linux-kernel - Re: [PATCH] migration/mm: Add WARN_ON to try_offline

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Message-ID: <20181002145922.GZ18290@dhcp22.suse.cz>
Date:   Tue, 2 Oct 2018 16:59:22 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Michael Bringmann <mwb@...ux.vnet.ibm.com>
Cc:     Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>,
        Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Mathieu Malaterre <malat@...ian.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Nicholas Piggin <npiggin@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>,
        Juliet Kim <minkim@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
        linuxppc-dev@...ts.ozlabs.org,
        Dan Williams <dan.j.williams@...el.com>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH] migration/mm: Add WARN_ON to try_offline_node

On Tue 02-10-18 09:51:40, Michael Bringmann wrote:
[...]
> When the device-tree affinity attributes have changed for memory,
> the 'nid' affinity calculated points to a different node for the
> memory block than the one used to install it, previously on the
> source system.  The newly calculated 'nid' affinity may not yet
> be initialized on the target system.  The current memory tracking
> mechanisms do not record the node to which a memory block was
> associated when it was added.  Nathan is looking at adding this
> feature to the new implementation of LMBs, but it is not there
> yet, and won't be present in earlier kernels without backporting a
> significant number of changes.

Then the patch you have proposed here just papers over a real issue, no?
IIUC then you simply do not remove the memory if you lose the race.
-- 
Michal Hocko
SUSE Labs