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
