Am 09.09.2019 um 10:25 hat Max Reitz geschrieben:
> On 05.09.19 16:05, Kevin Wolf wrote:
> > Am 09.08.2019 um 18:13 hat Max Reitz geschrieben:
> >> Use child access functions when iterating through backing chains so
> >> filters do not break the chain.
> >>
> >> Signed-off-by: Max Reitz <mre...@redhat.com>
> >> ---
> >> block.c | 40 ++++++++++++++++++++++++++++------------
> >> 1 file changed, 28 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/block.c b/block.c
> >> index 86b84bea21..42abbaf0ba 100644
> >> --- a/block.c
> >> +++ b/block.c
> >> @@ -4376,7 +4376,8 @@ int bdrv_change_backing_file(BlockDriverState *bs,
> >> }
> >>
> >> /*
> >> - * Finds the image layer in the chain that has 'bs' as its backing file.
> >> + * Finds the image layer in the chain that has 'bs' (or a filter on
> >> + * top of it) as its backing file.
> >> *
> >> * active is the current topmost image.
> >> *
> >> @@ -4388,11 +4389,18 @@ int bdrv_change_backing_file(BlockDriverState *bs,
> >> BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
> >> BlockDriverState *bs)
> >> {
> >> - while (active && bs != backing_bs(active)) {
> >> - active = backing_bs(active);
> >> + bs = bdrv_skip_rw_filters(bs);
> >> + active = bdrv_skip_rw_filters(active);
> >
> > This does more than the commit message says. In addition to iterating
> > through filters instead of stopping, it also changes the semantics of
> > the function to return the next non-filter on top of bs instead of the
> > next node.
>
> Which is to say the overlay.
>
> (I think we only ever use “overlay” in the COW sense.)I think we do, but so far also only ever for immediate COW childs, not for skipping through intermediate node. > > The block jobs seem to use it only for bdrv_is_allocated_above(), which > > should return the same thing in both cases, so the behaviour stays the > > same. qmp_block_commit() will check op blockers on a different node now, > > which could be a fix or a bug, I can't tell offhand. Probably the > > blocking doesn't really work anyway. > > You mean that the op blocker could have been on a block job filter node > before? I think that‘s pretty much the point of this fix; that that > doesn’t make sense. (We didn’t have mirror_top_bs and the like at > 058223a6e3b.) On the off chance that the op blocker actually works, it can't be a job filter. I was thinking more of throttling, blkdebug etc. > > All of this should be mentioned in the commit message at least. Maybe > > it's also worth splitting in two patches. > > I don’t know. The function was written when there were no filters. I doubt it. blkdebug is a really old filter. > This change would have been a no-op then. The fact that it isn’t to me > just means that introducing filters broke it. > > So I don’t know what I would write. Maybe “bdrv_find_overlay() now > actually finds the overlay, that is, it will not return a filter node. > This is the behavior that all callers expect (because they work on COW > backing chains).” Maybe just something like "In addition, filter nodes are not returned as overlays any more. Instead, the first non-filter node on top of bs is considered the overlay now."? Kevin
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