Messages in this thread | | | From | Dan Williams <> | Date | Mon, 11 Nov 2019 14:39:04 -0800 | Subject | Re: [PATCH v2 1/3] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved |
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On Mon, Nov 11, 2019 at 2:12 PM Sean Christopherson <sean.j.christopherson@intel.com> wrote: > > Explicitly exempt ZONE_DEVICE pages from kvm_is_reserved_pfn() and > instead manually handle ZONE_DEVICE on a case-by-case basis. For things > like page refcounts, KVM needs to treat ZONE_DEVICE pages like normal > pages, e.g. put pages grabbed via gup(). But for flows such as setting > A/D bits or shifting refcounts for transparent huge pages, KVM needs to > to avoid processing ZONE_DEVICE pages as the flows in question lack the > underlying machinery for proper handling of ZONE_DEVICE pages. > > This fixes a hang reported by Adam Borowski[*] in dev_pagemap_cleanup() > when running a KVM guest backed with /dev/dax memory, as KVM straight up > doesn't put any references to ZONE_DEVICE pages acquired by gup(). > > Note, Dan Williams proposed an alternative solution of doing put_page() > on ZONE_DEVICE pages immediately after gup() in order to simplify the > auditing needed to ensure is_zone_device_page() is called if and only if > the backing device is pinned (via gup()). But that approach would break > kvm_vcpu_{un}map() as KVM requires the page to be pinned from map() 'til > unmap() when accessing guest memory, unlike KVM's secondary MMU, which > coordinates with mmu_notifier invalidations to avoid creating stale > page references, i.e. doesn't rely on pages being pinned. > > [*] http://lkml.kernel.org/r/20190919115547.GA17963@angband.pl > > Reported-by: Adam Borowski <kilobyte@angband.pl> > Debugged-by: David Hildenbrand <david@redhat.com> > Cc: Dan Williams <dan.j.williams@intel.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
> Cc: stable@vger.kernel.org
Perhaps add:
Fixes: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
...since that was the first kernel that broke KVM's assumption about which pfn types needed to have the reference count managed.
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