Re: [Qemu-devel] [PATCH-for-4.1] virtio-balloon: fix QEMU crashes on pagesize > BALLOON_PAGE_SIZE

[David Hildenbrand]

On 17.07.19 13:10, Michael S. Tsirkin wrote:
> On Wed, Jul 17, 2019 at 10:42:55AM +0200, David Hildenbrand wrote:
>> We are using the wrong functions to set/clear bits, effectively touching
>> multiple bits, writing out of range of the bitmap, resulting in memory
>> corruptions. We have to use set_bit()/clear_bit() instead.
>>
>> Can easily be reproduced by starting a qemu guest on hugetlbfs memory,
>> inflating the balloon. QEMU crashes. This never could have worked
>> properly - especially, also pages would have been discarded when the
>> first sub-page would be inflated (the whole bitmap would be set).
>>
>> While testing I realized, that on hugetlbfs it is pretty much impossible
>> to discard a page - the guest just frees the 4k sub-pages in random order
>> most of the time. I was only able to discard a hugepage a handful of
>> times - so I hope that now works correctly.
> 
> I think this actually only works reasonably well on guests
> which have pages larger than 4K.
> So guest page size = host page size > balloon page size.
> 

Yes, otherwise it's pure luck (and therefore the printed warning is
appropriate).

-- 

Thanks,

David / dhildenb