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

[David Hildenbrand <david@redhat.com>]

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.
The issue is having effectively populated zones without manages pages
because everything is inflated in a balloon.

That can theoretically also happen when managing to fully inflate the
balloon in one zone and then, somehow, the zones get rebuilt.

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

> 
> 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 :/


-- 
Thanks,

David / dhildenb