Re: [PATCH] virtio-balloon: do not attempt to release more than available pages

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> Marcelo Tosatti wrote:
>   
>> Handle the case where the balloon target is larger than total ram size.
>>
>> BUG: unable to handle kernel paging request at 0000000000100100
>> IP: [<ffffffff881970f9>] :virtio_balloon:leak__balloon+0x2e/0xbe
>>
>> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>>
>> Index: virtio/virtio_balloon.c
>> ===================================================================
>> --- a/drivers/virtio/virtio_balloon.c
>> +++ b/drivers/virtio/virtio_balloon.c
>> @@ -122,10 +122,21 @@ static void release_pages_by_pfn(const u
>>  	}
>>  }
>>  
>> +static void update_target_size(struct virtio_balloon *vb)
>> +{
>> +	__le32 num_pages = cpu_to_le32(vb->num_pages);
>> +
>> +	vb->vdev->config->set(vb->vdev,
>> +			      offsetof(struct virtio_balloon_config, num_pages),
>> +			      &num_pages, sizeof(num_pages));
>> +}
>>   
>>     
>
> The target is host-owned; moreover the problem may be temporary, but 
> you've changed the target permanently.
>
> Suggest sending the host a message (like the page list) indicating it 
> couldn't allocate any more.
>   

This is what the actual field in the config space is meant for.

> Also, we may have driven the guest close to oom with this.  We need to 
> notify the host when the guest gets into a low-memory cannot swap condition.
>   

We need an OOM handler and a high seek-cost shrinker to ensure we do not 
go too low I imagine.  We can notify the host of when we refuse to go 
lower with actual in the config space.

Regards,

Anthony Liguori



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