On Thu, Oct 27, 2022, Oliver Upton wrote: > There is no real urgency to free a stage-2 subtree that was pruned. > Nonetheless, KVM does the tear down in the stage-2 fault path while > holding the MMU lock. > > Free removed stage-2 subtrees after an RCU grace period. To guarantee > all stage-2 table pages are freed before killing a VM, add an > rcu_barrier() to the flush path. This is _very_ misleading. The above paints RCU as an optimization of sorts to avoid doing work while holding mmu_lock. Freeing page tables in an RCU callback is _required_ for correctness when allowing parallel page faults to remove page tables, as holding mmu_lock for read in that case doesn't ensure no other CPU is accessing and/or holds a reference to the to-be-freed page table. IMO, this patch should to be squashed with the previous patch, "Protect stage-2 traversal with RCU". One doesn't make any sense without the other. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
