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Re: [Qemu-devel] [Qemu-block] [PATCH] block/qcow2: fix logic around dirt

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH] block/qcow2: fix logic around dirty_bitmaps_loaded
Date: Wed, 20 Jun 2018 20:20:58 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

20.06.2018 20:17, Vladimir Sementsov-Ogievskiy wrote:
13.06.2018 01:18, John Snow wrote:

On 06/12/2018 06:11 PM, John Snow wrote:

On 06/12/2018 01:26 PM, Vladimir Sementsov-Ogievskiy wrote:
First: this variable was introduced to handle reopens. We need it on
following qcow2_do_open, to don't try loading bitmaps again. So, we are
fixing qcow2_invalidate_cache().

However, if we fix only qcow2_invalidate_cache, iotest 169 fails on
case test__persistent__not_migbitmap__online_shared, because on first
target open, source is not yet closed, and is not yet stored bitmaps,
so, we are load nothing. And on second open, dirty_bitmaps_loaded is
already true, but we have no bitmaps to reopen with
qcow2_reopen_bitmaps_rw_hint(). So, we are fixing qcow2_do_open() too.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
  block/qcow2.c | 4 +++-
  1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 6fa5e1d71a..b4216a5830 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1499,7 +1499,7 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
              qcow2_reopen_bitmaps_rw_hint(bs, &header_updated, &local_err);
-    } else {
+    } else if (s->nb_bitmaps > 0) {
          header_updated = qcow2_load_dirty_bitmaps(bs, &local_err);
          s->dirty_bitmaps_loaded = true;
@@ -2178,6 +2178,7 @@ static void coroutine_fn qcow2_co_invalidate_cache(BlockDriverState *bs,
      QDict *options;
      Error *local_err = NULL;
      int ret;
+    bool dirty_bitmaps_loaded = s->dirty_bitmaps_loaded;
       * Backing files are read-only which makes all of their metadata immutable, @@ -2190,6 +2191,7 @@ static void coroutine_fn qcow2_co_invalidate_cache(BlockDriverState *bs,
        memset(s, 0, sizeof(BDRVQcow2State));
+    s->dirty_bitmaps_loaded = dirty_bitmaps_loaded;
      options = qdict_clone_shallow(bs->options);
        flags &= ~BDRV_O_INACTIVE;

Ah, tch... I didn't realize that invalidate completely wipes the qcow2

I guess the other three fields, nb_bitmaps, bitmap_directory_size and
bitmap_directory_offset get initialized again on re-open.

(I suppose this means I need to rethink caching bm_list more carefully,

I think this looks correct mechanically.

Oh, I meant:

Reviewed-by: John Snow <address@hidden>

It is interesting, this patch also fixes iotest 169. It's not broken formally, but if we look at vm output of vm_b after migration, we'll see

qemu-system-x86_64: Could not reopen qcow2 layer: Bitmap already exists: bitmap0

which leads to failed invalidation and bs->drv = NULL :)

I'll improve iotest somehow, to show this failure.

the bug can be shown with the following workaround:

diff --git a/tests/qemu-iotests/169 b/tests/qemu-iotests/169
index f243db9955..6dce24d29f 100755
--- a/tests/qemu-iotests/169
+++ b/tests/qemu-iotests/169
@@ -131,9 +131,13 @@ class TestDirtyBitmapMigration(iotests.QMPTestCase):

         self.check_bitmap(self.vm_b, sha256 if should_migrate else False)
+        self.vm_b.hmp_qemu_io('drive0', 'write 0 512')

         if should_migrate:
+            print '===='
+            print self.vm_b.get_log()
+            print '===='
             # recreate vm_b, as we don't want -incoming option (this will lead
             # to "cat" process left alive after test finish)
             self.vm_b = iotests.VM(path_suffix='b')
@@ -152,7 +156,8 @@ for cmb in list(itertools.product((True, False), repeat=4)):
     name += '_online' if cmb[2] else '_offline'
     name += '_shared' if cmb[3] else '_nonshared'

-    inject_test_case(TestDirtyBitmapMigration, name, 'do_test_migration',
+    if name == '_persistent__not_migbitmap__offline_shared':
+        inject_test_case(TestDirtyBitmapMigration, name, 'do_test_migration',

Best regards,

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