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

From: David Hildenbrand <david@redhat.com>
To: Juergen Gross <jgross@suse.com>,
	xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Cc: "Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	stable@vger.kernel.org,
	"Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>,
	"Wei Yang" <richard.weiyang@gmail.com>,
	"Michal Hocko" <mhocko@kernel.org>
Subject: Re: [PATCH] xen/balloon: fix page onlining when populating new zone
Date: Thu, 7 Apr 2022 11:00:33 +0200	[thread overview]
Message-ID: <f423e210-3e28-73f8-1082-869ef680b9b0@redhat.com> (raw)
In-Reply-To: <4f1908b5-5674-a772-3cd9-78e4dc40f776@suse.com>

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 https://lkml.kernel.org/r/20220201070044.zbm3obsoimhz3xd3@master
> 
> 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

https://lkml.kernel.org/r/20220329010901.1654-2-richard.weiyang@gmail.com

-- 
Thanks,

David / dhildenb