Merge remote-tracking branch 'remotes/jnsnow/tags/bitmaps-pull-request' into staging

Pull request

# gpg: Signature made Wed 16 Jan 2019 01:00:25 GMT
# gpg:                using RSA key 7DEF8106AAFC390E
# gpg: Good signature from "John Snow (John Huston) <jsnow@redhat.com>"
# Primary key fingerprint: FAEB 9711 A12C F475 812F  18F2 88A9 064D 1835 61EB
#      Subkey fingerprint: F9B7 ABDB BCAC DF95 BE76  CBD0 7DEF 8106 AAFC 390E

* remotes/jnsnow/tags/bitmaps-pull-request:
  Revert "hbitmap: Add @advance param to hbitmap_iter_next()"
  Revert "test-hbitmap: Add non-advancing iter_next tests"
  Revert "block/dirty-bitmap: Add bdrv_dirty_iter_next_area"
  block/mirror: fix and improve do_sync_target_write
  tests: add tests for hbitmap_next_dirty_area
  dirty-bitmap: add bdrv_dirty_bitmap_next_dirty_area
  tests: add tests for hbitmap_next_zero with specified end parameter
  dirty-bitmap: improve bdrv_dirty_bitmap_next_zero

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

8 files changed, 262 insertions, 121 deletions

diff --git a/block/backup.c b/block/backup.c
index b829b25..435414e 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -385,7 +385,7 @@ static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
     HBitmapIter hbi;
 
     hbitmap_iter_init(&hbi, job->copy_bitmap, 0);
-    while ((cluster = hbitmap_iter_next(&hbi, true)) != -1) {
+    while ((cluster = hbitmap_iter_next(&hbi)) != -1) {
         do {
             if (yield_and_check(job)) {
                 return 0;
@@ -422,7 +422,8 @@ static void backup_incremental_init_copy_bitmap(BackupBlockJob *job)
             break;
         }
 
-        offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset);
+        offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset,
+                                             UINT64_MAX);
         if (offset == -1) {
             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
             break;
diff --git a/block/dirty-bitmap.c b/block/dirty-bitmap.c
index 6b68839..00ea36f 100644
--- a/block/dirty-bitmap.c
+++ b/block/dirty-bitmap.c
@@ -515,62 +515,7 @@ void bdrv_dirty_iter_free(BdrvDirtyBitmapIter *iter)
 
 int64_t bdrv_dirty_iter_next(BdrvDirtyBitmapIter *iter)
 {
-    return hbitmap_iter_next(&iter->hbi, true);
-}
-
-/**
- * Return the next consecutively dirty area in the dirty bitmap
- * belonging to the given iterator @iter.
- *
- * @max_offset: Maximum value that may be returned for
- *              *offset + *bytes
- * @offset:     Will contain the start offset of the next dirty area
- * @bytes:      Will contain the length of the next dirty area
- *
- * Returns: True if a dirty area could be found before max_offset
- *          (which means that *offset and *bytes then contain valid
- *          values), false otherwise.
- *
- * Note that @iter is never advanced if false is returned.  If an area
- * is found (which means that true is returned), it will be advanced
- * past that area.
- */
-bool bdrv_dirty_iter_next_area(BdrvDirtyBitmapIter *iter, uint64_t max_offset,
-                               uint64_t *offset, int *bytes)
-{
-    uint32_t granularity = bdrv_dirty_bitmap_granularity(iter->bitmap);
-    uint64_t gran_max_offset;
-    int64_t ret;
-    int size;
-
-    if (max_offset == iter->bitmap->size) {
-        /* If max_offset points to the image end, round it up by the
-         * bitmap granularity */
-        gran_max_offset = ROUND_UP(max_offset, granularity);
-    } else {
-        gran_max_offset = max_offset;
-    }
-
-    ret = hbitmap_iter_next(&iter->hbi, false);
-    if (ret < 0 || ret + granularity > gran_max_offset) {
-        return false;
-    }
-
-    *offset = ret;
-    size = 0;
-
-    assert(granularity <= INT_MAX);
-
-    do {
-        /* Advance iterator */
-        ret = hbitmap_iter_next(&iter->hbi, true);
-        size += granularity;
-    } while (ret + granularity <= gran_max_offset &&
-             hbitmap_iter_next(&iter->hbi, false) == ret + granularity &&
-             size <= INT_MAX - granularity);
-
-    *bytes = MIN(size, max_offset - *offset);
-    return true;
+    return hbitmap_iter_next(&iter->hbi);
 }
 
