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Date: Fri, 14 Jul 2017 10:59:32 +0100
From: Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH 7/9] mm, page_alloc: remove stop_machine from
 build_all_zonelists
Message-ID: <20170714095932.jihl6h3y77hxyyiu@suse.de>
References: <20170714080006.7250-1-mhocko@kernel.org>
 <20170714080006.7250-8-mhocko@kernel.org>
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In-Reply-To: <20170714080006.7250-8-mhocko@kernel.org>
Sender: owner-linux-mm@kvack.org
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To: Michal Hocko <mhocko@kernel.org>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, Vlastimil Babka <vbabka@suse.cz>, LKML <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>

On Fri, Jul 14, 2017 at 10:00:04AM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> build_all_zonelists has been (ab)using stop_machine to make sure that
> zonelists do not change while somebody is looking at them. This is
> is just a gross hack because a) it complicates the context from which
> we can call build_all_zonelists (see 3f906ba23689 ("mm/memory-hotplug:
> switch locking to a percpu rwsem")) and b) is is not really necessary
> especially after "mm, page_alloc: simplify zonelist initialization".
> 
> Updates of the zonelists happen very seldom, basically only when a zone
> becomes populated during memory online or when it loses all the memory
> during offline. A racing iteration over zonelists could either miss a
> zone or try to work on one zone twice. Both of these are something we
> can live with occasionally because there will always be at least one
> zone visible so we are not likely to fail allocation too easily for
> example.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>

This patch is contingent on the last patch which updates in place
instead of zeroing the early part of the zonelist first but needs to fix
the stack usage issues. I think it's also worth pointing out in the
changelog that stop_machine never gave the guarantees it claimed as a
process iterating through the zonelist can be stopped so when it resumes
the zonelist has changed underneath it. Doing it online is roughly
equivalent in terms of safety.

-- 
Mel Gorman
SUSE Labs

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