv_create_dirty_bitmap(bs, granularity, NULL, errp);
     if (!s->dirty_bitmap) {
         goto fail;
     }
     if (s->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING) {
         bdrv_disable_dirty_bitmap(s->dirty_bitmap);
     }
 
     ret = block_job_add_bdrv(&s->common, "source", bs, 0,
                              BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE |
                              BLK_PERM_CONSISTENT_READ,
                              errp);
     if (ret < 0) {
         goto fail;
     }
 
     /* Required permissions are already taken with blk_new() */
     block_job_add_bdrv(&s->common, "target", target, 0, BLK_PERM_ALL,
                        &error_abort);
 
     /* In commit_active_start() all intermediate nodes disappear, so
      * any jobs in them must be blocked */
     if (target_is_backing) {
         BlockDriverState *iter, *filtered_target;
         uint64_t iter_shared_perms;
 
         /*
          * The topmost node with
          * bdrv_skip_filters(filtered_target) == bdrv_skip_filters(target)
          */
         filtered_target = bdrv_cow_bs(bdrv_find_overlay(bs, target));
 
         assert(bdrv_skip_filters(filtered_target) ==
                bdrv_skip_filters(target));
 
         /*
          * XXX BLK_PERM_WRITE needs to be allowed so we don't block
          * ourselves at s->base (if writes are blocked for a node, they are
          * also blocked for its backing file). The other options would be a
          * second filter driver above s->base (== target).
          */
         iter_shared_perms = BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE;
 
         for (iter = bdrv_filter_or_cow_bs(bs); iter != target;
              iter = bdrv_filter_or_cow_bs(iter))
         {
             if (iter == filtered_target) {
                 /*
                  * From here on, all nodes are filters on the base.
                  * This allows us to share BLK_PERM_CONSISTENT_READ.
                  */
                 iter_shared_perms |= BLK_PERM_CONSISTENT_READ;
             }
 
             ret = block_job_add_bdrv(&s->common, "intermediate node", iter, 0,
                                      iter_shared_perms, errp);
             if (ret < 0) {
                 goto fail;
             }
         }
 
         if (bdrv_freeze_backing_chain(mirror_top_bs, target, errp) < 0) {
             goto fail;
         }
     }
 
     QTAILQ_INIT(&s->ops_in_flight);
 
     trace_mirror_start(bs, s, opaque);
     job_start(&s->common.job);
 
     return &s->common;
 
 fail:
     if (s) {
         /* Make sure this BDS does not go away until we have completed the graph
          * changes below */
         bdrv_ref(mirror_top_bs);
 
         g_free(s->replaces);
         blk_unref(s->target);
         bs_opaque->job = NULL;
         if (s->dirty_bitmap) {
             bdrv_release_dirty_bitmap(s->dirty_bitmap);
         }
         job_early_fail(&s->common.job);
     }
 
     bs_opaque->stop = true;
     bdrv_child_refresh_perms(mirror_top_bs, mirror_top_bs->backing,
                              &error_abort);
     bdrv_replace_node(mirror_top_bs, mirror_top_bs->backing->bs, &error_abort);
 
     bdrv_unref(mirror_top_bs);
 
     return NULL;
 }
 
 void mirror_start(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *target, const char *replaces,
                   int creation_flags, int64_t speed,
                   uint32_t granularity, int64_t buf_size,
                   MirrorSyncMode mode, BlockMirrorBackingMode backing_mode,
                   bool zero_target,
                   BlockdevOnError on_source_error,
                   BlockdevOnError on_target_error,
                   bool unmap, const char *filter_node_name,
                   MirrorCopyMode copy_mode, Error **errp)
 {
     bool is_none_mode;
     BlockDriverState *base;
 
     GLOBAL_STATE_CODE();
 
     if ((mode == MIRROR_SYNC_MODE_INCREMENTAL) ||
         (mode == MIRROR_SYNC_MODE_BITMAP)) {
         error_setg(errp, "Sync mode '%s' not supported",
                    MirrorSyncMode_str(mode));
         return;
     }
     is_none_mode = mode == MIRROR_SYNC_MODE_NONE;
     base = mode == MIRROR_SYNC_MODE_TOP ? bdrv_backing_chain_next(bs) : NULL;
     mirror_start_job(job_id, bs, creation_flags, target, replaces,
                      speed, granularity, buf_size, backing_mode, zero_target,
                      on_source_error, on_target_error, unmap, NULL, NULL,
                      &mirror_job_driver, is_none_mode, base, false,
                      filter_node_name, true, copy_mode, errp);
 }
 
 BlockJob *commit_active_start(const char *job_id, BlockDriverState *bs,
                               BlockDriverState *base, int creation_flags,
                               int64_t speed, BlockdevOnError on_error,
                               const char *filter_node_name,
                               BlockCompletionFunc *cb, void *opaque,
                               bool auto_complete, Error **errp)
 {
     bool base_read_only;
     BlockJob *job;
 
     GLOBAL_STATE_CODE();
 
     base_read_only = bdrv_is_read_only(base);
 
     if (base_read_only) {
         if (bdrv_reopen_set_read_only(base, false, errp) < 0) {
             return NULL;
         }
     }
 
     job = mirror_start_job(
                      job_id, bs, creation_flags, base, NULL, speed, 0, 0,
                      MIRROR_LEAVE_BACKING_CHAIN, false,
                      on_error, on_error, true, cb, opaque,
                      &commit_active_job_driver, false, base, auto_complete,
                      filter_node_name, false, MIRROR_COPY_MODE_BACKGROUND,
                      errp);
     if (!job) {
         goto error_restore_flags;
     }
 
     return job;
 
 error_restore_flags:
     /* ignore error and errp for bdrv_reopen, because we want to propagate
      * the original error */
     if (base_read_only) {
         bdrv_reopen_set_read_only(base, true, NULL);
     }
     return NULL;
 }