Messages in this thread | | | Date | Thu, 7 Apr 2022 14:49:50 +0200 | From | Juergen Gross <> | Subject | Re: [PATCH] mm, page_alloc: fix build_zonerefs_node() |
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On 07.04.22 14:32, Mel Gorman wrote: > On Thu, Apr 07, 2022 at 01:17:19PM +0200, Juergen Gross wrote: >> 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. >> > > Is this new behaviour? I ask because keeping !managed_zones out of the
For some time (since kernel 5.9) Xen is using the zone device functionality with memremap_pages() and pgmap->type = MEMORY_DEVICE_GENERIC.
> zonelist and reclaim paths and the behaviour makes sense. Elsewhere you > state "zone can always happen to have no free memory left" and this is true > but it's usually a transient event. The difference between a populated
And if this "transient event" is just happening when the zonelists are being rebuilt the zone will be off the lists maybe forever.
> vs managed zone is usually permanent event where no memory will ever be > placed on the buddy lists because the memory was reserved early in boot > or a similar reason. The patch is probably harmless but it has the > potential to waste CPUs allocating or reclaiming from zones that will > never succeed.
I'd recommend to have an explicit flag per-zone for this case if you really care about that. This would be much cleaner than to imply from no free page being present at a specific point in time, that the zone will never be subject to memory allocation.
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |