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Date:   Tue, 12 Mar 2019 18:11:14 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Julien Grall <julien.grall@....com>
CC:     Juergen Gross <jgross@...e.com>, <k.khlebnikov@...sung.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        "VMware, Inc." <pv-drivers@...are.com>,
        osstest service owner <osstest-admin@...project.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, Julien Freche <jfreche@...are.com>,
        Nadav Amit <namit@...are.com>, <xen-devel@...ts.xenproject.org>
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 <willy@...radead.org>
>>>>>> 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@...sung.com>
>>>> 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.

~Andrew

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