Re: xen: Can't insert balloon page into VM userspace (WAS Re: [Xen-devel] [linux-linus bisection] complete test-arm64-arm64-xl-xsm)

[David Hildenbrand]

On 12.03.19 18:39, Julien Grall wrote:
> Hi David,
> 
> On 3/12/19 5:18 PM, David Hildenbrand wrote:
>> On 12.03.19 18:14, Matthew Wilcox wrote:
>>> On Tue, Mar 12, 2019 at 05:05:39PM +0000, Julien Grall wrote:
>>>> On 3/12/19 3:59 PM, Julien Grall wrote:
>>>>> It looks like all the arm test for linus [1] and next [2] tree
>>>>> are now failing. x86 seems to be mostly ok.
>>>>>
>>>>> The bisector fingered the following commit:
>>>>>
>>>>> commit 0ee930e6cafa048c1925893d0ca89918b2814f2c
>>>>> Author: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>>>>> Date:   Tue Mar 5 15:46:06 2019 -0800
>>>>>
>>>>>       mm/memory.c: prevent mapping typed pages to userspace
>>>>>       Pages which use page_type must never be mapped to userspace as it would
>>>>>       destroy their page type.  Add an explicit check for this instead of
>>>>>       assuming that kernel drivers always get this right.
>>>
>>> Oh good, it found a real problem.
>>>
>>>> It turns out the problem is because the balloon driver will call
>>>> __SetPageOffline() on allocated page. Therefore the page has a type and
>>>> vm_insert_pages will deny the insertion.
>>>>
>>>> My knowledge is quite limited in this area. So I am not sure how we can
>>>> solve the problem.
>>>>
>>>> I would appreciate if someone could provide input of to fix the mapping.
>>>
>>> I don't know the balloon driver, so I don't know why it was doing this,
>>> but what it was doing was Wrong and has been since 2014 with:
>>>
>>> commit d6d86c0a7f8ddc5b38cf089222cb1d9540762dc2
>>> Author: Konstantin Khlebnikov <k.khlebnikov@xxxxxxxxxxx>
>>> Date:   Thu Oct 9 15:29:27 2014 -0700
>>>
>>>      mm/balloon_compaction: redesign ballooned pages management
>>>
>>> If ballooned pages are supposed to be mapped into userspace, you can't mark
>>> them as ballooned pages using the mapcount field.
>>>
>>
>> Asking myself why anybody would want to map balloon inflated pages into
>> user space (this just sounds plain wrong but my understanding to what
>> XEN balloon driver does might be limited), but I assume the easy fix
>> would be to revert
> 
> Balloon pages are used to map foreign guest pages. As backend PV drivers 
> may live in userspace (e.g QEMU, Xenconsoled...) we need to be able to
> to insert balloon pages in the VM.

Okay, so this is really XEN specific (especially looking at Andrew's
reply). All other balloon drivers told the hypervisor that the inflated
page is dead and it is not going to be use before telling the hypervisor
otherwise. Mapping to user space would violate that contract (and even
be considered harmful in some hypervisor implementations).

> 
>>
>>
>> commit 2f085ff37d08ecbc7849d5abb9424bd7927dda1d
> 
> I guess you meant 77c4adf6a6df6f8f39807eaed48eb73d0eb4261e?

Yes indeed, no idea where that commit id came from :)

> 
> I have reverted the patch and can now access the guest console. Is there 
> a way to keep this patch and at the same time mapping the page in the 
> userspace?

Not without another page flag. And we all know that is unlikely to happen :)

Thanks!

> 
> 
>> Author: David Hildenbrand <david@xxxxxxxxxx>
>> Date:   Wed Mar 6 11:42:24 2019 +1100
>>
>>      xen/balloon: mark inflated pages PG_offline
>>
>>      Mark inflated and never onlined pages PG_offline, to tell the world that
>>      the content is stale and should not be dumped.
>>
>>
> 
> Cheers,
> 


-- 

Thanks,

David / dhildenb