linux-kernel - Re: [v2 PATCH 7/9] mm: vmscan: check if the demote target node is contended or not

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Date:   Thu, 11 Apr 2019 09:06:00 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>, mhocko@...e.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
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 7/9] mm: vmscan: check if the demote target node is
 contended or not

On 4/10/19 8:56 PM, Yang Shi wrote:
> When demoting to PMEM node, the target node may have memory pressure,
> then the memory pressure may cause migrate_pages() fail.
> 
> If the failure is caused by memory pressure (i.e. returning -ENOMEM),
> tag the node with PGDAT_CONTENDED.  The tag would be cleared once the
> target node is balanced again.
> 
> Check if the target node is PGDAT_CONTENDED or not, if it is just skip
> demotion.

This seems like an actively bad idea to me.

Why do we need an *active* note to say the node is contended?  Why isn't
just getting a failure back from migrate_pages() enough?  Have you
observed this in practice?