Messages in this thread | | | Date | Thu, 7 Apr 2022 13:17:19 +0200 | From | Juergen Gross <> | Subject | Re: [PATCH] mm, page_alloc: fix build_zonerefs_node() |
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On 07.04.22 13:07, Michal Hocko wrote: > On Thu 07-04-22 12:45:41, Juergen Gross wrote: >> On 07.04.22 12:34, Michal Hocko wrote: >>> Ccing Mel >>> >>> On Thu 07-04-22 11:32:21, Juergen Gross wrote: >>>> Since commit 9d3be21bf9c0 ("mm, page_alloc: simplify zonelist >>>> initialization") only zones with free memory are included in a built >>>> zonelist. This is problematic when e.g. all memory of a zone has been >>>> ballooned out. >>> >>> What is the actual problem there? >> >> When running as Xen guest new hotplugged memory will not be onlined >> automatically, but only on special request. This is done in order to >> support adding e.g. the possibility to use another GB of memory, while >> adding only a part of that memory initially. >> >> In case adding that memory is populating a new zone, the page allocator >> won't be able to use this memory when it is onlined, as the zone wasn't >> added to the zonelist, due to managed_zone() returning 0. > > How is that memory onlined? Because "regular" onlining (online_pages()) > does rebuild zonelists if their zone hasn't been populated before.
The Xen balloon driver has an own callback for onlining pages. The pages are just added to the ballooned-out page list without handing them to the allocator. This is done only when the guest is ballooned up.
So the problem is that a new zone is being populated, but it won't have free pages when the zonelists are rebuilt.
Juergen
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