"Huang, Ying" <ying.huang@xxxxxxxxx> writes: >>> The other way (suggested by this series) is to make dax/kmem call a >>> notifier chain, then CXL CDAT or ACPI HMAT can identify the type of >>> device and calculate the distance if the type is correct for them. I >>> don't think that it's good to make dax/kem to know every possible >>> types of memory devices. >> >> Do we expect there to be lots of different types of memory devices >> sharing a common dax/kmem driver though? Must admit I'm coming from a >> GPU background where we'd expect each type of device to have it's own >> driver anyway so wasn't expecting different types of memory devices to >> be handled by the same driver. > > Now, dax/kmem.c is used for > > - PMEM (Optane DCPMM, or AEP) > - CXL.mem > - HBM (attached to CPU) Thanks a lot for the background! I will admit to having a faily narrow focus here. >>> And, I don't think that we are forced to use the general notifier >>> chain interface in all memory device drivers. If the memory device >>> driver has better understanding of the memory device, it can use other >>> way to determine abstract distance. For example, a CXL memory device >>> driver can identify abstract distance by itself. While other memory >>> device drivers can use the general notifier chain interface at the >>> same time. >> >> Whilst I think personally I would find that flexibility useful I am >> concerned it means every driver will just end up divining it's own >> distance rather than ensuring data in HMAT/CDAT/etc. is correct. That >> would kind of defeat the purpose of it all then. > > But we have no way to enforce that too. Enforce that HMAT/CDAT/etc. is correct? Agree we can't enforce it, but we can influence it. If drivers can easily ignore the notifier chain and do their own thing that's what will happen. >>> While other memory device drivers can use the general notifier chain >>> interface at the same time. How would that work in practice though? The abstract distance as far as I can tell doesn't have any meaning other than establishing preferences for memory demotion order. Therefore all calculations are relative to the rest of the calculations on the system. So if a driver does it's own thing how does it choose a sensible distance? IHMO the value here is in coordinating all that through a standard interface, whether that is HMAT or something else. - Alistair
