On 10/03/2018 06:27 AM, Michael Bringmann wrote: > On 10/02/2018 02:45 PM, Tyrel Datwyler wrote: >> On 10/02/2018 11:13 AM, Michael Bringmann wrote: >>> >>> >>> 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. >> >> This is still basically a hack to get around a known race. In itself this >> patch is still worth while in that we shouldn't crash the kernel on a null >> pointer dereference. However, I think the actual problem still needs to be >> addressed. We shouldn't run any PRRN events for the source system on the >> target after a migration. The device tree update should have taken care of >> telling us about new affinities and what not. Can we just throw out any >> queued PRRN events when we wake up on the target? > > We are not talking about queued events provided on the source system, but > about > new PRRN events sent by phyp to the kernel on the target system to update the > kernel state after migration. No way to predict the content.
Okay, but either way shouldn't your other proposed patches to update memory affinity by re-adding memory and changing the time topology updates are stopped to include the post-mobility updates put things in the right nodes? Or, am I missing something? I would assume a PRRN on the target would assume the target was up-to-date with respect to where things are supposed to be located. -Tyrel > >> >> -Tyrel >>> >>> Regards, >>> > > Michael >
