Re: [PATCH 1/1] vhost: do not reset used_memslots when destroying vhost dev

[David Hildenbrand <david@redhat.com>]

On 30.05.25 13:18, Michael S. Tsirkin wrote:
> On Wed, May 14, 2025 at 11:26:05AM +0200, David Hildenbrand wrote:
>> On 14.05.25 11:12, Igor Mammedov wrote:
>>> On Tue, 13 May 2025 15:12:11 +0200
>>> David Hildenbrand <david@redhat.com> wrote:
>>>
>>>> On 13.05.25 14:13, Igor Mammedov wrote:
>>>>> On Mon,  3 Mar 2025 13:02:17 -0500
>>>>> yuanminghao <yuanmh12@chinatelecom.cn> wrote:
>>>>>>>> Global used_memslots or used_shared_memslots is updated to 0 unexpectly
>>>>>>>
>>>>>>> it shouldn't be 0 in practice, as it comes from number of RAM regions VM has.
>>>>>>> It's likely a bug somewhere else.
>>>>>
>>>>> I haven't touched this code for a long time, but I'd say if we consider multiple
>>>>> devices, we shouldn't do following:
>>>>>
>>>>> static void vhost_commit(MemoryListener *listener)
>>>>>        ...
>>>>>        if (dev->vhost_ops->vhost_backend_no_private_memslots &&
>>>>>            dev->vhost_ops->vhost_backend_no_private_memslots(dev)) {
>>>>>            used_shared_memslots = dev->mem->nregions;
>>>>>        } else {
>>>>>            used_memslots = dev->mem->nregions;
>>>>>        }
>>>>>
>>>>> where value dev->mem->nregions gets is well hidden/obscured
>>>>> and hard to trace where tail ends => fragile.
>>>>>
>>>>> CCing David (accidental victim) who rewrote this part the last time,
>>>>> perhaps he can suggest a better way to fix the issue.
>>>>
>>>> I think the original idea is that all devices (of on type: private vs.
>>>> non-private memslots) have the same number of memslots.
>>>>
>>>> This avoids having to loop over all devices to figure out the number of
>>>> memslots.
>>>>
>>>> ... but in vhost_get_free_memslots() we already loop over all devices.
>>>>
>>>> The check in vhost_dev_init() needs to be taken care of.
>>>>
>>>> So maybe we can get rid of both variables completely?
>>>
>>> looks reasonable to me,  (instead of current state which is
>>> juggling with  dev->mem->nregions that can become 0 on unplug
>>> as it was reported).
>>>
>>> David,
>>> do you have time to fix it?
>>
>> I can try, but I was wondering/hoping whether Yuanminghao could take a look
>> at that? I can provide guidance if necessary.
> 
> 
> Guys?

Is the original author not interested in fixing the problem?

-- 
Cheers,

David / dhildenb