On 10/02/2018 11:04 AM, Michal Hocko wrote: > On Tue 02-10-18 10:14:49, Michael Bringmann wrote: >> 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. > > I am sorry, but you are still too cryptic for me. Either there is a > correctness issue and the the patch doesn't really fix anything or the > final race doesn't make any difference and then the ppc code should be > explicit about that. Checking the node inside the hotplug core code just > looks as a wrong layer to mitigate an arch specific problem. I am not > saying the patch is a no-go but if anything we want a big fat comment > explaining how this is possible because right now it just points to an > incorrect API usage. > > That being said, this sounds pretty much ppc specific problem and I > would _prefer_ it to be handled there (along with a big fat comment of > course). Let me try again. Regardless of the path to which we get to this condition, we currently crash the kernel. This patch changes that to a WARN_ON notice and continues executing the kernel without shutting down the system. I saw the problem during powerpc testing, because that is the focus of my work. There are other paths to this function besides powerpc. I feel that the kernel should keep running instead of halting. Regards, -- Michael W. Bringmann Linux Technology Center IBM Corporation Tie-Line 363-5196 External: (512) 286-5196 Cell: (512) 466-0650 mwb@xxxxxxxxxxxxxxxxxx
