Re: [Qemu-devel] [PATCH v5 15/42] block: Re-evaluate backing file handling in reopen

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

14.06.2019 18:52, Max Reitz wrote:
> On 14.06.19 15:42, Vladimir Sementsov-Ogievskiy wrote:
>> 13.06.2019 1:09, Max Reitz wrote:
>>> Reopening a node's backing child needs a bit of special handling because
>>> the "backing" child has different defaults than all other children
>>> (among other things).  Adding filter support here is a bit more
>>> difficult than just using the child access functions.  In fact, we often
>>> have to directly use bs->backing because these functions are about the
>>> "backing" child (which may or may not be the COW backing file).
>>>
>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>> ---
>>>    block.c | 36 +++++++++++++++++++++++++++++-------
>>>    1 file changed, 29 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/block.c b/block.c
>>> index 505b3e9a01..db2759c10d 100644
>>> --- a/block.c
>>> +++ b/block.c
>>> @@ -3542,17 +3542,39 @@ static int bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
>>>            }
>>>        }
>>>    
>>> +    /*
>>> +     * Ensure that @bs can really handle backing files, because we are
>>> +     * about to give it one (or swap the existing one)
>>> +     */
>>> +    if (bs->drv->is_filter) {
>>> +        /* Filters always have a file or a backing child */
>>> +        if (!bs->backing) {
>>> +            error_setg(errp, "'%s' is a %s filter node that does not support a "
>>> +                       "backing child", bs->node_name, bs->drv->format_name);
>>> +            return -EINVAL;
>>> +        }
>>> +    } else if (!bs->drv->supports_backing) {
>>> +        error_setg(errp, "Driver '%s' of node '%s' does not support backing "
>>> +                   "files", bs->drv->format_name, bs->node_name);
>>> +        return -EINVAL;
>>> +    }
>>
>> hmm, shouldn't we have these checks for overlay_bs?
> 
> I think this is correct here because this is the only node the user has
> control over, so this is the only one we can reasonably complain about.
> 
> And I do think it is reasonable to complain about.
> 
>>> +
>>>        /*
>>>         * Find the "actual" backing file by skipping all links that point
>>>         * to an implicit node, if any (e.g. a commit filter node).
>>> +     * We cannot use any of the bdrv_skip_*() functions here because
>>> +     * those return the first explicit node, while we are looking for
>>> +     * its overlay here.
>>>         */
>>>        overlay_bs = bs;
>>> -    while (backing_bs(overlay_bs) && backing_bs(overlay_bs)->implicit) {
>>> -        overlay_bs = backing_bs(overlay_bs);
>>> +    while (bdrv_filtered_bs(overlay_bs) &&
>>> +           bdrv_filtered_bs(overlay_bs)->implicit)
>>> +    {
>>> +        overlay_bs = bdrv_filtered_bs(overlay_bs);
>>>        }
>>
>> here, overlay_bs may be some filter with file child ..
>>
>>>    
>>>        /* If we want to replace the backing file we need some extra checks */
>>> -    if (new_backing_bs != backing_bs(overlay_bs)) {
>>> +    if (new_backing_bs != bdrv_filtered_bs(overlay_bs)) {
>>>            /* Check for implicit nodes between bs and its backing file */
>>>            if (bs != overlay_bs) {
>>>                error_setg(errp, "Cannot change backing link if '%s' has "
>>> @@ -3560,8 +3582,8 @@ static int bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
>>>                return -EPERM;
>>>            }
>>>            /* Check if the backing link that we want to replace is frozen */
>>> -        if (bdrv_is_backing_chain_frozen(overlay_bs, backing_bs(overlay_bs),
>>> -                                         errp)) {
>>> +        if (bdrv_is_backing_chain_frozen(overlay_bs,
>>> +                                         child_bs(overlay_bs->backing), errp)) {
>>
>> .. and here we are doing wrong thing, as it don't have backing child
>>
>> Aha, you use the fact that we now don't have implicit filters with file child. Then, should
>> we add an assertion for this?
> 
> No, that wasn’t my intention.  The real reason is that all of this is a
> mess.
> 
> Here is the full context:
> 
>>      overlay_bs = bs;
>>      while (bdrv_filtered_bs(overlay_bs) &&
>>             bdrv_filtered_bs(overlay_bs)->implicit)
>>      {
>>          overlay_bs = bdrv_filtered_bs(overlay_bs);
>>      }
>>
>>      /* If we want to replace the backing file we need some extra checks */
>>      if (new_backing_bs != bdrv_filtered_bs(overlay_bs)) {
>>          /* Check for implicit nodes between bs and its backing file */
>>          if (bs != overlay_bs) {
>>              error_setg(errp, "Cannot change backing link if '%s' has "
>>                         "an implicit backing file", bs->node_name);
>>              return -EPERM;
>>          }
>>          /* Check if the backing link that we want to replace is frozen */
>>          if (bdrv_is_backing_chain_frozen(overlay_bs,
>>                                           child_bs(overlay_bs->backing), errp)) {
>>              return -EPERM;
>>          }
> 
> Note the “Check for implicit nodes” thing.  If we get to the frozen
> check, we have already confirmed that overlay_bs == bs, so we then know
> that overlay_bs->backing works.
> 
> I can add an additional comment to make that more clear.  It took myself
> quite a bit of digging to figure that out again...

Aha, I see it. Comment would be good.

> 
> (The reason for the loop is that we want to be able to recognize when
> the user tries to not change the backing file.  In that case, we don’t
> have to do anything, but because the user doesn’t know about implicit
> nodes, we have to skip them in order to check whether the user actually
> doesn’t want to change anything.)
> 
> Max
> 
>>>                return -EPERM;
>>>            }
>>>            reopen_state->replace_backing_bs = true;
>>> @@ -3712,7 +3734,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
>>>         * its metadata. Otherwise the 'backing' option can be omitted.
>>>         */
>>>        if (drv->supports_backing && reopen_state->backing_missing &&
>>> -        (backing_bs(reopen_state->bs) || reopen_state->bs->backing_file[0])) {
>>> +        (reopen_state->bs->backing || reopen_state->bs->backing_file[0])) {
>>>            error_setg(errp, "backing is missing for '%s'",
>>>                       reopen_state->bs->node_name);
>>>            ret = -EINVAL;
>>> @@ -3857,7 +3879,7 @@ void bdrv_reopen_commit(BDRVReopenState *reopen_state)
>>>         * from bdrv_set_backing_hd()) has the new values.
>>>         */
>>>        if (reopen_state->replace_backing_bs) {
>>> -        BlockDriverState *old_backing_bs = backing_bs(bs);
>>> +        BlockDriverState *old_backing_bs = child_bs(bs->backing);
>>>            assert(!old_backing_bs || !old_backing_bs->implicit);
>>>            /* Abort the permission update on the backing bs we're detaching */
>>>            if (old_backing_bs) {
>>>
>>
>>
> 
> 


-- 
Best regards,
Vladimir