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

From: David Hildenbrand
Date: Tue Mar 12 2019 - 13:23:53 EST

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.

> 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.



David / dhildenb