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:   Wed, 16 Aug 2023 06:39:20 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     bibo mao <maobibo@...ngson.cn>
Cc:     Yan Zhao <yan.y.zhao@...el.com>, 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 Wed, Aug 16, 2023, bibo mao wrote:
> 
> 
> 在 2023/8/16 15:18, Yan Zhao 写道:
> > On Wed, Aug 16, 2023 at 03:29:22PM +0800, bibo mao wrote:
> >>> Flush must be done before kvm->mmu_lock is unlocked, otherwise,
> >>> confusion will be caused when multiple threads trying to update the
> >>> secondary MMU.
> >> Since tlb flush is delayed after all pte entries are cleared, and currently
> >> there is no tlb flush range supported for secondary mmu. I do know why there
> >> is confusion before or after kvm->mmu_lock.
> > 
> > Oh, do you mean only do kvm_unmap_gfn_range() in .invalidate_range_end()?
> yes, it is just sketchy thought for numa balance scenery, 
> do kvm_unmap_gfn_range() in invalidate_range_end rather than
> invalidate_range_start.

That is not an option, it's a direction violation of the mmu_notifier contract.
Secondary MMUs must drop all references before returning from invalidate_range_start().