Re: [PATCH 1/2] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved

From: Paolo Bonzini <pbonzini@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Jim Mattson" <jmattson@google.com>,
	"Joerg Roedel" <joro@8bytes.org>,
	"KVM list" <kvm@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Adam Borowski" <kilobyte@angband.pl>,
	"David Hildenbrand" <david@redhat.com>
Subject: Re: [PATCH 1/2] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved
Date: Thu, 7 Nov 2019 12:12:03 +0100	[thread overview]
Message-ID: <0db7c328-1543-55db-bc02-c589deb3db22@redhat.com> (raw)
In-Reply-To: <CAPcyv4i3M18V9Gmx3x7Ad12VjXbq94NsaUG9o71j59mG9-6H9Q@mail.gmail.com>

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