linux-kernel - Re: [PATCH] mm, page_alloc: fix build_zonerefs

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Date:   Thu, 7 Apr 2022 13:17:19 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     xen-devel@...ts.xenproject.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, Mel Gorman <mgorman@...e.de>
Subject: 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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