 /* Called within bdrv_dirty_bitmap_lock..unlock */
@@ -781,9 +726,16 @@ char *bdrv_dirty_bitmap_sha256(const BdrvDirtyBitmap *bitmap, Error **errp)
     return hbitmap_sha256(bitmap->bitmap, errp);
 }
 
-int64_t bdrv_dirty_bitmap_next_zero(BdrvDirtyBitmap *bitmap, uint64_t offset)
+int64_t bdrv_dirty_bitmap_next_zero(BdrvDirtyBitmap *bitmap, uint64_t offset,
+                                    uint64_t bytes)
+{
+    return hbitmap_next_zero(bitmap->bitmap, offset, bytes);
+}
+
+bool bdrv_dirty_bitmap_next_dirty_area(BdrvDirtyBitmap *bitmap,
+                                       uint64_t *offset, uint64_t *bytes)
 {
-    return hbitmap_next_zero(bitmap->bitmap, offset);
+    return hbitmap_next_dirty_area(bitmap->bitmap, offset, bytes);
 }
 
 void bdrv_merge_dirty_bitmap(BdrvDirtyBitmap *dest, const BdrvDirtyBitmap *src,
diff --git a/block/mirror.c b/block/mirror.c
index f0b211a..24ede6f 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1185,25 +1185,23 @@ do_sync_target_write(MirrorBlockJob *job, MirrorMethod method,
                      uint64_t offset, uint64_t bytes,
                      QEMUIOVector *qiov, int flags)
 {
-    BdrvDirtyBitmapIter *iter;
     QEMUIOVector target_qiov;
-    uint64_t dirty_offset;
-    int dirty_bytes;
+    uint64_t dirty_offset = offset;
+    uint64_t dirty_bytes;
 
     if (qiov) {
         qemu_iovec_init(&target_qiov, qiov->niov);
     }
 
-    iter = bdrv_dirty_iter_new(job->dirty_bitmap);
-    bdrv_set_dirty_iter(iter, offset);
-
     while (true) {
         bool valid_area;
         int ret;
 
         bdrv_dirty_bitmap_lock(job->dirty_bitmap);
-        valid_area = bdrv_dirty_iter_next_area(iter, offset + bytes,
-                                               &dirty_offset, &dirty_bytes);
+        dirty_bytes = MIN(offset + bytes - dirty_offset, INT_MAX);
+        valid_area = bdrv_dirty_bitmap_next_dirty_area(job->dirty_bitmap,
+                                                       &dirty_offset,
+                                                       &dirty_bytes);
         if (!valid_area) {
             bdrv_dirty_bitmap_unlock(job->dirty_bitmap);
             break;
@@ -1259,9 +1257,10 @@ do_sync_target_write(MirrorBlockJob *job, MirrorMethod method,
                 break;
             }
         }
+
+        dirty_offset += dirty_bytes;
     }
 
