linux-kernel - Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

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Message-ID: <20190416183404.GA655@dhcp22.suse.cz>
Date:   Tue, 16 Apr 2019 20:34:04 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>, mgorman@...hsingularity.net,
        riel@...riel.com, hannes@...xchg.org, akpm@...ux-foundation.org,
        keith.busch@...el.com, dan.j.williams@...el.com,
        fengguang.wu@...el.com, fan.du@...el.com, ying.huang@...el.com,
        ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

On Tue 16-04-19 08:46:56, Dave Hansen wrote:
> On 4/16/19 7:39 AM, Michal Hocko wrote:
> >> Strict binding also doesn't keep another app from moving the
> >> memory.
> > I would consider that a bug.
> 
> A bug where, though?  Certainly not in the kernel.

Kernel should refrain from moving explicitly bound memory nilly willy. I
certainly agree that there are corner cases. E.g. memory hotplug. We do
break CPU affinity for CPU offline as well. So this is something user
should expect. But the kernel shouldn't move explicitly bound pages to a
different node implicitly. I am not sure whether we even do that during
compaction if we do then I would consider _this_ to be a bug. And NUMA
rebalancing under memory pressure falls into the same category IMO.
-- 
Michal Hocko
SUSE Labs