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

[Vladimir Sementsov-Ogievskiy]

12.07.2021 14:54, Lukas Straub wrote:
> Remove the workaround introduced in commit
> 6ecbc6c52672db5c13805735ca02784879ce8285
> "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.

So:

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)
>           return;
>       }
>
> -    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) {
>           return;
>       }
> --
> 2.20.1



--
Best regards,
Vladimir