On Thu, Feb 23, 2023, Yu Zhao wrote: > On Thu, Feb 23, 2023 at 12:58 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > 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"? > > Oops -- shouldn't have used "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... > > walk_mm() -> walk_pud_range() -> walk_pmd_range() -> walk_pte_range() > -> test_spte_young() -> mmu_notifier_test_clear_young(). > > MGLRU takes two passes: during the first pass, it sweeps entire VA > space on each MM (per MM/KVM); during the second pass, it uses the rmap on each > folio (per folio). Ah. IIUC, userspace can use LRU_GEN_SPTE_WALK to control whether or not to walk secondary MMUs, and the kernel further restricts LRU_GEN_SPTE_WALK to secondary MMUs that implement a lockless walk. And if the answer is "no", secondary MMUs are simply not consulted. If that's correct, then the proper way to handle this is by extending mmu_notifier_ops to query (a) if there's at least one register listeners that implements test_clear_young() and (b) if all registered listeners that implement test_clear_young() support lockless walks. That avoids direct dependencies on KVM, and avoids making assumptions that may not always hold true, e.g. that KVM is the only mmu_notifier user that supports the young APIs. P.S. all of this info absolutely belongs in documentation and/or changelogs.
