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
