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

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Message-ID: <ZNZtdKXslPlKYRWY@nvidia.com>
Date:   Fri, 11 Aug 2023 14:18:44 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Yan Zhao <yan.y.zhao@...el.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, 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

> However, *if* it's ok to invoke MMU notifiers while holding PMD/PTE locks, then
> I think we can achieve what you want without losing batching, and without changing
> secondary MMUs.

No, this is not legal.

> +void change_pmd_range_notify_secondary_mmus(unsigned long addr,
> +					    struct mmu_notifier_range *range)
> +{
> +	if (range->start)
> +		return;
> +
> +	VM_WARN_ON(addr >= range->end);
> +	range->start = addr;
> +	mmu_notifier_invalidate_range_start_nonblock(range);
> +}

The 'nonblock' entry point is advisory, if it fails the caller must
not change the mm.

Jason