On Thu, May 23, 2013 at 03:55:59AM +0800, Xiao Guangrong wrote:
> kvm_zap_obsolete_pages uses lock-break technique to zap pages,
> it will flush tlb every time when it does lock-break
>
> We can reload mmu on all vcpus after updating the generation
> number so that the obsolete pages are not used on any vcpus,
> after that we do not need to flush tlb when obsolete pages
> are zapped
After that point batching is also not relevant anymore?
Still concerned about a similar case mentioned earlier:
"
Note the account for pages freed step after pages are actually
freed: as discussed with Takuya, having pages freed and freed page
accounting out of sync across mmu_lock is potentially problematic:
kvm->arch.n_used_mmu_pages and friends do not reflect reality which can
cause problems for SLAB freeing and page allocation throttling.
"
This is a real problem, if you decrease n_used_mmu_pages at
kvm_mmu_prepare_zap_page, but only actually free pages later
at kvm_mmu_commit_zap_page, there is the possibility of allowing
a huge number to be retained. There should be a maximum number of pages
at invalid_list.
(even higher possibility if you schedule without freeing pages reported
as released!).
> Note: kvm_mmu_commit_zap_page is still needed before free
> the pages since other vcpus may be doing locklessly shadow
> page walking
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
> arch/x86/kvm/mmu.c | 32 ++++++++++++++++++++++----------
> 1 files changed, 22 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index e676356..5e34056 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4237,8 +4237,6 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
> restart:
> list_for_each_entry_safe_reverse(sp, node,
> &kvm->arch.active_mmu_pages, link) {
> - int ret;
> -
> /*
> * No obsolete page exists before new created page since
> * active_mmu_pages is the FIFO list.
> @@ -4254,21 +4252,24 @@ restart:
> if (sp->role.invalid)
> continue;
>
> + /*
> + * Need not flush tlb since we only zap the sp with invalid
> + * generation number.
> + */
> if (batch >= BATCH_ZAP_PAGES &&
> - (need_resched() || spin_needbreak(&kvm->mmu_lock))) {
> + cond_resched_lock(&kvm->mmu_lock)) {
> batch = 0;
> - kvm_mmu_commit_zap_page(kvm, &invalid_list);
> - cond_resched_lock(&kvm->mmu_lock);
> goto restart;
> }
>
> - ret = kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
> - batch += ret;
> -
> - if (ret)
> - goto restart;
> + batch += kvm_mmu_prepare_zap_obsolete_page(kvm, sp,
> + &invalid_list);
> }
>
> + /*
> + * Should flush tlb before free page tables since lockless-walking
> + * may use the pages.
> + */
> kvm_mmu_commit_zap_page(kvm, &invalid_list);
> }
>
> @@ -4287,6 +4288,17 @@ void kvm_mmu_invalidate_zap_all_pages(struct kvm *kvm)
> trace_kvm_mmu_invalidate_zap_all_pages(kvm);
> kvm->arch.mmu_valid_gen++;
>
> + /*
> + * Notify all vcpus to reload its shadow page table
> + * and flush TLB. Then all vcpus will switch to new
> + * shadow page table with the new mmu_valid_gen.
> + *
> + * Note: we should do this under the protection of
> + * mmu-lock, otherwise, vcpu would purge shadow page
> + * but miss tlb flush.
> + */
> + kvm_reload_remote_mmus(kvm);
> +
> kvm_zap_obsolete_pages(kvm);
> spin_unlock(&kvm->mmu_lock);
> }
> --
> 1.7.7.6
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