linux-kernel - Re: [PATCH 1/2] KVM: MMU: Do not treat ZONE

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Date:   Thu, 7 Nov 2019 12:12:03 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, KVM list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Adam Borowski <kilobyte@...band.pl>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 1/2] KVM: MMU: Do not treat ZONE_DEVICE pages as being
 reserved

On 07/11/19 06:48, Dan Williams wrote:
>> How do mmu notifiers get held off by page references and does that
>> machinery work with ZONE_DEVICE? Why is this not a concern for the
>> VM_IO and VM_PFNMAP case?
> Put another way, I see no protection against truncate/invalidate
> afforded by a page pin. If you need guarantees that the page remains
> valid in the VMA until KVM can install a mmu notifier that needs to
> happen under the mmap_sem as far as I can see. Otherwise gup just
> weakly asserts "this pinned page was valid in this vma at one point in
> time".

The MMU notifier is installed before gup, so any invalidation will be
preceded by a call to the MMU notifier.  In turn,
invalidate_range_start/end is called with mmap_sem held so there should
be no race.

However, as Sean mentioned, early put_page of ZONE_DEVICE pages would be
racy, because we need to keep the reference between the gup and the last
time we use the corresponding struct page.

Based on this, I think Sean's patches should work fine, and I prefer
them over David's approach.  Either way, adding some documentation is in
order.

Paolo