diff --git a/block.c b/block.c
index c77cfd1..c344fd7 100644
--- a/block.c
+++ b/block.c
@@ -2025,18 +2025,11 @@ BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
     return overlay;
 }
 
-typedef struct BlkIntermediateStates {
-    BlockDriverState *bs;
-    QSIMPLEQ_ENTRY(BlkIntermediateStates) entry;
-} BlkIntermediateStates;
-
-
 /*
- * Drops images above 'base' up to and including 'top', and sets the image
- * above 'top' to have base as its backing file.
- *
- * Requires that the overlay to 'top' is opened r/w, so that the backing file
- * information in 'bs' can be properly updated.
+ * Drops images above '*base' up to and including '*top', and sets new '*base'
+ * as backing_hd of top_overlay (the image orignally has 'top' as backing
+ * file). top_overlay may be NULL if '*top' is active, no such update needed.
+ * Requires that the top_overlay to 'top' is opened r/w.
  *
  * E.g., this will convert the following chain:
  * bottom <- base <- intermediate <- top <- active
@@ -2053,82 +2046,47 @@ typedef struct BlkIntermediateStates {
  *
  * base <- active
  *
- * Error conditions:
- *  if active == top, that is considered an error
+ * It also allows active==top, in which case it converts:
+ *
+ * base <- intermediate <- active (also top)
+ *
+ * to
+ *
+ * base == active == top, i.e. only base remains: *top == *base when return.
  *
  */
-int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
-                           BlockDriverState *base)
+int bdrv_drop_intermediate(BlockDriverState *top_overlay,
+                           BlockDriverState **top,
+                           BlockDriverState **base)
 {
-    BlockDriverState *intermediate;
+    BlockDriverState *pbs;
     BlockDriverState *base_bs = NULL;
-    BlockDriverState *new_top_bs = NULL;
-    BlkIntermediateStates *intermediate_state, *next;
     int ret = -EIO;
 
-    QSIMPLEQ_HEAD(states_to_delete, BlkIntermediateStates) states_to_delete;
-    QSIMPLEQ_INIT(&states_to_delete);
-
-    if (!top->drv || !base->drv) {
+    if (!(*top)->drv || !(*base)->drv) {
         goto exit;
     }
 
-    new_top_bs = bdrv_find_overlay(active, top);
-
-    if (new_top_bs == NULL) {
-        /* we could not find the image above 'top', this is an error */
-        goto exit;
+    for (pbs = (*top)->backing_hd; pbs != *base; pbs = base_bs) {
+        assert(pbs);
+        base_bs = pbs->backing_hd;
+        pbs->backing_hd = NULL;
+        bdrv_delete(pbs);
     }
 
-    /* special case of new_top_bs->backing_hd already pointing to base - nothing
-     * to do, no intermediate images */
-    if (new_top_bs->backing_hd == base) {
-        ret = 0;
-        goto exit;
-    }
+    bdrv_swap(*base, *top);
 
-    intermediate = top;
+    (*base)->backing_hd = NULL;
+    bdrv_delete(*base);
+    *base = *top;
 
-    /* now we will go down through the list, and add each BDS we find
-     * into our deletion queue, until we hit the 'base'
-     */
-    while (intermediate) {
-        intermediate_state = g_malloc0(sizeof(BlkIntermediateStates));
-        intermediate_state->bs = intermediate;
-        QSIMPLEQ_INSERT_TAIL(&states_to_delete, intermediate_state, entry);
-
-        if (intermediate->backing_hd == base) {
-            base_bs = intermediate->backing_hd;
-            break;
-        }
-        intermediate = intermediate->backing_hd;
-    }
-    if (base_bs == NULL) {
-        /* something went wrong, we did not end at the base. safely
-         * unravel everything, and exit with error */
-        goto exit;
-    }
-
-    /* success - we can delete the intermediate states, and link top->base */
-    ret = bdrv_change_backing_file(new_top_bs, base_bs->filename,
-                                   base_bs->drv ? base_bs->drv->format_name : "");
-    if (ret) {
-        goto exit;
-    }
-    new_top_bs->backing_hd = base_bs;
-
-
-    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, next) {
-        /* so that bdrv_close() does not recursively close the chain */
-        intermediate_state->bs->backing_hd = NULL;
-        bdrv_delete(intermediate_state->bs);
+    /* overlay exists when active != top, need to change backing file for it */
+    if (top_overlay) {
+        ret = bdrv_change_backing_file(top_overlay, (*base)->filename,
+                                       (*base)->drv ?
+                                            (*base)->drv->format_name : "");
     }
-    ret = 0;
-
 exit:
-    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, next) {
-        g_free(intermediate_state);
-    }
     return ret;
 }
 
