On Tue 02-10-18 12:45:50, 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? And until a proper fix is developed can we have NODE_DATA test in the affected code rather than pollute the generic code with something that is essentially a wrong usage of the API? With a big fat warning explaining what is going on here? -- Michal Hocko SUSE Labs