-    bdrv_dirty_iter_free(iter);
     if (qiov) {
         qemu_iovec_destroy(&target_qiov);
     }
diff --git a/include/block/dirty-bitmap.h b/include/block/dirty-bitmap.h
index 8f38a3d..04a117f 100644
--- a/include/block/dirty-bitmap.h
+++ b/include/block/dirty-bitmap.h
@@ -83,8 +83,6 @@ void bdrv_set_dirty_bitmap_locked(BdrvDirtyBitmap *bitmap,
 void bdrv_reset_dirty_bitmap_locked(BdrvDirtyBitmap *bitmap,
                                     int64_t offset, int64_t bytes);
 int64_t bdrv_dirty_iter_next(BdrvDirtyBitmapIter *iter);
-bool bdrv_dirty_iter_next_area(BdrvDirtyBitmapIter *iter, uint64_t max_offset,
-                               uint64_t *offset, int *bytes);
 void bdrv_set_dirty_iter(BdrvDirtyBitmapIter *hbi, int64_t offset);
 int64_t bdrv_get_dirty_count(BdrvDirtyBitmap *bitmap);
 int64_t bdrv_get_meta_dirty_count(BdrvDirtyBitmap *bitmap);
@@ -99,7 +97,10 @@ bool bdrv_has_changed_persistent_bitmaps(BlockDriverState *bs);
 BdrvDirtyBitmap *bdrv_dirty_bitmap_next(BlockDriverState *bs,
                                         BdrvDirtyBitmap *bitmap);
 char *bdrv_dirty_bitmap_sha256(const BdrvDirtyBitmap *bitmap, Error **errp);
-int64_t bdrv_dirty_bitmap_next_zero(BdrvDirtyBitmap *bitmap, uint64_t start);
+int64_t bdrv_dirty_bitmap_next_zero(BdrvDirtyBitmap *bitmap, uint64_t offset,
+                                    uint64_t bytes);
+bool bdrv_dirty_bitmap_next_dirty_area(BdrvDirtyBitmap *bitmap,
+                                       uint64_t *offset, uint64_t *bytes);
 BdrvDirtyBitmap *bdrv_reclaim_dirty_bitmap_locked(BlockDriverState *bs,
                                                   BdrvDirtyBitmap *bitmap,
                                                   Error **errp);
diff --git a/include/qemu/hbitmap.h b/include/qemu/hbitmap.h
index a7cb780..4afbe62 100644
--- a/include/qemu/hbitmap.h
+++ b/include/qemu/hbitmap.h
@@ -300,12 +300,32 @@ void hbitmap_iter_init(HBitmapIter *hbi, const HBitmap *hb, uint64_t first);
 unsigned long hbitmap_iter_skip_words(HBitmapIter *hbi);
 
 /* hbitmap_next_zero:
+ *
+ * Find next not dirty bit within selected range. If not found, return -1.
+ *
  * @hb: The HBitmap to operate on
  * @start: The bit to start from.
- *
- * Find next not dirty bit.
+ * @count: Number of bits to proceed. If @start+@count > bitmap size, the whole
+ * bitmap is looked through. You can use UINT64_MAX as @count to search up to
+ * the bitmap end.
+ */
+int64_t hbitmap_next_zero(const HBitmap *hb, uint64_t start, uint64_t count);
+
+/* hbitmap_next_dirty_area:
+ * @hb: The HBitmap to operate on
+ * @start: in-out parameter.
+ *         in: the offset to start from
+ *         out: (if area found) start of found area
+ * @count: in-out parameter.
+ *         in: length of requested region
+ *         out: length of found area
+ *
+ * If dirty area found within [@start, @start + @count), returns true and sets
+ * @offset and @bytes appropriately. Otherwise returns false and leaves @offset
+ * and @bytes unchanged.
  */
-int64_t hbitmap_next_zero(const HBitmap *hb, uint64_t start);
+bool hbitmap_next_dirty_area(const HBitmap *hb, uint64_t *start,
+                             uint64_t *count);
 
 /* hbitmap_create_meta:
  * Create a "meta" hbitmap to track dirtiness of the bits in this HBitmap.
@@ -331,14 +351,11 @@ void hbitmap_free_meta(HBitmap *hb);
 /**
  * hbitmap_iter_next:
  * @hbi: HBitmapIter to operate on.
- * @advance: If true, advance the iterator.  Otherwise, the next call
- *           of this function will return the same result (if that
- *           position is still dirty).
  *
  * Return the next bit that is set in @hbi's associated HBitmap,
  * or -1 if all remaining bits are zero.
  */
-int64_t hbitmap_iter_next(HBitmapIter *hbi, bool advance);
+int64_t hbitmap_iter_next(HBitmapIter *hbi);
 
