linux-kernel - Re: [RFC PATCH 3/3] KVM: x86/mmu: skip zap maybe-dma-pinned pages for NUMA migration

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Date:   Wed, 9 Aug 2023 10:58:28 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Sean Christopherson <seanjc@...gle.com>, <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>
Subject: Re: [RFC PATCH 3/3] KVM: x86/mmu: skip zap maybe-dma-pinned pages
 for NUMA migration

On Tue, Aug 08, 2023 at 11:32:37AM -0300, Jason Gunthorpe wrote:
.... 
> > This really needs to be fixed in the primary MMU and not require any direct
> > involvement from secondary MMUs, e.g. the mmu_notifier invalidation itself needs
> > to be skipped.
> 
> This likely has the same issue you just described, we don't know if it
> can be skipped until we iterate over the PTEs and by then it is too
> late to invoke the notifier. Maybe some kind of abort and restart
The problem is that KVM currently performs the zap in handler of .invalidate_range_start(),
so before abort in mm, KVM has done the zap in secondary MMU.

Or, could we move the zap in KVM side to handler of .invalidate_range_end() only for
MMU_NOTIFY_PROTECTION_VMA and MMU_NOTIFIER_RANGE_NUMA?

Then, in mm side, we could do the abort and update the range to contain only successful
subrange .invalidate_range_end().

Is that acceptable?

> scheme could work?
>