linux-kernel - Re: [RFC PATCH v2 5/5] KVM: Unmap pages only when it's indeed protected for NUMA migration

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Date:   Mon, 14 Aug 2023 15:44:54 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        bibo mao <maobibo@...ngson.cn>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <pbonzini@...hat.com>, <mike.kravetz@...cle.com>,
        <apopple@...dia.com>, <jgg@...dia.com>, <rppt@...nel.org>,
        <akpm@...ux-foundation.org>, <kevin.tian@...el.com>,
        <david@...hat.com>
Subject: Re: [RFC PATCH v2 5/5] KVM: Unmap pages only when it's indeed
 protected for NUMA migration

On Mon, Aug 14, 2023 at 02:52:07PM +0800, Yan Zhao wrote:
> I wonder if we could loose the frequency to check for rescheduling in
> tdp_mmu_iter_cond_resched() if the zap range is wide, e.g.
> 
> if (iter->next_last_level_gfn ==
>     iter->yielded_gfn + KVM_PAGES_PER_HPAGE(PG_LEVEL_2M))
> 	return false;
Correct:

@@ -712,7 +713,8 @@ static inline bool __must_check tdp_mmu_iter_cond_resched(struct kvm *kvm,
        WARN_ON(iter->yielded);

        /* Ensure forward progress has been made before yielding. */
-       if (iter->next_last_level_gfn == iter->yielded_gfn)
+       if (iter->next_last_level_gfn >= iter->yielded_gfn &&
+          iter->next_last_level_gfn < iter->yielded_gfn + KVM_PAGES_PER_HPAGE(PG_LEVEL_2M))
                return false;

        if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) {


This can reduce kvm_flush_remote_tlbs() a lot in one kvm_unmap_gfn_range() in KVM x86 TDP MMU.