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 Linux Technology Center IBM Corporation Tie-Line 363-5196 External: (512) 286-5196 Cell: (512) 466-0650 mwb@xxxxxxxxxxxxxxxxxx