 /**
  * hbitmap_iter_next_word:
diff --git a/nbd/server.c b/nbd/server.c
index e8c5660..6b13601 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -2014,7 +2014,7 @@ static unsigned int bitmap_to_extents(BdrvDirtyBitmap *bitmap, uint64_t offset,
         bool next_dirty = !dirty;
 
         if (dirty) {
-            end = bdrv_dirty_bitmap_next_zero(bitmap, begin);
+            end = bdrv_dirty_bitmap_next_zero(bitmap, begin, UINT64_MAX);
         } else {
             bdrv_set_dirty_iter(it, begin);
             end = bdrv_dirty_iter_next(it);
diff --git a/tests/test-hbitmap.c b/tests/test-hbitmap.c
index 5e67ac1..592d821 100644
--- a/tests/test-hbitmap.c
+++ b/tests/test-hbitmap.c
@@ -30,18 +30,6 @@ typedef struct TestHBitmapData {
 } TestHBitmapData;
 
 
-static int64_t check_hbitmap_iter_next(HBitmapIter *hbi)
-{
-    int next0, next1;
-
-    next0 = hbitmap_iter_next(hbi, false);
-    next1 = hbitmap_iter_next(hbi, true);
-
-    g_assert_cmpint(next0, ==, next1);
-
-    return next0;
-}
-
 /* Check that the HBitmap and the shadow bitmap contain the same data,
  * ignoring the same "first" bits.
  */
@@ -58,7 +46,7 @@ static void hbitmap_test_check(TestHBitmapData *data,
 
     i = first;
     for (;;) {
-        next = check_hbitmap_iter_next(&hbi);
+        next = hbitmap_iter_next(&hbi);
         if (next < 0) {
             next = data->size;
         }
@@ -447,25 +435,25 @@ static void test_hbitmap_iter_granularity(TestHBitmapData *data,
     /* Note that hbitmap_test_check has to be invoked manually in this test.  */
     hbitmap_test_init(data, 131072 << 7, 7);
     hbitmap_iter_init(&hbi, data->hb, 0);
-    g_assert_cmpint(check_hbitmap_iter_next(&hbi), <, 0);
+    g_assert_cmpint(hbitmap_iter_next(&hbi), <, 0);
 
     hbitmap_test_set(data, ((L2 + L1 + 1) << 7) + 8, 8);
     hbitmap_iter_init(&hbi, data->hb, 0);
-    g_assert_cmpint(check_hbitmap_iter_next(&hbi), ==, (L2 + L1 + 1) << 7);
-    g_assert_cmpint(check_hbitmap_iter_next(&hbi), <, 0);
+    g_assert_cmpint(hbitmap_iter_next(&hbi), ==, (L2 + L1 + 1) << 7);
+    g_assert_cmpint(hbitmap_iter_next(&hbi), <, 0);
 
     hbitmap_iter_init(&hbi, data->hb, (L2 + L1 + 2) << 7);
-    g_assert_cmpint(hbitmap_iter_next(&hbi, true), <, 0);
+    g_assert_cmpint(hbitmap_iter_next(&hbi), <, 0);
 
     hbitmap_test_set(data, (131072 << 7) - 8, 8);
     hbitmap_iter_init(&hbi, data->hb, 0);
-    g_assert_cmpint(check_hbitmap_iter_next(&hbi), ==, (L2 + L1 + 1) << 7);
-    g_assert_cmpint(check_hbitmap_iter_next(&hbi), ==, 131071 << 7);
-    g_assert_cmpint(check_hbitmap_iter_next(&hbi), <, 0);
+    g_assert_cmpint(hbitmap_iter_next(&hbi), ==, (L2 + L1 + 1) << 7);
+    g_assert_cmpint(hbitmap_iter_next(&hbi), ==, 131071 << 7);
+    g_assert_cmpint(hbitmap_iter_next(&hbi), <, 0);
 
     hbitmap_iter_init(&hbi, data->hb, (L2 + L1 + 2) << 7);
-    g_assert_cmpint(check_hbitmap_iter_next(&hbi), ==, 131071 << 7);
-    g_assert_cmpint(check_hbitmap_iter_next(&hbi), <, 0);
+    g_assert_cmpint(hbitmap_iter_next(&hbi), ==, 131071 << 7);
+    g_assert_cmpint(hbitmap_iter_next(&hbi), <, 0);
 }
 
