KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved

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>
Analyzed-by: David Hildenbrand <david@redhat.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Cc: stable@vger.kernel.org
Fixes: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 24c23c6..bf82b1f 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3306,7 +3306,7 @@ static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
         * here.
         */
        if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn) &&
-           level == PT_PAGE_TABLE_LEVEL &&
+           !kvm_is_zone_device_pfn(pfn) && level == PT_PAGE_TABLE_LEVEL &&
            PageTransCompoundMap(pfn_to_page(pfn)) &&
            !mmu_gfn_lpage_is_disallowed(vcpu, gfn, PT_DIRECTORY_LEVEL)) {
                unsigned long mask;
@@ -5914,9 +5914,9 @@ restart:
                 * the guest, and the guest page table is using 4K page size
                 * mapping if the indirect sp has level = 1.
                 */
-               if (sp->role.direct &&
-                       !kvm_is_reserved_pfn(pfn) &&
-                       PageTransCompoundMap(pfn_to_page(pfn))) {
+               if (sp->role.direct && !kvm_is_reserved_pfn(pfn) &&
+                   !kvm_is_zone_device_pfn(pfn) &&
+                   PageTransCompoundMap(pfn_to_page(pfn))) {
                        pte_list_remove(rmap_head, sptep);
 
                        if (kvm_available_flush_tlb_with_range())

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 719fc3e..290dbe3 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -966,6 +966,7 @@ int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu);
 void kvm_vcpu_kick(struct kvm_vcpu *vcpu);
 
 bool kvm_is_reserved_pfn(kvm_pfn_t pfn);
+bool kvm_is_zone_device_pfn(kvm_pfn_t pfn);
 
 struct kvm_irq_ack_notifier {
        struct hlist_node link;

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e7a0713..0dac149 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -149,10 +149,30 @@ __weak int kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
        return 0;
 }
 
+bool kvm_is_zone_device_pfn(kvm_pfn_t pfn)
+{
+       /*
+        * The metadata used by is_zone_device_page() to determine whether or
+        * not a page is ZONE_DEVICE is guaranteed to be valid if and only if
+        * the device has been pinned, e.g. by get_user_pages().  WARN if the
+        * page_count() is zero to help detect bad usage of this helper.
+        */
+       if (!pfn_valid(pfn) || WARN_ON_ONCE(!page_count(pfn_to_page(pfn))))
+               return false;
+
+       return is_zone_device_page(pfn_to_page(pfn));
+}
+
 bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
 {
+       /*
+        * ZONE_DEVICE pages currently set PG_reserved, but from a refcounting
+        * perspective they are "normal" pages, albeit with slightly different
+        * usage rules.
+        */
        if (pfn_valid(pfn))
-               return PageReserved(pfn_to_page(pfn));
+               return PageReserved(pfn_to_page(pfn)) &&
+                      !kvm_is_zone_device_pfn(pfn);
 
        return true;
 }
@@ -1857,7 +1877,7 @@ EXPORT_SYMBOL_GPL(kvm_release_pfn_dirty);
 
 void kvm_set_pfn_dirty(kvm_pfn_t pfn)
 {
-       if (!kvm_is_reserved_pfn(pfn)) {
+       if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn)) {
                struct page *page = pfn_to_page(pfn);
 
                SetPageDirty(page);
@@ -1867,7 +1887,7 @@ EXPORT_SYMBOL_GPL(kvm_set_pfn_dirty);
 
 void kvm_set_pfn_accessed(kvm_pfn_t pfn)
 {
-       if (!kvm_is_reserved_pfn(pfn))
+       if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn))
                mark_page_accessed(pfn_to_page(pfn));
 }
 EXPORT_SYMBOL_GPL(kvm_set_pfn_accessed);