Re: [PATCH v2 26/36] block/backup-top: drop .active

[Kevin Wolf]

Am 04.02.2021 um 13:33 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 04.02.2021 15:26, Kevin Wolf wrote:
> > Am 27.11.2020 um 15:45 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > > We don't need this workaround anymore: bdrv_append is already smart
> > > enough and we can use new bdrv_drop_filter().
> > > 
> > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > > ---
> > >   block/backup-top.c         | 38 +-------------------------------------
> > >   tests/qemu-iotests/283.out |  2 +-
> > >   2 files changed, 2 insertions(+), 38 deletions(-)
> > > 
> > > diff --git a/block/backup-top.c b/block/backup-top.c
> > > index 650ed6195c..84eb73aeb7 100644
> > > --- a/block/backup-top.c
> > > +++ b/block/backup-top.c
> > > @@ -37,7 +37,6 @@
> > >   typedef struct BDRVBackupTopState {
> > >       BlockCopyState *bcs;
> > >       BdrvChild *target;
> > > -    bool active;
> > >       int64_t cluster_size;
> > >   } BDRVBackupTopState;
> > > @@ -127,21 +126,6 @@ static void backup_top_child_perm(BlockDriverState 
> > > *bs, BdrvChild *c,
> > >                                     uint64_t perm, uint64_t shared,
> > >                                     uint64_t *nperm, uint64_t *nshared)
> > >   {
> > > -    BDRVBackupTopState *s = bs->opaque;
> > > -
> > > -    if (!s->active) {
> > > -        /*
> > > -         * The filter node may be in process of bdrv_append(), which 
> > > firstly do
> > > -         * bdrv_set_backing_hd() and then bdrv_replace_node(). This 
> > > means that
> > > -         * we can't unshare BLK_PERM_WRITE during bdrv_append() 
> > > operation. So,
> > > -         * let's require nothing during bdrv_append() and refresh 
> > > permissions
> > > -         * after it (see bdrv_backup_top_append()).
> > > -         */
> > > -        *nperm = 0;
> > > -        *nshared = BLK_PERM_ALL;
> > > -        return;
> > > -    }
> > > -
> > >       if (!(role & BDRV_CHILD_FILTERED)) {
> > >           /*
> > >            * Target child
> > > @@ -229,18 +213,6 @@ BlockDriverState 
> > > *bdrv_backup_top_append(BlockDriverState *source,
> > >       }
> > >       appended = true;
> > > -    /*
> > > -     * bdrv_append() finished successfully, now we can require 
> > > permissions
> > > -     * we want.
> > > -     */
> > > -    state->active = true;
> > > -    bdrv_child_refresh_perms(top, top->backing, &local_err);
> > 
> > bdrv_append() uses bdrv_refresh_perms() for the whole node. Is it doing
> > unnecessary extra work there and should really do the same as backup-top
> > did here, i.e. bdrv_child_refresh_perms(bs_new->backing)?
> > 
> > (Really a comment for an earlier patch. This patch itself looks fine.)
> > 
> 
> You mean how backup-top code works at the point when we modified
> bdrv_append()? Actually all works, as we use state->active. We set it
> to true and should call refresh_perms. Now we drop _refresh_perms
> _together_ with state->active variable, so filter is always "active",
> but new bdrv_append can handle it now. I.e., before this patch
> backup-top.c code is correct but over-complicated with logic which is
> not necessary after bdrv_append() improvement (and of-course we need
> also bdrv_drop_filter() to drop the whole state->active related
> logic).

No, I just mean that bdrv_child_refresh_perms(bs, bs->backing) is enough
when adding a new image to the chain. A full bdrv_child_refresh_perms()
like we now have in bdrv_append() is doing more work than is necessary.

It doesn't make a difference for backup-top (because the filter has only
a single child), but if you append a new qcow2 snapshot, you would also
recalculate permissions for the bs->file subtree even though nothing has
changed there.

It's only a small detail anyway, not very important in a slow path.

Kevin