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

[John Snow <jsnow@redhat.com>]

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.

--js