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

Matthew Wilcox
2019-03-12T17:14:31Z

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@infradead.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@samsung.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.