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

Juergen Gross
2019-03-14T08:37:56Z

On 12/03/2019 20:46, David Hildenbrand wrote:
> On 12.03.19 19:23, David Hildenbrand wrote:
>> On 12.03.19 19:02, 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@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.
>>>>>>
>>>>> 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().
>>>
>>
>> Haven't had a look at the code yet, but would another temporary fix be
>> to clear/set PG_offline when allocating/freeing a ballooned page?
>> (assuming here that only such pages will be mapped to user space)
>>
>
> I guess something like this could do the trick if I understood it correctly:
>
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 39b229f9e256..d37dd5bb7a8f 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -604,6 +604,7 @@ int alloc_xenballooned_pages(int nr_pages, struct
> page **pages)
> while (pgno < nr_pages) {
> page = balloon_retrieve(true);
> if (page) {
> + __ClearPageOffline(page);
> pages[pgno++] = page;
> #ifdef CONFIG_XEN_HAVE_PVMMU
> /*
> @@ -645,8 +646,10 @@ void free_xenballooned_pages(int nr_pages, struct
> page **pages)
> mutex_lock(&balloon_mutex);
>
> for (i = 0; i < nr_pages; i++) {
> - if (pages[i])
> + if (pages[i]) {
> + __SetPageOffline(pages[i]);
> balloon_append(pages[i]);
> + }
> }
>
> balloon_stats.target_unpopulated -= nr_pages;
>
>
> At least this way, the pages allocated (and thus eventually mapped to
> user space) would not be marked, but the other ones would remain marked
> and could be excluded by makedumptool.
>

I think this patch should do the trick. Julien, could you give it a
try? On x86 I can't reproduce your problem easily as dom0 is PV with
plenty of unpopulated pages for grant memory not suffering from
missing "offline" bit.

Juergen