On Thu, Feb 23, 2023, Yu Zhao wrote: > On Thu, Feb 23, 2023 at 12:11 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > On Thu, Feb 23, 2023, Yu Zhao wrote: > > > > As alluded to in patch 1, unless batching the walks even if KVM does _not_ support > > > > a lockless walk is somehow _worse_ than using the existing mmu_notifier_clear_flush_young(), > > > > I think batching the calls should be conditional only on LRU_GEN_SPTE_WALK. Or > > > > if we want to avoid batching when there are no mmu_notifier listeners, probe > > > > mmu_notifiers. But don't call into KVM directly. > > > > > > I'm not sure I fully understand. Let's present the problem on the MM > > > side: assuming KVM supports lockless walks, batching can still be > > > worse (very unlikely), because GFNs can exhibit no memory locality at > > > all. So this option allows userspace to disable batching. > > > > I'm asking the opposite. Is there a scenario where batching+lock is worse than > > !batching+lock? If not, then don't make batching depend on lockless walks. > > Yes, absolutely. batching+lock means we take/release mmu_lock for > every single PTE in the entire VA space -- each small batch contains > 64 PTEs but the entire batch is the whole KVM. Who is "we"? I don't see anything in the kernel that triggers walking the whole VMA, e.g. lru_gen_look_around() limits the walk to a single PMD. I feel like I'm missing something...
