Re: [PATCH v2 3/6] block: Clarify that @bytes is no limit on *pnum

[Max Reitz <mreitz@redhat.com>]

On 24.06.21 12:25, Vladimir Sementsov-Ogievskiy wrote:
> 24.06.2021 13:16, Max Reitz wrote:
>> On 24.06.21 11:15, Vladimir Sementsov-Ogievskiy wrote:
>>> 23.06.2021 18:01, Max Reitz wrote:
>>>> .bdrv_co_block_status() implementations are free to return a *pnum 
>>>> that
>>>> exceeds @bytes, because bdrv_co_block_status() in block/io.c will 
>>>> clamp
>>>> *pnum as necessary.
>>>>
>>>> On the other hand, if drivers' implementations return values for *pnum
>>>> that are as large as possible, our recently introduced block-status
>>>> cache will become more effective.
>>>>
>>>> So, make a note in block_int.h that @bytes is no upper limit for 
>>>> *pnum.
>>>>
>>>> Suggested-by: Eric Blake <eblake@redhat.com>
>>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>>> ---
>>>>   include/block/block_int.h | 5 +++++
>>>>   1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/include/block/block_int.h b/include/block/block_int.h
>>>> index fcb599dd1c..f85b96ed99 100644
>>>> --- a/include/block/block_int.h
>>>> +++ b/include/block/block_int.h
>>>> @@ -347,6 +347,11 @@ struct BlockDriver {
>>>>        * clamped to bdrv_getlength() and aligned to request_alignment,
>>>>        * as well as non-NULL pnum, map, and file; in turn, the driver
>>>>        * must return an error or set pnum to an aligned non-zero 
>>>> value.
>>>> +     *
>>>> +     * Note that @bytes is just a hint on how big of a region the
>>>> +     * caller wants to inspect.  It is not a limit on *pnum.
>>>> +     * Implementations are free to return larger values of *pnum if
>>>> +     * doing so does not incur a performance penalty.
>>>
>>> Worth mention that the cache will benefit of it?
>>
>> Oh, right, absolutely.  Like so:
>>
>> "block/io.c's bdrv_co_block_status() will clamp *pnum before 
>> returning it to its caller, but it itself can still make use of the 
>> unclamped *pnum value.  Specifically, the block-status cache for 
>> protocol nodes will benefit from storing as large a region as possible."
>>
>
> Sounds good. Do you mean this as an addition or substitution? If the 
> latter, I'd keep "if doing so does not incur a performance penalty 

I meant it as an addition, just a new paragraph.

Max