Re: [PATCH v2 23/36] block: adapt bdrv_append() for inserting filters

[Vladimir Sementsov-Ogievskiy]

04.02.2021 00:33, Kevin Wolf wrote:
> Am 27.11.2020 um 15:45 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > bdrv_append is not very good for inserting filters: it does extra
> > permission update as part of bdrv_set_backing_hd(). During this update
> > filter may conflict with other parents of top_bs.
> >
> > Instead, let's first do all graph modifications and after it update
> > permissions.
>
> This sounds like it fixes a bug. If so, should we have a test like for
> the other cases fixed by this series?

Hm. I considered it mostly like a lack not a bug. We just have to workaround this lack by 
"inactive" mode of filters. But adding a test is good idea anyway. Will do.

> > Note: bdrv_append() is still only works for backing-child based
> > filters. It's something to improve later.
> >
> > It simplifies the fact that bdrv_append() used to append new nodes,
> > without backing child. Let's add an assertion.
> >
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > ---
> >   block.c | 28 +++++++++++++++++-----------
> >   1 file changed, 17 insertions(+), 11 deletions(-)
> >
> > diff --git a/block.c b/block.c
> > index 02da1a90bc..7094922509 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -4998,22 +4998,28 @@ int bdrv_replace_node(BlockDriverState *from, 
> > BlockDriverState *to,
> >   int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
> >                   Error **errp)
> >   {
> > -    Error *local_err = NULL;
> > +    int ret;
> > +    GSList *tran = NULL;
> >   
> > -    bdrv_set_backing_hd(bs_new, bs_top, &local_err);
> > -    if (local_err) {
> > -        error_propagate(errp, local_err);
> > -        return -EPERM;
> > +    assert(!bs_new->backing);
> > +
> > +    ret = bdrv_attach_child_noperm(bs_new, bs_top, "backing",
> > +                                   &child_of_bds, bdrv_backing_role(bs_new),
> > +                                   &bs_new->backing, &tran, errp);
> > +    if (ret < 0) {
> > +        goto out;
> >       }
>
> I don't think changing bs->backing without bdrv_set_backing_hd() is
> correct at the moment. We lose a few things:
>
> 1. The bdrv_is_backing_chain_frozen() check
> 2. Updating backing_hd->inherits_from if necessary
> 3. bdrv_refresh_limits()
>
> If I'm not missing anything, all of these are needed in the context of
> bdrv_append().

I decided that bdrv_append() is only for appending new nodes, so frozen and 
inherts_from checks are not needed. And I've added assert(!bs_new->backing)...

Checking this now:

- appending filters is obvious
- bdrv_append_temp_snapshot() creates new qcow2 node based on tmp file, don't 
see any backing initialization (and it would be rather strange)
- external_snapshot_prepare() do check if (bdrv_cow_child(state->new_bs)) {  
error-out }

So everything is OK. I should describe it in commit message and add a comment 
to bdrv_append.

> > -    bdrv_replace_node(bs_top, bs_new, &local_err);
> > -    if (local_err) {
> > -        error_propagate(errp, local_err);
> > -        bdrv_set_backing_hd(bs_new, NULL, &error_abort);
> > -        return -EPERM;
> > +    ret = bdrv_replace_node_noperm(bs_top, bs_new, true, &tran, errp);
> > +    if (ret < 0) {
> > +        goto out;
> >       }
> >   
> > -    return 0;
> > +    ret = bdrv_refresh_perms(bs_new, errp);
> > +out:
> > +    tran_finalize(tran, ret);
> > +
> > +    return ret;
> >   }
>
> Kevin


--
Best regards,
Vladimir