Re: [PATCH] virtio-mem: Fix the bitmap index of the section offset

[Chenyi Qiang]

On 12/16/2022 6:30 PM, David Hildenbrand wrote:
> On 16.12.22 09:52, David Hildenbrand wrote:
>> On 16.12.22 07:22, Chenyi Qiang wrote:
>>> vmem->bitmap indexes the memory region of the virtio-mem backend at a
>>> granularity of block_size. To calculate the index of target section
>>> offset,
>>> the block_size should be divided instead of the bitmap_size.
>>
>> I'm curious, what's the user-visible effect and how did you identify
>> this issue?
>>
>> IIUC, we could end up our search for a plugged/unplugged block "too
>> late", such that we miss to process blocks.
>>
>> That would be the case if the bitmap_size < block_size, which should
>> effectively always happen ...
>>
>>
>> unplug_all and migration would be affected, which is why a simple test
>> case without a guest reboot/migration wouldn't run into it.
> 
> I just realized that unplug_all is fine because only vfio implements the
> ram_discard_listener so far and always sets
> double_discard_supported=true. So migration should be the issue (and
> IIRC migration with VFIO is still shaky).

Yes, actually, no obvious visible effect on my side. I was just learning
the RamDiscardManager interface and found this issue. :)

>