Re: [PATCH] migration/mm: Add WARN_ON to try_offline_node

[Michael Bringmann]

On 10/02/2018 09:59 AM, Michal Hocko wrote:
> 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.

The problem occurs when removing memory after an affinity change references a node that was previously unreferenced.  Other code in 'kernel/mm/memory_hotplug.c' deals with initializing an empty node when adding memory to a system.  The   'removing memory' case is specific to systems that perform LPM and allow device-tree changes.  The powerpc kernel does not have the option of accepting some PRRN requests and accepting others.  It must perform them all.

The kernel/mm code that removes memory blocks does not (before this patch) recognize that the affinity of a memory block could have changed to a previously unused node.  If every path to try_offline_node made such a check, then this patch would be unnecessary.  However, putting a patch at a single location to check for a relatively rare occurrence, would seem to be a more efficient implementation.

Michael

-- 
Michael W. Bringmann
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