 static void hbitmap_test_set_boundary_bits(TestHBitmapData *data, ssize_t diff)
@@ -905,7 +893,7 @@ static void test_hbitmap_serialize_zeroes(TestHBitmapData *data,
     for (i = 0; i < num_positions; i++) {
         hbitmap_deserialize_zeroes(data->hb, positions[i], min_l1, true);
         hbitmap_iter_init(&iter, data->hb, 0);
-        next = check_hbitmap_iter_next(&iter);
+        next = hbitmap_iter_next(&iter);
         if (i == num_positions - 1) {
             g_assert_cmpint(next, ==, -1);
         } else {
@@ -931,37 +919,55 @@ static void test_hbitmap_iter_and_reset(TestHBitmapData *data,
 
     hbitmap_iter_init(&hbi, data->hb, BITS_PER_LONG - 1);
 
-    check_hbitmap_iter_next(&hbi);
+    hbitmap_iter_next(&hbi);
 
     hbitmap_reset_all(data->hb);
-    check_hbitmap_iter_next(&hbi);
+    hbitmap_iter_next(&hbi);
 }
 
-static void test_hbitmap_next_zero_check(TestHBitmapData *data, int64_t start)
+static void test_hbitmap_next_zero_check_range(TestHBitmapData *data,
+                                               uint64_t start,
+                                               uint64_t count)
 {
-    int64_t ret1 = hbitmap_next_zero(data->hb, start);
+    int64_t ret1 = hbitmap_next_zero(data->hb, start, count);
     int64_t ret2 = start;
-    for ( ; ret2 < data->size && hbitmap_get(data->hb, ret2); ret2++) {
+    int64_t end = start >= data->size || data->size - start < count ?
+                data->size : start + count;
+
+    for ( ; ret2 < end && hbitmap_get(data->hb, ret2); ret2++) {
         ;
     }
-    if (ret2 == data->size) {
+    if (ret2 == end) {
         ret2 = -1;
     }
 
     g_assert_cmpint(ret1, ==, ret2);
 }
 
+static void test_hbitmap_next_zero_check(TestHBitmapData *data, int64_t start)
+{
+    test_hbitmap_next_zero_check_range(data, start, UINT64_MAX);
+}
+
 static void test_hbitmap_next_zero_do(TestHBitmapData *data, int granularity)
 {
     hbitmap_test_init(data, L3, granularity);
     test_hbitmap_next_zero_check(data, 0);
     test_hbitmap_next_zero_check(data, L3 - 1);
+    test_hbitmap_next_zero_check_range(data, 0, 1);
+    test_hbitmap_next_zero_check_range(data, L3 - 1, 1);
 
     hbitmap_set(data->hb, L2, 1);
     test_hbitmap_next_zero_check(data, 0);
     test_hbitmap_next_zero_check(data, L2 - 1);
     test_hbitmap_next_zero_check(data, L2);
     test_hbitmap_next_zero_check(data, L2 + 1);
+    test_hbitmap_next_zero_check_range(data, 0, 1);
+    test_hbitmap_next_zero_check_range(data, 0, L2);
+    test_hbitmap_next_zero_check_range(data, L2 - 1, 1);
+    test_hbitmap_next_zero_check_range(data, L2 - 1, 2);
+    test_hbitmap_next_zero_check_range(data, L2, 1);
+    test_hbitmap_next_zero_check_range(data, L2 + 1, 1);
 
     hbitmap_set(data->hb, L2 + 5, L1);
     test_hbitmap_next_zero_check(data, 0);
@@ -970,6 +976,10 @@ static void test_hbitmap_next_zero_do(TestHBitmapData *data, int granularity)
     test_hbitmap_next_zero_check(data, L2 + 5);
     test_hbitmap_next_zero_check(data, L2 + L1 - 1);
     test_hbitmap_next_zero_check(data, L2 + L1);
+    test_hbitmap_next_zero_check_range(data, L2, 6);
+    test_hbitmap_next_zero_check_range(data, L2 + 1, 3);
+    test_hbitmap_next_zero_check_range(data, L2 + 4, L1);
+    test_hbitmap_next_zero_check_range(data, L2 + 5, L1);
 
