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

[Lukas Straub]

On Mon, 12 Jul 2021 13:06:19 +0300
Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> wrote:

> 11.07.2021 23:33, Lukas Straub wrote:
> > 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.  
> 
> Ah, OK. So we need this check in _child_perm().. Then, maybe, leave check or 
> assertion in secondary_do_checkpoint, that hidden_disk is writable?

Good Idea. I will add assertions to secondary_do_checkpoint and 
replication_co_writev too.

> >   
> >>>
> >>> 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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