Re: [Qemu-devel] [PATCH] util/hbitmap: fix unaligned reset

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

05.08.2019 12:26, Vladimir Sementsov-Ogievskiy wrote:
> 02.08.2019 22:21, John Snow wrote:
>>
>>
>> On 8/2/19 2:58 PM, Vladimir Sementsov-Ogievskiy wrote:
>>> hbitmap_reset is broken: it rounds up the requested region. It leads to
>>> the following bug, which is shown by fixed test:
>>>
>>> assume granularity = 2
>>> set(0, 3) # count becomes 4
>>> reset(0, 1) # count becomes 2
>>>
>>> But user of the interface assume that virtual bit 1 should be still
>>> dirty, so hbitmap should report count to be 4!
>>>
>>> In other words, because of granularity, when we set one "virtual" bit,
>>> yes, we make all "virtual" bits in same chunk to be dirty. But this
>>> should not be so for reset.
>>>
>>> Fix this, aligning bound correctly.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>
>>> Hi all!
>>>
>>> Hmm, is it a bug or feature? :)
>>
>> Very, very good question.
>>
>>> I don't have a test for mirror yet, but I think that sync mirror may be broken
>>> because of this, as do_sync_target_write() seems to be using unaligned reset.
>>>
>>
>> Honestly I was worried about this -- if you take a look at my patches
>> where I add new bitmap sync modes, I bent over backwards to align
>> requests for the sync=top bitmap initialization methods because I was
>> worried about this possibly being the case.
>>
>>
>> I'm not sure what the "right" behavior ought to be.
>>
>> Let's say you have a granularity of 8 bytes:
>>
>> if you reset 0-3 in one call, and then 4-7 in the next, what happens? If
>> the caller naively thinks there's a 1:1 relationship, it might actually
>> expect that to reflect a cleared bit. With alignment protection, we'll
>> just fail to clear it both times and it remains set.
>>
>> On the other hand, if you do allow partial clears, the first reset for
>> 0-3 will toggle off 4-7 too, where we might rely on the fact that it's
>> actually still dirty.
>>
>> Whether or not that's dangerous depends on the context, and only the
>> caller knows the context. I think we need to make the semantic effect of
>> the reset "obvious" to the caller.
>>
>>
>> I envision this:
>>
>> - hbitmap_reset(bitmap, start, length)
>>    returns -EINVAL if the range is not properly aligned
> 
> hbitmap_reset don't return, I thinks it should be an assertion

don't return any value

> 
>>
>> - hbitmap_reset_flags(bitmap, flags, start, length)
>>    if (flags & HBITMAP_ALIGN_DOWN) align request to only full bits
>>    if (flags & HBITMAP_ALIGN_UP) align request to cover any bit even
>> partially touched by the specified range
>>    otherwise, pass range through as-is to hbitmap_reset (and possibly get
>> -EINVAL if caller did not align the request.)
>>
>>
>> That way the semantics are always clear to the caller.
> 
> Hmm, I doubt, is there any use of ALIGN_UP? In most cases it's safe to thing that
> something clear is dirty (and this is how hbitmap actually works on set/get), but
> it seems always unsafe to ALIGN_UP reset..
> 
> So, I think that it should be default to ALIGN_DOWN, or just an assertion that request
> is aligned (which anyway leads to implementing a helper hbitmap_reset_align_up)..

hbitmap_reset_align_down I mean.

> 
>>
>> --js
>>
>>>   tests/test-hbitmap.c |  2 +-
>>>   util/hbitmap.c       | 24 +++++++++++++++++++-----
>>>   2 files changed, 20 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/tests/test-hbitmap.c b/tests/test-hbitmap.c
>>> index 592d8219db..0008025a9f 100644
>>> --- a/tests/test-hbitmap.c
>>> +++ b/tests/test-hbitmap.c
>>> @@ -424,7 +424,7 @@ static void test_hbitmap_granularity(TestHBitmapData *data,
>>>       hbitmap_test_set(data, 0, 3);
>>>       g_assert_cmpint(hbitmap_count(data->hb), ==, 4);
>>>       hbitmap_test_reset(data, 0, 1);
>>> -    g_assert_cmpint(hbitmap_count(data->hb), ==, 2);
>>> +    g_assert_cmpint(hbitmap_count(data->hb), ==, 4);
>>>   }
>>>   static void test_hbitmap_iter_granularity(TestHBitmapData *data,
>>> diff --git a/util/hbitmap.c b/util/hbitmap.c
>>> index 7905212a8b..61a813994a 100644
>>> --- a/util/hbitmap.c
>>> +++ b/util/hbitmap.c
>>> @@ -473,15 +473,29 @@ void hbitmap_reset(HBitmap *hb, uint64_t start, uint64_t count)
>>>   {
>>>       /* Compute range in the last layer.  */
>>>       uint64_t first;
>>> -    uint64_t last = start + count - 1;
>>> +    uint64_t last;
>>> +    uint64_t end = start + count;
>>> +    uint64_t gran = UINT64_C(1) << hb->granularity;
>>> -    trace_hbitmap_reset(hb, start, count,
>>> -                        start >> hb->granularity, last >> hb->granularity);
>>> +    /*
>>> +     * We should clear only bits, fully covered by requested region. Otherwise
>>> +     * we may clear something that is actually still dirty.
>>> +     */
>>> +    first = DIV_ROUND_UP(start, gran);
>>> -    first = start >> hb->granularity;
>>> -    last >>= hb->granularity;
>>> +    if (end == hb->orig_size) {
>>> +        end = DIV_ROUND_UP(end, gran);
>>> +    } else {
>>> +        end = end >> hb->granularity;
>>> +    }
>>> +    if (end <= first) {
>>> +        return;
>>> +    }
>>> +    last = end - 1;
>>>       assert(last < hb->size);
>>> +    trace_hbitmap_reset(hb, start, count, first, last);
>>> +
>>>       hb->count -= hb_count_between(hb, first, last);
>>>       if (hb_reset_between(hb, HBITMAP_LEVELS - 1, first, last) &&
>>>           hb->meta) {
>>>
> 
> 


-- 
Best regards,
Vladimir