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Re: [PATCH v5 5/5] replication: Remove workaround

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v5 5/5] replication: Remove workaround
Date: Thu, 15 Jul 2021 16:48:06 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

12.07.2021 14:54, Lukas Straub wrote:
Remove the workaround introduced in commit
"replication: Avoid blk_make_empty() on read-only child".

It is not needed anymore since s->hidden_disk is guaranteed to be
writable when secondary_do_checkpoint() runs. Because replication_start(),
_do_checkpoint() and _stop() are only called by COLO migration code
and COLO-migration activates all disks via bdrv_invalidate_cache_all()
before it calls these functions.

In replication_child_perm()
we have

 if ((bs->open_flags & (BDRV_O_INACTIVE | BDRV_O_RDWR)) == BDRV_O_RDWR) {
     *nperm |= BLK_PERM_WRITE;

That's probably possible

1. configure a block graph like described in replicatio doc, but all disks 
opened read-only. so we don't have WRITE permission

2. start replication

3. crash on trying to make disk empty in do_checkpoint with no WRITE permission

Still, I think if it possible, we'll crash on first bdrv_make_empty of active 
disk, and that's preexisting.


Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
  block/replication.c | 12 +-----------
  1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/block/replication.c b/block/replication.c
index 772bb63374..1e9dc4d309 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -356,17 +356,7 @@ static void secondary_do_checkpoint(BlockDriverState *bs, 
Error **errp)

-    BlockBackend *blk = blk_new(qemu_get_current_aio_context(),
-                                BLK_PERM_WRITE, BLK_PERM_ALL);
-    blk_insert_bs(blk, s->hidden_disk->bs, &local_err);
-    if (local_err) {
-        error_propagate(errp, local_err);
-        blk_unref(blk);
-        return;
-    }
-    ret = blk_make_empty(blk, errp);
-    blk_unref(blk);
+    ret = bdrv_make_empty(s->hidden_disk, errp);
      if (ret < 0) {

Best regards,

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