On 09.09.19 11:55, Kevin Wolf wrote:
> 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.Yes, because it’s broken so far. >>> 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. Well, that’s just broken altogether right now. >>> 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."? Isn’t that basically the same? :-) Max
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