On 10/2/24 10:35 AM, Dave Hansen wrote:
We were just chatting about this on David Rientjes's MM alignment call. I thought I'd try to give a little brain Let's start by thinking about KVM and secondary MMUs. KVM has a primary mm: the QEMU (or whatever) process mm. The virtualization (EPT/NPT) tables get entries that effectively mirror the primary mm page tables and constitute a secondary MMU. If the primary page tables change, mmu_notifiers ensure that the changes get reflected into the virtualization tables and also that the virtualization paging structure caches are flushed. msharefs is doing something very similar. But, in the msharefs case, the secondary MMUs are actually normal CPU MMUs. The page tables are normal old page tables and the caches are the normal old TLB. That's what makes it so confusing: we have lots of infrastructure for dealing with that "stuff" (CPU page tables and TLB), but msharefs has short-circuited the infrastructure and it doesn't work any more. Basically, I think it makes a lot of sense to check what KVM (or another mmu_notifier user) is doing and make sure that msharefs is following its lead. For instance, KVM _should_ have the exact same "page free" flushing issue where it gets the MMU notifier call but the page may still be in the secondary MMU. I _think_ KVM fixes it with an extra page refcount that it takes when it first walks the primary page tables. But the short of it is that the msharefs host mm represents a "secondary MMU". I don't think it is really that special of an MMU other than the fact that it has an mm_struct.
Thanks, Dave. This is helpful. I'll look at what other mmu notifier users are doing. This does align with the comments in mmu_notifier.h regarding invalidate_range_start/end.
Anthony
