Re: [PATCH 09/17] block: Refactor bdrv_has_zero_init{,_truncate}

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

04.02.2020 18:49, Eric Blake wrote:
> On 2/4/20 9:35 AM, Vladimir Sementsov-Ogievskiy wrote:
>> 31.01.2020 20:44, Eric Blake wrote:
>>> Having two slightly-different function names for related purposes is
>>> unwieldy, especially since I envision adding yet another notion of
>>> zero support in an upcoming patch.  It doesn't help that
>>> bdrv_has_zero_init() is a misleading name (I originally thought that a
>>> driver could only return 1 when opening an already-existing image
>>> known to be all zeroes; but in reality many drivers always return 1
>>> because it only applies to a just-created image).  Refactor all uses
>>> to instead have a single function that returns multiple bits of
>>> information, with better naming and documentation.
>>
>> Sounds good
>>
>>>
>>> No semantic change, although some of the changes (such as to qcow2.c)
>>> require a careful reading to see how it remains the same.
>>>
>>
>> ...
>>
>>> diff --git a/include/block/block.h b/include/block/block.h
>>> index 6cd566324d95..a6a227f50678 100644
>>> --- a/include/block/block.h
>>> +++ b/include/block/block.h
>>
>> Hmm, header file in the middle of the patch, possibly you don't use
>> [diff]
>>      orderFile = scripts/git.orderfile
>>
>> in git config.. Or it is broken.
> 
> I do have it set up, so I'm not sure why it didn't work as planned. I'll make sure v2 follows the order I intended.
> 
>>
>>> @@ -85,6 +85,28 @@ typedef enum {
>>>       BDRV_REQ_MASK               = 0x3ff,
>>>   } BdrvRequestFlags;
>>>
>>> +typedef enum {
>>> +    /*
>>> +     * bdrv_known_zeroes() should include this bit if the contents of
>>> +     * a freshly-created image with no backing file reads as all
>>> +     * zeroes without any additional effort.  If .bdrv_co_truncate is
>>> +     * set, then this must be clear if BDRV_ZERO_TRUNCATE is clear.
>>
>> I understand that this is preexisting logic, but could I ask: why? What's wrong
>> if driver can guarantee that created file is all-zero, but is not sure about
>> file resizing? I agree that it's normal for these flags to have the same value,
>> but what is the reason for this restriction?..
> 
> For _this_ patch, my goal is to preserve pre-existing practice. Where we think pre-existing practice is wrong, we can then improve it in other patches (see patch 6, for example).

This is OK, of course, I'm just trying to understand existing logic.

> 
> I _think_ the reason for this original limitation is as follows: If an image can be resized, we could choose to perform 'create(size=0), truncate(size=final)' instead of 'create(size=final)', and we want to guarantee the same behavior. If truncation can't guarantee a zero read, then why is creation doing so?

If we want to guarantee the same behavior, we should restrict any difference between these flags :)

> 
> But as I did not write the original patch, I would welcome Max's input with regards to the thought behind commit ceaca56f.
> 
>>
>> So, the only possible combination of flags, when they differs, is create=0 and
>> truncate=1.. How is it possible?
> 
> qcow2 had that mode, at least before patch 5.

yes, it reported even for encrypted images truncate=1...

> 
>>
>>> +     * Since this bit is only reliable at image creation, a driver may
>>> +     * return this bit even for existing images that do not currently
>>> +     * read as zero.
>>> +     */
>>> +    BDRV_ZERO_CREATE        = 0x1,
>>> +
>>> +    /*
>>> +     * bdrv_known_zeroes() should include this bit if growing an image
>>> +     * with PREALLOC_MODE_OFF (either with no backing file, or beyond
>>> +     * the size of the backing file) will read the new data as all
>>> +     * zeroes without any additional effort.  This bit only matters
>>> +     * for drivers that set .bdrv_co_truncate.
>>> +     */
>>> +    BDRV_ZERO_TRUNCATE      = 0x2,
>>> +} BdrvZeroFlags;
>>> +
>>
>> ...
>>
>>
> 


-- 
Best regards,
Vladimir