     hbitmap_set(data->hb, L2 * 2, L3 - L2 * 2);
     test_hbitmap_next_zero_check(data, L2 * 2 - L1);
@@ -977,6 +987,8 @@ static void test_hbitmap_next_zero_do(TestHBitmapData *data, int granularity)
     test_hbitmap_next_zero_check(data, L2 * 2 - 1);
     test_hbitmap_next_zero_check(data, L2 * 2);
     test_hbitmap_next_zero_check(data, L3 - 1);
+    test_hbitmap_next_zero_check_range(data, L2 * 2 - L1, L1 + 1);
+    test_hbitmap_next_zero_check_range(data, L2 * 2, L2);
 
     hbitmap_set(data->hb, 0, L3);
     test_hbitmap_next_zero_check(data, 0);
@@ -992,6 +1004,106 @@ static void test_hbitmap_next_zero_4(TestHBitmapData *data, const void *unused)
     test_hbitmap_next_zero_do(data, 4);
 }
 
+static void test_hbitmap_next_dirty_area_check(TestHBitmapData *data,
+                                               uint64_t offset,
+                                               uint64_t count)
+{
+    uint64_t off1, off2;
+    uint64_t len1 = 0, len2;
+    bool ret1, ret2;
+    int64_t end;
+
+    off1 = offset;
+    len1 = count;
+    ret1 = hbitmap_next_dirty_area(data->hb, &off1, &len1);
+
+    end = offset > data->size || data->size - offset < count ? data->size :
+                                                               offset + count;
+
+    for (off2 = offset; off2 < end && !hbitmap_get(data->hb, off2); off2++) {
+        ;
+    }
+
+    for (len2 = 1; off2 + len2 < end && hbitmap_get(data->hb, off2 + len2);
+         len2++) {
+        ;
+    }
+
+    ret2 = off2 < end;
+    if (!ret2) {
+        /* leave unchanged */
+        off2 = offset;
+        len2 = count;
+    }
+
+    g_assert_cmpint(ret1, ==, ret2);
+    g_assert_cmpint(off1, ==, off2);
+    g_assert_cmpint(len1, ==, len2);
+}
+
+static void test_hbitmap_next_dirty_area_do(TestHBitmapData *data,
+                                            int granularity)
+{
+    hbitmap_test_init(data, L3, granularity);
+    test_hbitmap_next_dirty_area_check(data, 0, UINT64_MAX);
+    test_hbitmap_next_dirty_area_check(data, 0, 1);
+    test_hbitmap_next_dirty_area_check(data, L3 - 1, 1);
+
+    hbitmap_set(data->hb, L2, 1);
+    test_hbitmap_next_dirty_area_check(data, 0, 1);
+    test_hbitmap_next_dirty_area_check(data, 0, L2);
+    test_hbitmap_next_dirty_area_check(data, 0, UINT64_MAX);
+    test_hbitmap_next_dirty_area_check(data, L2 - 1, UINT64_MAX);
+    test_hbitmap_next_dirty_area_check(data, L2 - 1, 1);
+    test_hbitmap_next_dirty_area_check(data, L2 - 1, 2);
+    test_hbitmap_next_dirty_area_check(data, L2 - 1, 3);
+    test_hbitmap_next_dirty_area_check(data, L2, UINT64_MAX);
+    test_hbitmap_next_dirty_area_check(data, L2, 1);
+    test_hbitmap_next_dirty_area_check(data, L2 + 1, 1);
+
+    hbitmap_set(data->hb, L2 + 5, L1);
+    test_hbitmap_next_dirty_area_check(data, 0, UINT64_MAX);
+    test_hbitmap_next_dirty_area_check(data, L2 - 2, 8);
+    test_hbitmap_next_dirty_area_check(data, L2 + 1, 5);
+    test_hbitmap_next_dirty_area_check(data, L2 + 1, 3);
+    test_hbitmap_next_dirty_area_check(data, L2 + 4, L1);
+    test_hbitmap_next_dirty_area_check(data, L2 + 5, L1);
+    test_hbitmap_next_dirty_area_check(data, L2 + 7, L1);
+    test_hbitmap_next_dirty_area_check(data, L2 + L1, L1);
+    test_hbitmap_next_dirty_area_check(data, L2, 0);
+    test_hbitmap_next_dirty_area_check(data, L2 + 1, 0);
+
+    hbitmap_set(data->hb, L2 * 2, L3 - L2 * 2);
+    test_hbitmap_next_dirty_area_check(data, 0, UINT64_MAX);
+    test_hbitmap_next_dirty_area_check(data, L2, UINT64_MAX);
+    test_hbitmap_next_dirty_area_check(data, L2 + 1, UINT64_MAX);
+    test_hbitmap_next_dirty_area_check(data, L2 + 5 + L1 - 1, UINT64_MAX);
+    test_hbitmap_next_dirty_area_check(data, L2 + 5 + L1, 5);
+    test_hbitmap_next_dirty_area_check(data, L2 * 2 - L1, L1 + 1);
+    test_hbitmap_next_dirty_area_check(data, L2 * 2, L2);
+
+    hbitmap_set(data->hb, 0, L3);
+    test_hbitmap_next_dirty_area_check(data, 0, UINT64_MAX);
+}
+
+static void test_hbitmap_next_dirty_area_0(TestHBitmapData *data,
+                                           const void *unused)
+{
+    test_hbitmap_next_dirty_area_do(data, 0);
+}
+
+static void test_hbitmap_next_dirty_area_1(TestHBitmapData *data,
+                                           const void *unused)
+{
+    test_hbitmap_next_dirty_area_do(data, 1);
+}
+
+static void test_hbitmap_next_dirty_area_4(TestHBitmapData *data,
+                                           const void *unused)
+{
+    test_hbitmap_next_dirty_area_do(data, 4);
+}
+
 int main(int argc, char **argv)
 {
     g_test_init(&argc, &argv, NULL);
@@ -1058,6 +1170,13 @@ int main(int argc, char **argv)
     hbitmap_test_add("/hbitmap/next_zero/next_zero_4",
                      test_hbitmap_next_zero_4);
 
