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

[Boris Ostrovsky]

On 3/12/19 1:24 PM, Andrew Cooper wrote:
> On 12/03/2019 17:18, 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
> I suspect the bug here is that the balloon driver is (ab)used for a
> second purpose

Yes. And its name is alloc_xenballooned_pages().

-boris

>  - to create a hole in pfn space to map some other bits of
> shared memory into.
>
> I think at the end of the day, what is needed is a struct page_info
> which looks like normal RAM, but the backing for which can be altered by
> hypercall to map other things.
>
> ~Andrew