Re: [PATCH v3 22/36] block: add bdrv_remove_filter_or_cow transaction action

[Kevin Wolf <kwolf@redhat.com>]

Am 26.04.2021 um 19:18 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 26.04.2021 19:26, Kevin Wolf wrote:
> > Am 17.03.2021 um 15:35 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > > ---
> > >   block.c | 78 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
> > >   1 file changed, 76 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/block.c b/block.c
> > > index 11f7ad0818..2fca1f2ad5 100644
> > > --- a/block.c
> > > +++ b/block.c
> > > @@ -2929,12 +2929,19 @@ static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs)
> > >       }
> > >   }
> > > +static void bdrv_child_free(void *opaque)
> > > +{
> > > +    BdrvChild *c = opaque;
> > > +
> > > +    g_free(c->name);
> > > +    g_free(c);
> > > +}
> > > +
> > >   static void bdrv_remove_empty_child(BdrvChild *child)
> > >   {
> > >       assert(!child->bs);
> > >       QLIST_SAFE_REMOVE(child, next);
> > > -    g_free(child->name);
> > > -    g_free(child);
> > > +    bdrv_child_free(child);
> > >   }
> > >   typedef struct BdrvAttachChildCommonState {
> > > @@ -4956,6 +4963,73 @@ static bool should_update_child(BdrvChild *c, BlockDriverState *to)
> > >       return ret;
> > >   }
> > > +typedef struct BdrvRemoveFilterOrCowChild {
> > > +    BdrvChild *child;
> > > +    bool is_backing;
> > > +} BdrvRemoveFilterOrCowChild;
> > > +
> > > +/* this doesn't restore original child bs, only the child itself */
> > 
> > Hm, this comment tells me that it's intentional, but why is it correct?
> 
> that's because bdrv_remove_filter_or_cow_child_abort() aborts only
> part of  bdrv_remove_filter_or_cow_child().

I see that it aborts only part of it, but why?

Normally, a function getting a Transaction object suggests to me that on
failure, all changes the function made will be reverted, not just parts
of the changes.

> Look: bdrv_remove_filter_or_cow_child() firstly do
> bdrv_replace_child_safe(child, NULL, tran);, so bs would be restored
> by .abort() of bdrv_replace_child_safe() action.

Ah! So the reason is not that we don't want to restore child->bs, but
that bdrv_replace_child_safe() is already transactionable and will
automatically do this.

> So, improved comment may look like:
> 
> This doesn't restore original child bs, only the child itself. The bs
> would be restored by .abort() bdrv_replace_child_safe() subation of
> bdrv_remove_filter_or_cow_child() action.

"subation" is a typo for "subaction"?

Maybe something like this:

    We don't have to restore child->bs here to undo bdrv_replace_child()
    because that function is already transactionable and will do so in
    its own .abort() callback.

> Probably it would be more correct to rename
> 
> BdrvRemoveFilterOrCowChild -> BdrvRemoveFilterOrCowChildNoBs
> bdrv_remove_filter_or_cow_child_abort -> bdrv_remove_filter_or_cow_child_no_bs_abort
> bdrv_remove_filter_or_cow_child_commit -> bdrv_remove_filter_or_cow_child_no_bs_commit
> 
> and assert on .abort() and .commit() that s->child->bs is NULL.

I wouldn't bother with that. It was just that the comment confused me
because it seemed to suggest that we actually don't want to restore
child->bs, when its real intention was to say that it's already restored
somewhere else.

Kevin