On 04.02.20 16:35, 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.
>
>> @@ -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?..If areas added by truncation (or growth, rather) are always zero, then the file can always be created with size 0 and grown from there. Thus, images where truncation adds zeroed areas will generally always be zero after creation. > So, the only possible combination of flags, when they differs, is > create=0 and > truncate=1.. How is it possible? For preallocated qcow2 images, it depends on the storage whether they are actually 0 after creation. Hence qcow2_has_zero_init() then defers to bdrv_has_zero_init() of s->data_file->bs. But when you truncate them (with PREALLOC_MODE_OFF, as BlockDriver.bdrv_has_zero_init_truncate()’s comment explains), the new area is always going to be 0, regardless of initial preallocation. I just noticed a bug there, though: Encrypted qcow2 images will not see areas added through growth as 0. Hence, qcow2’s bdrv_has_zero_init_truncate() implementation should not return true unconditionally, but only for unencrypted images. Max
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