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

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Message-ID: <f08c1493-9238-0009-56b4-dc0ab3571b33@suse.com>
Date:   Thu, 7 Apr 2022 12:45:41 +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 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.

Note that a similar problem could occur in case the zonelists are
rebuilt and a zone happens to have all memory in use. That zone will
then be dropped from the rebuilt zonelists.

> 
>> Use populated_zone() when building a zonelist as it has been done
>> before that commit.
>>
>> Cc: stable@...r.kernel.org
>> Fixes: 9d3be21bf9c0 ("mm, page_alloc: simplify zonelist initialization")
> 
> Did you mean to refer to
> 6aa303defb74 ("mm, vmscan: only allocate and reclaim from zones with
> pages managed by the buddy allocator")?

Yes. I really can't explain where this other reference is coming from.
I must have selected the wrong line from a git annotate output

Juergen

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