diff --git a/block/commit.c b/block/commit.c
index 2227fc2..5bbd955 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -17,14 +17,13 @@
 #include "block/blockjob.h"
 #include "qemu/ratelimit.h"
 
-enum {
-    /*
-     * Size of data buffer for populating the image file.  This should be large
-     * enough to process multiple clusters in a single call, so that populating
-     * contiguous regions of the image is efficient.
-     */
-    COMMIT_BUFFER_SIZE = 512 * 1024, /* in bytes */
-};
+/*
+ * Size of data buffer for populating the image file.  This should be large
+ * enough to process multiple clusters in a single call, so that populating
+ * contiguous regions of the image is efficient.
+ */
+#define COMMIT_BUFFER_SECTORS 128
+#define COMMIT_BUFFER_BYTES (COMMIT_BUFFER_SECTORS * BDRV_SECTOR_SIZE)
 
 #define SLICE_TIME 100000000ULL /* ns */
 
@@ -34,11 +33,27 @@ typedef struct CommitBlockJob {
     BlockDriverState *active;
     BlockDriverState *top;
     BlockDriverState *base;
+    BlockDriverState *overlay;
     BlockdevOnError on_error;
     int base_flags;
     int orig_overlay_flags;
+    bool should_complete;
+    bool ready;
 } CommitBlockJob;
 
+static void commit_complete(BlockJob *job, Error **errp)
+{
+    CommitBlockJob *s = container_of(job, CommitBlockJob, common);
+
+    if (!s->ready) {
+        error_set(errp, QERR_BLOCK_JOB_NOT_READY, job->bs->device_name);
+        return;
+    }
+
+    s->should_complete = true;
+    block_job_resume(job);
+}
+
 static int coroutine_fn commit_populate(BlockDriverState *bs,
                                         BlockDriverState *base,
                                         int64_t sector_num, int nb_sectors,
@@ -65,100 +80,125 @@ static void coroutine_fn commit_run(void *opaque)
     BlockDriverState *active = s->active;
     BlockDriverState *top = s->top;
     BlockDriverState *base = s->base;
-    BlockDriverState *overlay_bs;
     int64_t sector_num, end;
     int ret = 0;
     int n = 0;
     void *buf;
-    int bytes_written = 0;
     int64_t base_len;
+    int64_t next_dirty;
+    HBitmapIter hbi;
 
+    buf = qemu_blockalign(top, COMMIT_BUFFER_BYTES);
     ret = s->common.len = bdrv_getlength(top);
 
-
     if (s->common.len < 0) {
-        goto exit_restore_reopen;
+        goto exit;
     }
 
     ret = base_len = bdrv_getlength(base);
     if (base_len < 0) {
-        goto exit_restore_reopen;
+        goto exit;
     }
 
     if (base_len < s->common.len) {
         ret = bdrv_truncate(base, s->common.len);
         if (ret) {
-            goto exit_restore_reopen;
+            goto exit;
         }
     }
 
     end = s->common.len >> BDRV_SECTOR_BITS;
-    buf = qemu_blockalign(top, COMMIT_BUFFER_SIZE);
 
     for (sector_num = 0; sector_num < end; sector_num += n) {
-        uint64_t delay_ns = 0;
-        bool copy;
 
