linux-kernel - Re: [PATCH 6/9] mm, page_alloc: simplify zonelist initialization

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Date:   Thu, 20 Jul 2017 08:55:42 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 6/9] mm, page_alloc: simplify zonelist initialization

On 07/14/2017 10:00 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> build_zonelists gradually builds zonelists from the nearest to the most
> distant node. As we do not know how many populated zones we will have in
> each node we rely on the _zoneref to terminate initialized part of the
> zonelist by a NULL zone. While this is functionally correct it is quite
> suboptimal because we cannot allow updaters to race with zonelists
> users because they could see an empty zonelist and fail the allocation
> or hit the OOM killer in the worst case.
> 
> We can do much better, though. We can store the node ordering into an
> already existing node_order array and then give this array to
> build_zonelists_in_node_order and do the whole initialization at once.
> zonelists consumers still might see halfway initialized state but that
> should be much more tolerateable because the list will not be empty and
> they would either see some zone twice or skip over some zone(s) in the
> worst case which shouldn't lead to immediate failures.
> 
> This patch alone doesn't introduce any functional change yet, though, it
> is merely a preparatory work for later changes.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>

I've collected the fold-ups from this thread and looked at the result as
single patch. Sems OK, just two things:
- please rename variable "i" in build_zonelists() to e.g. "nr_nodes"
- the !CONFIG_NUMA variant of build_zonelists() won't build, because it
doesn't declare nr_zones variable