On Mon, Jun 03, 2024, James Houghton wrote: > On Thu, May 30, 2024 at 11:06 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > What I don't think is acceptable is simplifying those optimizations > > out without documenting your justifications (I would even call it a > > design change, rather than simplification, from v3 to v4). > > I'll put back something similar to what you had before (like a > test_clear_young() with a "fast" parameter instead of "bitmap"). I > like the idea of having a new mmu notifier, like > fast_test_clear_young(), while leaving test_young() and clear_young() > unchanged (where "fast" means "prioritize speed over accuracy"). Those two statements are contradicting each other, aren't they? Anyways, I vote for a "fast only" variant, e.g. test_clear_young_fast_only() or so. gup() has already established that terminology in mm/, so hopefully it would be familiar to readers. We could pass a param, but then the MGLRU code would likely end up doing a bunch of useless indirect calls into secondary MMUs, whereas a dedicated hook allows implementations to nullify the pointer if the API isn't supported for whatever reason. And pulling in Oliver's comments about locking, I think it's important that the mmu_notifier API express it's requirement that the operation be "fast", not that it be lockless. E.g. if a secondary MMU can guarantee that a lock will be contented only in rare, slow cases, then taking a lock is a-ok. Or a secondary MMU could do try-lock and bail if the lock is contended. That way KVM can honor the intent of the API with an implementation that works best for KVM _and_ for MGRLU. I'm sure there will be future adjustments and fixes, but that's just more motivation for using something like "fast only" instead of "lockless". > > > I made this logic change as part of removing batching. > > > > > > I'd really appreciate guidance on what the correct thing to do is. > > > > > > In my mind, what would work great is: by default, do aging exactly > > > when KVM can do it locklessly, and then have a Kconfig to always have > > > MGLRU to do aging with KVM if a user really cares about proactive > > > reclaim (when the feature bit is set). The selftest can check the > > > Kconfig + feature bit to know for sure if aging will be done. > > > > I still don't see how that Kconfig helps. Or why the new static branch > > isn't enough? > > Without a special Kconfig, the feature bit just tells us that aging > with KVM is possible, not that it will necessarily be done. For the > self-test, it'd be good to know exactly when aging is being done or > not, so having a Kconfig like LRU_GEN_ALWAYS_WALK_SECONDARY_MMU would > help make the self-test set the right expectations for aging. > > The Kconfig would also allow a user to know that, no matter what, > we're going to get correct age data for VMs, even if, say, we're using > the shadow MMU. Heh, unless KVM flushes, you won't get "correct" age data. > This is somewhat important for me/Google Cloud. Is that reasonable? Maybe > there's a better solution. Hmm, no? There's no reason to use a Kconfig, e.g. if we _really_ want to prioritize accuracy over speed, then a KVM (x86?) module param to have KVM walk nested TDP page tables would give us what we want. But before we do that, I think we need to perform due dilegence (or provide data) showing that having KVM take mmu_lock for write in the "fast only" API provides better total behavior. I.e. that the additional accuracy is indeed worth the cost.
