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Date: Mon, 16 Nov 2015 11:51:15 +0900 From: Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp> To: Marcelo Tosatti <mtosatti@...hat.com> Cc: pbonzini@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 05/10] KVM: x86: MMU: Use for_each_rmap_spte macro instead of pte_list_walk() On 2015/11/14 18:20, Marcelo Tosatti wrote: > The actual issue is this: a higher level page that had, under its children, > no out of sync pages, now, due to your addition, a child that is unsync: > > initial state: > level1 > > final state: > > level1 -x-> level2 -x-> level3 > > Where -x-> are the links created by this pagefault fixing round. > > If _any_ page under you is unsync (not necessarily the ones this > pagefault is accessing), you have to mark parents unsync. I understand this, but I don't think my patch will break this. What kvm_mmu_mark_parents_unsync() does is: for each p_i in sp->parent_ptes rmap chain mark_unsync(p_i); Then, mark_unsync() finds the parent sp including that p_i to set ->unsync_child_bitmap and increment ->unsync_children if necessary. It may also call kvm_mmu_mark_parents_unsync() recursively. I understand we need to tell the parents "you have an unsync child/descendant" until this information reaches the top level by that recursive calls. But since these recursive calls cannot come back to the starting sp, the child->parent graph has no loop, each mark_unsync(p_i) will not be affected by other parents in that sp->parent_ptes rmap chain, from which we started the recursive calls. As the following code shows, my patch does mark_unsync(parent_pte) separately, and then mmu_page_add_parent_pte(vcpu, sp, parent_pte): > - } else if (sp->unsync) > + if (parent_pte) > + mark_unsync(parent_pte); > + } else if (sp->unsync) { > kvm_mmu_mark_parents_unsync(sp); > + if (parent_pte) > + mark_unsync(parent_pte); > + } > + mmu_page_add_parent_pte(vcpu, sp, parent_pte); So, as you worried, during each mark_unsync(p_i) is processed, this parent_pte does not exist in that sp->parent_ptes rmap chain. But as I explained above, this does not change anything about what each mark_unsync(p_i) call does, so keeps the original behaviour. By the way, I think "kvm_mmu_mark_parents_unsync" and "mark_unsync" do not tell what they actually do well. When I first saw the names, I thought they would just set the parents' sp->unsync. To reflect the following meaning better, it should be propagate_unsync(_to_parents) or something: Tell the parents "you have an unsync child/descendant" until this unsync information reaches the top level Thanks, Takuya -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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