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

[Nadav Amit]

> On Mar 12, 2019, at 10:23 AM, David Hildenbrand <david@xxxxxxxxxx> 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:
> 
> Just to clarify on that point, XEN balloon does not use balloon
> compaction as far as I know (only virtio-balloon and as far as I know
> now also vmware balloon). Both of them don't map any such pages to user
> space, so it never was and isn't a problem.

I still need to submit the next version of the patches, but yes, we are not
about to map them to userspace (dah).