-wait:
-        /* Note that even when no rate limit is applied we need to yield
-         * with no pending I/O here so that bdrv_drain_all() returns.
-         */
-        block_job_sleep_ns(&s->common, rt_clock, delay_ns);
-        if (block_job_is_cancelled(&s->common)) {
-            break;
-        }
         /* Copy if allocated above the base */
         ret = bdrv_co_is_allocated_above(top, base, sector_num,
-                                         COMMIT_BUFFER_SIZE / BDRV_SECTOR_SIZE,
+                                         COMMIT_BUFFER_SECTORS,
                                          &n);
-        copy = (ret == 1);
-        trace_commit_one_iteration(s, sector_num, n, ret);
-        if (copy) {
-            if (s->common.speed) {
-                delay_ns = ratelimit_calculate_delay(&s->limit, n);
-                if (delay_ns > 0) {
-                    goto wait;
-                }
+        if (ret) {
+            bdrv_set_dirty(top, sector_num, n);
+        }
+    }
+
+    for (;;) {
+        uint64_t delay_ns = 0;
+        int64_t cnt = bdrv_get_dirty_count(s->top);
+        if (cnt == 0) {
+            if (!s->overlay && !s->ready) {
+                s->ready = true;
+                block_job_ready(&s->common);
             }
-            ret = commit_populate(top, base, sector_num, n, buf);
-            bytes_written += n * BDRV_SECTOR_SIZE;
+            /* We can complete if user called complete job or the job is
+             * committing non-active image */
+            if (s->should_complete || s->overlay) {
+                break;
+            }
+            block_job_sleep_ns(&s->common, rt_clock, delay_ns);
         }
-        if (ret < 0) {
-            if (s->on_error == BLOCKDEV_ON_ERROR_STOP ||
-                s->on_error == BLOCKDEV_ON_ERROR_REPORT||
-                (s->on_error == BLOCKDEV_ON_ERROR_ENOSPC && ret == -ENOSPC)) {
-                goto exit_free_buf;
-            } else {
-                n = 0;
-                continue;
+
+        if (block_job_is_cancelled(&s->common)) {
+            goto exit;
+        }
+
+        bdrv_dirty_iter_init(s->top, &hbi);
+        for (next_dirty = hbitmap_iter_next(&hbi);
+                next_dirty >= 0;
+                next_dirty = hbitmap_iter_next(&hbi)) {
+            sector_num = next_dirty;
+            if (block_job_is_cancelled(&s->common)) {
+                goto exit;
+            }
+            delay_ns = ratelimit_calculate_delay(&s->limit,
+                                                 COMMIT_BUFFER_SECTORS);
+            /* Note that even when no rate limit is applied we need to yield
+             * with no pending I/O here so that bdrv_drain_all() returns.
+             */
+            block_job_sleep_ns(&s->common, rt_clock, delay_ns);
+            trace_commit_one_iteration(s, sector_num,
+                                       COMMIT_BUFFER_SECTORS, ret);
+            ret = commit_populate(top, base, sector_num,
+                                  COMMIT_BUFFER_SECTORS, buf);
+            if (ret < 0) {
+                if (s->on_error == BLOCKDEV_ON_ERROR_STOP ||
+                    s->on_error == BLOCKDEV_ON_ERROR_REPORT ||
+                    (s->on_error == BLOCKDEV_ON_ERROR_ENOSPC &&
+                         ret == -ENOSPC)) {
+                    goto exit;
+                } else {
+                    continue;
+                }
             }
+            /* Publish progress */
+            s->common.offset += COMMIT_BUFFER_BYTES;
+            bdrv_reset_dirty(top, sector_num, COMMIT_BUFFER_SECTORS);
         }
-        /* Publish progress */
-        s->common.offset += n * BDRV_SECTOR_SIZE;
     }
+    s->common.offset = end;
 
-    ret = 0;
-
-    if (!block_job_is_cancelled(&s->common) && sector_num == end) {
-        /* success */
-        ret = bdrv_drop_intermediate(active, top, base);
+    bdrv_flush(base);
+    if (!block_job_is_cancelled(&s->common)) {
+        /* Drop intermediate: [top, base) */
+        ret = bdrv_drop_intermediate(s->overlay, &top, &base);
+        s->common.offset = s->common.len;
     }
 
-exit_free_buf:
-    qemu_vfree(buf);
+    ret = 0;
+
+exit:
+    bdrv_set_dirty_tracking(active, 0);
 
