Re: [Qemu-block] [PATCH v9 1/2] qemu-img info lists bitmap directory entries

[Vladimir Sementsov-Ogievskiy]

29.01.2019 17:35, Vladimir Sementsov-Ogievskiy wrote:
> 29.01.2019 17:23, Kevin Wolf wrote:
>> Am 29.01.2019 um 15:06 hat Vladimir Sementsov-Ogievskiy geschrieben:
>>> 29.01.2019 16:17, Kevin Wolf wrote:
>>>> Am 29.01.2019 um 13:50 hat Vladimir Sementsov-Ogievskiy geschrieben:
>>>>> 29.01.2019 15:38, Kevin Wolf wrote:
>>>>>> Am 29.01.2019 um 13:04 hat Andrey Shinkevich geschrieben:
>>>>>>>>>
>>>>>>>>> diff --git a/block/qapi.c b/block/qapi.c
>>>>>>>>> index c66f949..0fde98c 100644
>>>>>>>>> --- a/block/qapi.c
>>>>>>>>> +++ b/block/qapi.c
>>>>>>>>> @@ -38,6 +38,7 @@
>>>>>>>>>      #include "qapi/qmp/qstring.h"
>>>>>>>>>      #include "sysemu/block-backend.h"
>>>>>>>>>      #include "qemu/cutils.h"
>>>>>>>>> +#include "qemu/error-report.h"
>>>>>>>>>      BlockDeviceInfo *bdrv_block_device_info(BlockBackend *blk,
>>>>>>>>>                                              BlockDriverState *bs, 
>>>>>>>>> Error **errp)
>>>>>>>>> @@ -868,6 +869,11 @@ void bdrv_image_info_dump(fprintf_function 
>>>>>>>>> func_fprintf, void *f,
>>>>>>>>>          if (info->has_format_specific) {
>>>>>>>>>              func_fprintf(f, "Format specific information:\n");
>>>>>>>>> +        if (info->format_specific &&
>>>>>>>>> +            info->format_specific->type == 
>>>>>>>>> IMAGE_INFO_SPECIFIC_KIND_QCOW2 &&
>>>>>>>>> +            info->format_specific->u.qcow2.data->has_bitmaps == 
>>>>>>>>> false) {
>>>>>>>>> +            warn_report("Failed to load bitmap list");
>>>>>>>>> +        }
>>>>>>>>>              bdrv_image_info_specific_dump(func_fprintf, f, 
>>>>>>>>> info->format_specific);
>>>>>>>>>          }
>>>>>>>>>      }
>>>>>>>>
>>>>>>>> Is it really necessary to introduce qcow2 specific knowledge in
>>>>>>>> block/qapi.c (where it definitely doesn't belong), just to emit a
>>>>>>>> warning?
>>>>>>>>
>>>>>>>> Why can't you print the warning in qcow2_get_specific_info()?
>>>>>>>
>>>>>>> Dear Kevin,
>>>>>>> The reason behind the idea to move the warning to qapi is that we
>>>>>>> wouldn't like to print that in case of JSON format.
>>>>>>>
>>>>>>>>
>>>>>>>>> diff --git a/block/qcow2.c b/block/qcow2.c
>>>>>>>>> index 4897aba..07b99ee 100644
>>>>>>>>> --- a/block/qcow2.c
>>>>>>>>> +++ b/block/qcow2.c
>>>>>>>>> @@ -4386,8 +4386,14 @@ static ImageInfoSpecific 
>>>>>>>>> *qcow2_get_specific_info(BlockDriverState *bs)
>>>>>>>>>              *spec_info->u.qcow2.data = (ImageInfoSpecificQCow2){
>>>>>>>>>                  .compat             = g_strdup("0.10"),
>>>>>>>>>                  .refcount_bits      = s->refcount_bits,
>>>>>>>>> +            .has_bitmaps        = true, /* To handle error check 
>>>>>>>>> properly */
>>>>>>>>> +            .bitmaps            = NULL, /* Unsupported for version 2 
>>>>>>>>> */
>>>>>>>>
>>>>>>>> .has_bitmaps = false would be nicer if the format doesn't even support
>>>>>>>> bitmaps. You only need this here because you put the warning in the
>>>>>>>> wrong place.
>>>>>>>>
>>>>>>>>>              };
>>>>>>>>>          } else if (s->qcow_version == 3) {
>>>>>>>>> +        Qcow2BitmapInfoList *bitmaps;
>>>>>>>>> +        Error *local_err = NULL;
>>>>>>>>> +
>>>>>>>>> +        bitmaps = qcow2_get_bitmap_info_list(bs, &local_err);
>>>>>>>>
>>>>>>>> Here you even had a good error message to print...
>>>>>>>>
>>>>>>>>>              *spec_info->u.qcow2.data = (ImageInfoSpecificQCow2){
>>>>>>>>>                  .compat             = g_strdup("1.1"),
>>>>>>>>>                  .lazy_refcounts     = s->compatible_features &
>>>>>>>>> @@ -4397,7 +4403,14 @@ static ImageInfoSpecific 
>>>>>>>>> *qcow2_get_specific_info(BlockDriverState *bs)
>>>>>>>>>                                        QCOW2_INCOMPAT_CORRUPT,
>>>>>>>>>                  .has_corrupt        = true,
>>>>>>>>>                  .refcount_bits      = s->refcount_bits,
>>>>>>>>> +            .has_bitmaps        = !local_err,
>>>>>>>>> +            .bitmaps            = bitmaps,
>>>>>>>>>              };
>>>>>>>>> +        /*
>>>>>>>>> +         * If an error occurs in obtaining bitmaps, ignore
>>>>>>>>> +         * it to show other QCOW2 specific information.
>>>>>>>>> +         */
>>>>>>>>> +        error_free(local_err);
>>>>>>>>
>>>>>>>> ...but you decided to discard it.
>>>>>>>>
>>>>>>>> How about you do this here:
>>>>>>>>
