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

[David Hildenbrand <david@redhat.com>]

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).

-- 
Thanks,

David / dhildenb