-exit_restore_reopen:
     /* restore base open flags here if appropriate (e.g., change the base back
      * to r/o). These reopens do not need to be atomic, since we won't abort
      * even on failure here */
-    if (s->base_flags != bdrv_get_flags(base)) {
+    if (s->overlay && s->base_flags != bdrv_get_flags(base)) {
         bdrv_reopen(base, s->base_flags, NULL);
     }
-    overlay_bs = bdrv_find_overlay(active, top);
-    if (overlay_bs && s->orig_overlay_flags != bdrv_get_flags(overlay_bs)) {
-        bdrv_reopen(overlay_bs, s->orig_overlay_flags, NULL);
+
+    if (s->overlay && s->orig_overlay_flags != bdrv_get_flags(s->overlay)) {
+        bdrv_reopen(s->overlay, s->orig_overlay_flags, NULL);
     }
 
+    qemu_vfree(buf);
     block_job_completed(&s->common, ret);
 }
 
@@ -177,6 +217,7 @@ static const BlockJobType commit_job_type = {
     .instance_size = sizeof(CommitBlockJob),
     .job_type      = "commit",
     .set_speed     = commit_set_speed,
+    .complete      = commit_complete,
 };
 
 void commit_start(BlockDriverState *bs, BlockDriverState *base,
@@ -198,13 +239,6 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base,
         return;
     }
 
-    /* Once we support top == active layer, remove this check */
-    if (top == bs) {
-        error_setg(errp,
-                   "Top image as the active layer is currently unsupported");
-        return;
-    }
-
     if (top == base) {
         error_setg(errp, "Invalid files for merge: top and base are the same");
         return;
@@ -212,23 +246,20 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base,
 
     overlay_bs = bdrv_find_overlay(bs, top);
 
-    if (overlay_bs == NULL) {
-        error_setg(errp, "Could not find overlay image for %s:", top->filename);
-        return;
-    }
-
     orig_base_flags    = bdrv_get_flags(base);
-    orig_overlay_flags = bdrv_get_flags(overlay_bs);
+    if (overlay_bs) {
+        orig_overlay_flags = bdrv_get_flags(overlay_bs);
+        if (!(orig_overlay_flags & BDRV_O_RDWR)) {
+            reopen_queue = bdrv_reopen_queue(reopen_queue, overlay_bs,
+                    orig_overlay_flags | BDRV_O_RDWR);
+        }
+    }
 
     /* convert base & overlay_bs to r/w, if necessary */
     if (!(orig_base_flags & BDRV_O_RDWR)) {
         reopen_queue = bdrv_reopen_queue(reopen_queue, base,
                                          orig_base_flags | BDRV_O_RDWR);
     }
-    if (!(orig_overlay_flags & BDRV_O_RDWR)) {
-        reopen_queue = bdrv_reopen_queue(reopen_queue, overlay_bs,
-                                         orig_overlay_flags | BDRV_O_RDWR);
-    }
     if (reopen_queue) {
         bdrv_reopen_multiple(reopen_queue, &local_err);
         if (local_err != NULL) {
@@ -237,7 +268,6 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base,
         }
     }
 
-
     s = block_job_create(&commit_job_type, bs, speed, cb, opaque, errp);
     if (!s) {
         return;
@@ -246,13 +276,19 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base,
     s->base   = base;
     s->top    = top;
     s->active = bs;
+    s->overlay = overlay_bs;
 
     s->base_flags          = orig_base_flags;
-    s->orig_overlay_flags  = orig_overlay_flags;
+    if (overlay_bs) {
+        s->orig_overlay_flags  = orig_overlay_flags;
+    }
 
     s->on_error = on_error;
     s->common.co = qemu_coroutine_create(commit_run);
 
     trace_commit_start(bs, base, top, s, s->common.co, opaque);
+
+    bdrv_set_dirty_tracking(top, COMMIT_BUFFER_BYTES);
+
     qemu_coroutine_enter(s->common.co, s);
 }
diff --git a/include/block/block.h b/include/block/block.h
index 742fce5..a5d05ab 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -199,8 +199,9 @@ int bdrv_commit_all(void);
 int bdrv_change_backing_file(BlockDriverState *bs,
     const char *backing_file, const char *backing_fmt);
 void bdrv_register(BlockDriver *bdrv);
-int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
-                           BlockDriverState *base);
+int bdrv_drop_intermediate(BlockDriverState *top_overlay,
+                           BlockDriverState **top,
+                           BlockDriverState **base);
 BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
                                     BlockDriverState *bs);
 BlockDriverState *bdrv_find_base(BlockDriverState *bs);