>>>>>>>>         warn_reportf_err(local_err, "Failed to load bitmap list: ");
>>>>>>>
>>>>>>> In case of JSON format, we can fail here.
>>>>>>> It will happen in the current implementation of the new test #239.
>>>>>>
>>>>>> Why do you want to silently leave out the bitmap list for JSON output?
>>>>>>
>>>>>> Essentially, this is the same question I asked here:
>>>>>>
>>>>>>>> And then get rid of the code in block/qapi.c and the version 2 path?
>>>>>>>>
>>>>>>>> Actually, should this really only be a warning, or in fact an error?
>>>>>>>> What's the case where we expect that loading the bitmap list can fail,
>>>>>>>> but the management tool doesn't need to know that and we just want to
>>>>>>>> log a message?
>>>>>>
>>>>>> I don't understand why it's okay not to print bitmaps without making it
>>>>>> an error. Do you have a specific use case in mind for this behaviour?
>>>>>>
>>>>>
>>>>>
>>>>> We just can't print anything here, as this code is executed from qmp 
>>>>> command.
>>>>>
>>>>> It was discussed, that we don't want to fail the whole query, if failed to
>>>>> obtain bitmaps.
>>>>
>>>> It's obvious that you don't want to fail the query command, but I
>>>> haven't found any explanation for _why_ you want this.
>>>>
>>>> The thing that is closest to an explanation is "it may be sad to fail
>>>> get any information because of repeating some disk/qcow2 error", written
>>>> by you in the v4 thread.
>>>>
>>>> I don't think "may be sad" is a good justification for non-standard
>>>> behaviour. If we can't load the bitmaps, the image is broken. And if the
>>>> image is broken, the rest of the information may be invalid, too.
>>>
>>> So, you mean that we go wrong way. And it was my idea. That is sad too.
>>>
>>> Than, seems like we should add errp to the function and to the full stack
>>> down to qmp_query_block. And drop extra partial-success qapi logic.
>>
>> I'm not necessarily saying that it's the wrong way, though possibly it
>> is wrong indeed.
>>
>> But ignoring some kind of failures is special, so what I was looking for
>> were comments or documentation to explain the reason behind it at least.
>> Now from the replies I got so far it looks to me that possibly the
>> reasons are not only undocumented, but we might actually not have any.
>>
>>>>> So, to show, that there were error during bitmaps querying
>>>>> we decided to use the following semantics:
>>>>>
>>>>> empty list (has_bitmaps=true) means that there no bitmaps, and there were
>>>>> no errors during bitmaps quering (if it was)
>>>>>
>>>>> absence of the field (has_bitmaps=false) means that there _were_ errors 
>>>>> during
>>>>> bitmaps querying.
>>>>
>>>> ...or that you're running an old QEMU version. I'm not sure if making
>>>> old QEMU versions and image errors look the same is a good move.
>>>
>>> No, for old versions we return empty list, to show that there were no 
>>> errors.
>>
>> You mean old image format versions?
> 
> yes.
> 
> I'm talking about old QEMU versions
>> that don't even know the 'bitmaps' field.
> 
> hmm. Yes, that's a problem, which we didn't considered.
> 
>>
>> A QMP client would have to parse the schema to understand whether a
>> missing 'bitmaps' field means that it's talking to an old QEMU (then
>> 'bitmaps' doesn't exist in the schema), or that an error happened (then
>> the field does exist in the schema).
>>
>> Looking at the schema is not impossible, so if we require this for a
>> good reason, it's okay. But it's not trivial either. If we really want
>> to go with this semantics, we should probably mention both the old and
>> the new meaning in the QAPI documentation, with the recommendation that
>> you parse the schema to determine the meaning of a missing 'bitmaps'
>> field.
> 
> Hm, now I think that if we really face the case when we need partial success
> of info querying, it should be additional optional parameter which enables 
> it, like
> skip-failed=true (default false) or something, which is more clear than 
> parsing the
> schema. And, which we can add later if needed.
> 
> 

Note also, that we already skip some errors, for example, 
block_crypto_get_specific_info_luks
will return NULL on errors (unlike qcow2_get_specific_info, which aborts), and
bdrv_query_image_info skip these errors, in same manner as for formats which 
don't support
bdrv_get_specific_info.

Looks like a good moment to fix this too, do you agree?

-- 
Best regards,
Vladimir