Re: [PATCH v3 4/4] replication: Remove workaround

[Lukas Straub <lukasstraub2@web.de>]

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On Fri, 9 Jul 2021 10:49:23 +0300
Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> wrote:

> 07.07.2021 21:15, 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 doesn't inactivate disks.  
> 
> If look at replication_child_perm() you should also be sure that it always works only with RW disks..
> 
> Actually, I think that it would be correct just require BLK_PERM_WRITE in replication_child_perm() unconditionally. Let generic layer care about all these RD/WR things. In _child_perm() we can require WRITE and don't care. If something goes wrong and we can't get WRITE permission we should see clean error-out.
> 
> Opposite, if we don't require WRITE permission in some case and still do WRITE request, it may crash.
> 
> Still, this may be considered as a preexisting problem of replication_child_perm() and fixed separately.

Hmm, unconditionally requesting write doesn't work, since qemu on the
secondary side is started with "-miration incoming", it goes into
runstate RUN_STATE_INMIGRATE from the beginning and then blockdev_init()
opens every blockdev with BDRV_O_INACTIVE and then it errors out with
-drive driver=replication,...: Block node is read-only.

> > 
> > Signed-off-by: Lukas Straub <lukasstraub2@web.de>  
> 
> So, for this one commit (with probably updated commit message accordingly to my comments, or even rebased on fixed replication_child_perm()):
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> 
> 
> > ---
> >   block/replication.c | 12 +-----------
> >   1 file changed, 1 insertion(+), 11 deletions(-)
> > 
> > diff --git a/block/replication.c b/block/replication.c
> > index c0d4a6c264..68b46d65a8 100644
> > --- a/block/replication.c
> > +++ b/block/replication.c
> > @@ -348,17 +348,7 @@ 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
> >   
> 
> 



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