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

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Date:   Mon, 1 Oct 2018 16:20:30 -0700
From:   Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Michael Bringmann <mwb@...ux.vnet.ibm.com>
Cc:     Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Mathieu Malaterre <malat@...ian.org>,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>, 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 10/01/2018 01:27 PM, Michal Hocko wrote:
> On Mon 01-10-18 13:56:25, Michael Bringmann wrote:
>> In some LPAR migration scenarios, device-tree modifications are
>> made to the affinity of the memory in the system.  For instance,
>> it may occur that memory is installed to nodes 0,3 on a source
>> system, and to nodes 0,2 on a target system.  Node 2 may not
>> have been initialized/allocated on the target system.
>>
>> After migration, if a RTAS PRRN memory remove is made to a
>> memory block that was in node 3 on the source system, then
>> try_offline_node tries to remove it from node 2 on the target.
>> The NODE_DATA(2) block would not be initialized on the target,
>> and there is no validation check in the current code to prevent
>> the use of a NULL pointer.
> 
> I am not familiar with ppc and the above doesn't really help me
> much. Sorry about that. But from the above it is not clear to me whether
> it is the caller which does something unexpected or the hotplug code
> being not robust enough. From your changelog I would suggest the later
> but why don't we see the same problem for other archs? Is this a problem
> of unrolling a partial failure?
> 
> dlpar_remove_lmb does the following
> 
> 	nid = memory_add_physaddr_to_nid(lmb->base_addr);
> 
> 	remove_memory(nid, lmb->base_addr, block_sz);
> 
> 	/* Update memory regions for memory remove */
> 	memblock_remove(lmb->base_addr, block_sz);
> 
> 	dlpar_remove_device_tree_lmb(lmb);
> 
> Is the whole operation correct when remove_memory simply backs off
> silently. Why don't we have to care about memblock resp
> dlpar_remove_device_tree_lmb parts? In other words how come the physical
> memory range is valid while the node association is not?
> 

I think the issue here is a race between the LPM code updating affinity and PRRN events being processed. Does your other patch[1] not fix the issue? Or is it that the LPM affinity updates don't do any of the initialization/allocation you mentioned?

-Tyrel

[1] https://lore.kernel.org/linuxppc-dev/20181001185603.11373.61650.stgit@ltcalpine2-lp9.aus.stglabs.ibm.com/T/#u