+    hbitmap_test_add("/hbitmap/next_dirty_area/next_dirty_area_0",
+                     test_hbitmap_next_dirty_area_0);
+    hbitmap_test_add("/hbitmap/next_dirty_area/next_dirty_area_1",
+                     test_hbitmap_next_dirty_area_1);
+    hbitmap_test_add("/hbitmap/next_dirty_area/next_dirty_area_4",
+                     test_hbitmap_next_dirty_area_4);
+
     g_test_run();
 
     return 0;
diff --git a/util/hbitmap.c b/util/hbitmap.c
index 8d402c5..7905212 100644
--- a/util/hbitmap.c
+++ b/util/hbitmap.c
@@ -53,6 +53,9 @@
  */
 
 struct HBitmap {
+    /* Size of the bitmap, as requested in hbitmap_alloc. */
+    uint64_t orig_size;
+
     /* Number of total bits in the bottom level.  */
     uint64_t size;
 
@@ -141,7 +144,7 @@ unsigned long hbitmap_iter_skip_words(HBitmapIter *hbi)
     return cur;
 }
 
-int64_t hbitmap_iter_next(HBitmapIter *hbi, bool advance)
+int64_t hbitmap_iter_next(HBitmapIter *hbi)
 {
     unsigned long cur = hbi->cur[HBITMAP_LEVELS - 1] &
             hbi->hb->levels[HBITMAP_LEVELS - 1][hbi->pos];
@@ -154,12 +157,8 @@ int64_t hbitmap_iter_next(HBitmapIter *hbi, bool advance)
         }
     }
 
