Date:   Thu, 7 Apr 2022 10:50:56 +0200
From:   Juergen Gross <jgross@...e.com>
To:     David Hildenbrand <david@...hat.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        stable@...r.kernel.org,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH] xen/balloon: fix page onlining when populating new zone

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.

> 
>>
>> This will result in the new memory being assigned to the guest, but
>> without the allocator being able to use it.
>>
>> When running as a PV guest the situation is even worse: when having
>> been started with less memory than allowed, and the upper limit being
>> lower than 4G, ballooning up will have the same effect as hotplugging
>> new memory. This is due to the usage of the zone device functionality
>> since commit 9e2369c06c8a ("xen: add helpers to allocate unpopulated
>> memory") for creating mappings of other guest's pages, which as a side
>> effect is being used for PV guest ballooning, too.
>>
>> Fix this by checking in xen_online_page() whether the new memory page
>> will be the first in a new zone. If this is the case, add another page
>> to the balloon and use the first memory page of the new chunk as a
>> replacement for this now ballooned out page. This will result in the
>> newly populated zone containing one page being available for the page
>> allocator, which in turn will lead to the zone being added to the
>> allocator.
> 
> This somehow feels like a hack for something that should be handled in
> the core instead :/

Okay, I'll rework the patch (better wording might be: replace) to switch
build_zonerefs_node() to use populated_zone() instead of managed_zone().


Juergen

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