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:

quoted

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 (local)