-    if (advance) {
-        /* The next call will resume work from the next bit.  */
-        hbi->cur[HBITMAP_LEVELS - 1] = cur & (cur - 1);
-    } else {
-        hbi->cur[HBITMAP_LEVELS - 1] = cur;
-    }
+    /* The next call will resume work from the next bit.  */
+    hbi->cur[HBITMAP_LEVELS - 1] = cur & (cur - 1);
     item = ((uint64_t)hbi->pos << BITS_PER_LEVEL) + ctzl(cur);
 
     return item << hbi->granularity;
@@ -192,16 +191,28 @@ void hbitmap_iter_init(HBitmapIter *hbi, const HBitmap *hb, uint64_t first)
     }
 }
 
-int64_t hbitmap_next_zero(const HBitmap *hb, uint64_t start)
+int64_t hbitmap_next_zero(const HBitmap *hb, uint64_t start, uint64_t count)
 {
     size_t pos = (start >> hb->granularity) >> BITS_PER_LEVEL;
     unsigned long *last_lev = hb->levels[HBITMAP_LEVELS - 1];
-    uint64_t sz = hb->sizes[HBITMAP_LEVELS - 1];
     unsigned long cur = last_lev[pos];
-    unsigned start_bit_offset =
-            (start >> hb->granularity) & (BITS_PER_LONG - 1);
+    unsigned start_bit_offset;
+    uint64_t end_bit, sz;
     int64_t res;
 
+    if (start >= hb->orig_size || count == 0) {
+        return -1;
+    }
+
+    end_bit = count > hb->orig_size - start ?
+                hb->size :
+                ((start + count - 1) >> hb->granularity) + 1;
+    sz = (end_bit + BITS_PER_LONG - 1) >> BITS_PER_LEVEL;
+
+    /* There may be some zero bits in @cur before @start. We are not interested
+     * in them, let's set them.
+     */
+    start_bit_offset = (start >> hb->granularity) & (BITS_PER_LONG - 1);
     cur |= (1UL << start_bit_offset) - 1;
     assert((start >> hb->granularity) < hb->size);
 
@@ -218,7 +229,7 @@ int64_t hbitmap_next_zero(const HBitmap *hb, uint64_t start)
     }
 
     res = (pos << BITS_PER_LEVEL) + ctol(cur);
-    if (res >= hb->size) {
+    if (res >= end_bit) {
         return -1;
     }
 
@@ -231,6 +242,45 @@ int64_t hbitmap_next_zero(const HBitmap *hb, uint64_t start)
     return res;
 }
 
+bool hbitmap_next_dirty_area(const HBitmap *hb, uint64_t *start,
+                             uint64_t *count)
+{
+    HBitmapIter hbi;
+    int64_t firt_dirty_off, area_end;
+    uint32_t granularity = 1UL << hb->granularity;
+    uint64_t end;
+
+    if (*start >= hb->orig_size || *count == 0) {
+        return false;
+    }
+
+    end = *count > hb->orig_size - *start ? hb->orig_size : *start + *count;
+
+    hbitmap_iter_init(&hbi, hb, *start);
+    firt_dirty_off = hbitmap_iter_next(&hbi);
+
+    if (firt_dirty_off < 0 || firt_dirty_off >= end) {
+        return false;
+    }
+
+    if (firt_dirty_off + granularity >= end) {
+        area_end = end;
+    } else {
+        area_end = hbitmap_next_zero(hb, firt_dirty_off + granularity,
+                                     end - firt_dirty_off - granularity);
+        if (area_end < 0) {
+            area_end = end;
+        }
+    }
+
+    if (firt_dirty_off > *start) {
+        *start = firt_dirty_off;
+    }
+    *count = area_end - *start;
+
+    return true;
+}
+
 bool hbitmap_empty(const HBitmap *hb)
 {
     return hb->count == 0;
@@ -652,6 +702,8 @@ HBitmap *hbitmap_alloc(uint64_t size, int granularity)
     HBitmap *hb = g_new0(struct HBitmap, 1);
     unsigned i;
 
+    hb->orig_size = size;
+
     assert(granularity >= 0 && granularity < 64);
     size = (size + (1ULL << granularity) - 1) >> granularity;
     assert(size <= ((uint64_t)1 << HBITMAP_LOG_MAX_SIZE));