Re: [Qemu-block] [PATCH v6 14/42] block: Use CAFs when working with backing chains

[Max Reitz]

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 <address@hidden>
>> ---
>>  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.)

> 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.)

> 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.
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).”

>> +    while (active) {
>> +        BlockDriverState *next = bdrv_backing_chain_next(active);
>> +        if (bs == next) {
>> +            return active;
>> +        }
>> +        active = next;
>>      }
>>  
>> -    return active;
>> +    return NULL;
>>  }
>>  
>>  /* Given a BDS, searches for the base layer. */
>> @@ -4544,9 +4552,7 @@ int bdrv_drop_intermediate(BlockDriverState *top, 
>> BlockDriverState *base,
>>       * other intermediate nodes have been dropped.
>>       * If 'top' is an implicit node (e.g. "commit_top") we should skip
>>       * it because no one inherits from it. We use explicit_top for that. */
>> -    while (explicit_top && explicit_top->implicit) {
>> -        explicit_top = backing_bs(explicit_top);
>> -    }
>> +    explicit_top = bdrv_skip_implicit_filters(explicit_top);
>>      update_inherits_from = bdrv_inherits_from_recursive(base, explicit_top);
>>  
>>      /* success - we can delete the intermediate states, and link top->base 
>> */
>> @@ -5014,7 +5020,7 @@ BlockDriverState *bdrv_lookup_bs(const char *device,
>>  bool bdrv_chain_contains(BlockDriverState *top, BlockDriverState *base)
>>  {
>>      while (top && top != base) {
>> -        top = backing_bs(top);
>> +        top = bdrv_filtered_bs(top);
>>      }
>>  
>>      return top != NULL;
>> @@ -5253,7 +5259,17 @@ BlockDriverState 
>> *bdrv_find_backing_image(BlockDriverState *bs,
>>  
>>      is_protocol = path_has_protocol(backing_file);
>>  
>> -    for (curr_bs = bs; curr_bs->backing; curr_bs = curr_bs->backing->bs) {
>> +    /*
>> +     * Being largely a legacy function, skip any filters here
>> +     * (because filters do not have normal filenames, so they cannot
>> +     * match anyway; and allowing json:{} filenames is a bit out of
>> +     * scope).
>> +     */
>> +    for (curr_bs = bdrv_skip_rw_filters(bs);
>> +         bdrv_filtered_cow_child(curr_bs) != NULL;
>> +         curr_bs = bdrv_backing_chain_next(curr_bs))
> 
> This could just use bs_below instead of recalculating the node if you
> moved the declaration of bs_below to the function scope.

Indeed, thanks.

Max

>> +    {
>> +        BlockDriverState *bs_below = bdrv_backing_chain_next(curr_bs);
>>  
>>          /* If either of the filename paths is actually a protocol, then
>>           * compare unmodified paths; otherwise make paths relative */

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