Re: [Qemu-devel] [PATCH v2] util/hbitmap: strict hbitmap_reset

[John Snow <jsnow@redhat.com>]

On 9/12/19 4:20 AM, Vladimir Sementsov-Ogievskiy wrote:
> 11.09.2019 20:59, John Snow wrote:
>>
>>
>> On 9/11/19 11:13 AM, Vladimir Sementsov-Ogievskiy wrote:
>>> 07.08.2019 19:27, John Snow wrote:
>>>>
>>>>
>>>> On 8/6/19 12:19 PM, Vladimir Sementsov-Ogievskiy wrote:
>>>>> 06.08.2019 19:09, Max Reitz wrote:
>>>>>> On 06.08.19 17:26, Vladimir Sementsov-Ogievskiy wrote:
>>>>>>> hbitmap_reset has an unobvious property: it rounds requested region up.
>>>>>>> It may provoke bugs, like in recently fixed write-blocking mode of
>>>>>>> mirror: user calls reset on unaligned region, not keeping in mind that
>>>>>>> there are possible unrelated dirty bytes, covered by rounded-up region
>>>>>>> and information of this unrelated "dirtiness" will be lost.
>>>>>>>
>>>>>>> Make hbitmap_reset strict: assert that arguments are aligned, allowing
>>>>>>> only one exception when @start + @count == hb->orig_size. It's needed
>>>>>>> to comfort users of hbitmap_next_dirty_area, which cares about
>>>>>>> hb->orig_size.
>>>>>>>
>>>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>>>>>> ---
>>>>>>>
>>>>>>> v2 based on Max's https://github.com/XanClic/qemu.git block
>>>>>>> which will be merged soon to 4.1, and this patch goes to 4.2
>>>>>>> Based-on: https://github.com/XanClic/qemu.git block
>>>>>>>
>>>>>>> v1 was "[PATCH] util/hbitmap: fix unaligned reset", and as I understand
>>>>>>> we all agreed to just assert alignment instead of aligning down
>>>>>>> automatically.
>>>>>>>
>>>>>>>     include/qemu/hbitmap.h | 5 +++++
>>>>>>>     tests/test-hbitmap.c   | 2 +-
>>>>>>>     util/hbitmap.c         | 4 ++++
>>>>>>>     3 files changed, 10 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/include/qemu/hbitmap.h b/include/qemu/hbitmap.h
>>>>>>> index 4afbe6292e..7865e819ca 100644
>>>>>>> --- a/include/qemu/hbitmap.h
>>>>>>> +++ b/include/qemu/hbitmap.h
>>>>>>> @@ -132,6 +132,11 @@ void hbitmap_set(HBitmap *hb, uint64_t start, uint64_t count);
>>>>>>>      * @count: Number of bits to reset.
>>>>>>>      *
>>>>>>>      * Reset a consecutive range of bits in an HBitmap.
>>>>>>> + * @start and @count must be aligned to bitmap granularity. The only exception
>>>>>>> + * is resetting the tail of the bitmap: @count may be equal to @start +
>>>>>>> + * hb->orig_size,
>>>>>>
>>>>>> s/@start + hb->orig_size/hb->orig_size - @start/, I think.
>>>>>
>>>>> Ha, I wanted to say start + count equal to orig_size. Yours is OK too of course.
>>>>>
>>>>>>
>>>>>>>        in this case @count may be not aligned. @start + @count
>>>>>>
>>>>>> +are
>>>>>>
>>>>>> With those fixed:
>>>>>>
>>>>>> Reviewed-by: Max Reitz <mreitz@redhat.com>
>>>>>
>>>>> Thanks!
>>>>>
>>>>
>>>> I'll add this to the pile for 4.2, after I fix the rebase conflicts that
>>>> arose from 4.1-rc4.
>>>>
>>>
>>> Hi!
>>>
>>> Didn't you forget, or should I resend?
>>>
>>>
>>
>> I must have dropped the patch by accident during the rebasing. As an
>> apology, I squashed in Max's suggestions from the list. Check that they
>> look OK, please?
>>
>> Thanks, applied to my bitmaps tree:
>>
>> https://github.com/jnsnow/qemu/commits/bitmaps
>> https://github.com/jnsnow/qemu.git
>>
> 
> Thanks! Still:
> 
> Quote from your branch:
> 
>  >   * Reset a consecutive range of bits in an HBitmap.
>  > + * @start and @count must be aligned to bitmap granularity. The only exception
>  > + * is resetting the tail of the bitmap: @count may be equal to hb->orig_size -
>  > + * start, in this case @count may be not aligned. @start + @count are
> 
> s/start/@start/ (corresponds to Max's comment, too)
> 

OK, you got it.

> Also, I'm not sure about "are" suggested by Max. "are" is for plural, but here I meant
> one object: sum of @start and @count.
> 

There's not great agreement universally about how to treat things like
collective nouns. Sometimes "Data" is singular, but sometimes it's
plural. "It depends."

In this case, "start + count" refers to one sum, but two constituent
pieces, so it's functioning like a collective noun.

We might say "a + b (together) /are/ ..." but also "the sum of a + b /is/".

> So, you may use exactly "Sum of @start and @count is" or "(@start + @count) sum is" or
> just "(@start + @count) is", whichever you like more.
> 

I like using "the sum of @x and @y is" for being grammatically unambiguous.

updated and pushed.

(Sorry about my language again! --js)