Re: [PATCH] xen/balloon: fix page onlining when populating new zone

From: David Hildenbrand
Date: Thu Apr 07 2022 - 05:01:53 EST

On 07.04.22 10:50, Juergen Gross wrote:
> On 07.04.22 10:23, David Hildenbrand wrote:
>> On 06.04.22 15:32, Juergen Gross wrote:
>>> When onlining a new memory page in a guest the Xen balloon driver is
>>> adding it to the ballooned pages instead making it available to be
>>> used immediately. This is meant to enable to add a new upper memory
>>> limit to a guest via hotplugging memory, without having to assign the
>>> new memory in one go.
>>> In case the upper memory limit will be raised above 4G, the new memory
>>> will populate the ZONE_NORMAL memory zone, which wasn't populated
>>> before. The newly populated zone won't be added to the list of zones
>>> looked at by the page allocator though, as only zones with available
>>> memory are being added, and the memory isn't yet available as it is
>>> ballooned out.
>> I think we just recently discussed these corner cases on the -mm list.
> Indeed.
>> The issue is having effectively populated zones without manages pages
>> because everything is inflated in a balloon.
> Correct.
>> That can theoretically also happen when managing to fully inflate the
>> balloon in one zone and then, somehow, the zones get rebuilt.
> I think you are right. I didn't think of that scenario.
>> build_zonerefs_node() documents "Add all populated zones of a node to
>> the zonelist" but checks for managed zones, which is wrong.
>> See
> I found commit 6aa303defb7454 which introduced this test. I thought
> it was needed due to the problem this commit tried to solve. Maybe I
> was wrong and that commit shouldn't have changed the condition when
> building the zonelist, but just the ones in the allocation paths.

In regard to kswapd, that is currently being worked on via


David / dhildenb