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Subject: Re: [PATCH 6/9] mm, page_alloc: simplify zonelist initialization
References: <20170714080006.7250-1-mhocko@kernel.org>
 <20170714080006.7250-7-mhocko@kernel.org>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <d23a3570-e39c-d708-c9d1-80258d45a97f@suse.cz>
Date: Thu, 20 Jul 2017 08:55:42 +0200
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Sender: owner-linux-mm@kvack.org
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To: Michal Hocko <mhocko@kernel.org>, linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@suse.de>, Johannes Weiner <hannes@cmpxchg.org>, LKML <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>

On 07/14/2017 10:00 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.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@suse.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

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