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From:   Nadav Amit <namit@vmware.com>
To:     David Hildenbrand <david@redhat.com>
CC:     Matthew Wilcox <willy@infradead.org>,
        Julien Grall <julien.grall@arm.com>,
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        Julien Freche <jfreche@vmware.com>,
        Pv-drivers <Pv-drivers@vmware.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: xen: Can't insert balloon page into VM userspace (WAS Re:
 [Xen-devel] [linux-linus bisection] complete test-arm64-arm64-xl-xsm)
Thread-Topic: xen: Can't insert balloon page into VM userspace (WAS Re:
 [Xen-devel] [linux-linus bisection] complete test-arm64-arm64-xl-xsm)
Thread-Index: AQHU2PcQ+2WuG1OSZEK1UAN5w8teIKYIPsCAgAAAfIA=
Date:   Tue, 12 Mar 2019 17:25:28 +0000
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 <46118631-61d4-adb6-6ffc-4e7c62ea3da9@arm.com>
 <20190312171421.GJ19508@bombadil.infradead.org>
 <28a72642-d0db-214c-2bf2-d1a6c6e03d92@redhat.com>
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> On Mar 12, 2019, at 10:23 AM, David Hildenbrand <david@redhat.com> wrote:
>=20
> 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.
>>>>=20
>>>> The bisector fingered the following commit:
>>>>=20
>>>> commit 0ee930e6cafa048c1925893d0ca89918b2814f2c
>>>> Author: Matthew Wilcox <willy@infradead.org>
>>>> Date:   Tue Mar 5 15:46:06 2019 -0800
>>>>=20
>>>>     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 o=
f
>>>>     assuming that kernel drivers always get this right.
>>=20
>> Oh good, it found a real problem.
>>=20
>>> 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.
>>>=20
>>> My knowledge is quite limited in this area. So I am not sure how we can
>>> solve the problem.
>>>=20
>>> I would appreciate if someone could provide input of to fix the mapping=
.
>>=20
>> 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:
>=20
> 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).