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

[Juergen Gross <jgross@suse.com>]

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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

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