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Date:   Tue, 12 Mar 2019 18:11:14 +0000
From:   Andrew Cooper <>
To:     Boris Ostrovsky <>,
        David Hildenbrand <>,
        Matthew Wilcox <>,
        Julien Grall <>
CC:     Juergen Gross <>, <>,
        Stefano Stabellini <>,
        Kees Cook <>,
        Konrad Rzeszutek Wilk <>,
        "VMware, Inc." <>,
        osstest service owner <>,
        "" <>,
        <>, Julien Freche <>,
        Nadav Amit <>, <>
Subject: Re: [Xen-devel] xen: Can't insert balloon page into VM userspace (WAS
 Re: [linux-linus bisection] complete test-arm64-arm64-xl-xsm)

On 12/03/2019 18:02, Boris Ostrovsky wrote:
> 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 <>
>>>>>> 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 <>
>>>> 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().

FWIW, I did express my views that this was a BadIdea(tm) when that logic
was first introduced.

But yes - now is clearly the time to fix this properly.


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