On Aug 3, 2013, at 1:09 PM, Takuya Yoshikawa <takuya.yoshikawa@xxxxxxxxx> wrote: > On Tue, 30 Jul 2013 21:01:58 +0800 > Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx> wrote: > >> Background >> ========== >> Currently, when mark memslot dirty logged or get dirty page, we need to >> write-protect large guest memory, it is the heavy work, especially, we need to >> hold mmu-lock which is also required by vcpu to fix its page table fault and >> mmu-notifier when host page is being changed. In the extreme cpu / memory used >> guest, it becomes a scalability issue. >> >> This patchset introduces a way to locklessly write-protect guest memory. > > Nice improvements! Thank you! > > If I read the patch set correctly, this work contains the following changes: > > Cleanups: > Patch 1 and patch 12. > > Lazy large page dropping for dirty logging: > Patch 2-3. > Patch 2 is preparatory to patch 3. > > This does not look like an RFC if you address Marcelo's comment. > Any reason to include this in an RFC patch set? Right, these two patches are not really RFC since you guys have reviewed the idea. The reason i put these into this patchset is that they are also the preparing work for implementing lockless writ-protection since after that we do not need to remove a spte from the rmap any more. (only need to write-protect a spte.) > > Making remote TLBs flushable outside of mmu_lock for dirty logging: > Patch 6. > > This is nice. I'm locally using a similar patch for my work, but yours > is much cleaner and better. I hope this will get merged soon. Thanks! > > New Pte-list handling: > Patch 7-9. > > Still reading the details. > > RCU-based lockless write protection. > Patch 10-11. > > If I understand RCU correctly, the current implementation has a problem: > read-side critical sections can become too long. > > See the following LWN's article: > "Sleepable RCU" > https://lwn.net/Articles/202847/ > > Especially, kvm_mmu_slot_remove_write_access() can take hundreds of > milliseconds, or even a few seconds for guests using shadow paging. > Is it possible to break the read-side critical section after protecting > some pages? -- I guess so. Yes. we can use the break-tech in the code if it is needed, like this: if (need_resched()) { kvm_use_rcu_free_page_end(); kvm_use_rcu_free_page_begin(); } > > Anyway, I want to see the following non-RFC quality patches get merged first: > - Lazy large page dropping for dirty logging: > - Making remote TLBs flushable outside of mmu_lock for dirty logging > > As you are doing in patch 11, the latter can eliminate the TLB flushes before > cond_resched_lock(). So this alone is an optimization, and since my work is > based on this TLB flush-less lock breaking, I would appriciate if you make this > change first in your clean way. Okay, i will move these patches to the front then the maintainers can merge them easily. > > The remaining patches, pte-list refactoring and lock-less ones, also look > interesting, but I need to read more to understand them. > > Thanks for the nice work! Thanks for your review and the comments! :) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
