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Re: [PATCH] mm, page_alloc: fix build_zonerefs_node()
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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