KVM: Don't grab reference on VM_MIXEDMAP pfns that have a "struct page"

Now that KVM no longer relies on an ugly heuristic to find its struct page
references, i.e. now that KVM can't get false positives on VM_MIXEDMAP
pfns, remove KVM's hack to elevate the refcount for pfns that happen to
have a valid struct page.  In addition to removing a long-standing wart
in KVM, this allows KVM to map non-refcounted struct page memory into the
guest, e.g. for exposing GPU TTM buffers to KVM guests.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-86-seanjc@google.com>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d045f8310a481e27c27ddaa710850448235c08f1..02f0206fd2dc11389580fd5eb28c0c64c329eee7 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1730,9 +1730,6 @@ void kvm_arch_sync_events(struct kvm *kvm);
 
 int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu);
 
-struct page *kvm_pfn_to_refcounted_page(kvm_pfn_t pfn);
-bool kvm_is_zone_device_page(struct page *page);
-
 struct kvm_irq_ack_notifier {
        struct hlist_node link;
        unsigned gsi;

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 396ca14f18f39e692cbb2fb600d3de0b241ac93a..b1b10dc408a0968fb1d6c339a4eb7dd92197f4c2 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -160,52 +160,6 @@ __weak void kvm_arch_guest_memory_reclaimed(struct kvm *kvm)
 {
 }
 
-bool kvm_is_zone_device_page(struct page *page)
-{
-       /*
-        * 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 (WARN_ON_ONCE(!page_count(page)))
-               return false;
-
-       return is_zone_device_page(page);
-}
-
-/*
- * Returns a 'struct page' if the pfn is "valid" and backed by a refcounted
- * page, NULL otherwise.  Note, the list of refcounted PG_reserved page types
- * is likely incomplete, it has been compiled purely through people wanting to
- * back guest with a certain type of memory and encountering issues.
- */
-struct page *kvm_pfn_to_refcounted_page(kvm_pfn_t pfn)
-{
-       struct page *page;
-
-       if (!pfn_valid(pfn))
-               return NULL;
-
-       page = pfn_to_page(pfn);
-       if (!PageReserved(page))
-               return page;
-
-       /* The ZERO_PAGE(s) is marked PG_reserved, but is refcounted. */
-       if (is_zero_pfn(pfn))
-               return page;
-
-       /*
-        * ZONE_DEVICE pages currently set PG_reserved, but from a refcounting
-        * perspective they are "normal" pages, albeit with slightly different
-        * usage rules.
-        */
-       if (kvm_is_zone_device_page(page))
-               return page;
-
-       return NULL;
-}
-
 /*
  * Switches to specified vcpu, until a matching vcpu_put()
  */
@@ -2804,35 +2758,10 @@ static kvm_pfn_t kvm_resolve_pfn(struct kvm_follow_pfn *kfp, struct page *page,
        if (kfp->map_writable)
                *kfp->map_writable = writable;
 
-       /*
-        * FIXME: Remove this once KVM no longer blindly calls put_page() on
-        *        every pfn that points at a struct page.
-        *
-        * Get a reference for follow_pte() pfns if they happen to point at a
-        * struct page, as KVM will ultimately call kvm_release_pfn_clean() on
-        * the returned pfn, i.e. KVM expects to have a reference.
-        *
-        * Certain IO or PFNMAP mappings can be backed with valid struct pages,
-        * but be allocated without refcounting, e.g. tail pages of
-        * non-compound higher order allocations.  Grabbing and putting a
-        * reference to such pages would cause KVM to prematurely free a page
-        * it doesn't own (KVM gets and puts the one and only reference).
-        * Don't allow those pages until the FIXME is resolved.
-        *
-        * Don't grab a reference for pins, callers that pin pages are required
-        * to check refcounted_page, i.e. must not blindly release the pfn.
-        */
-       if (map) {
+       if (map)
                pfn = map->pfn;
-
-               if (!kfp->pin) {
-                       page = kvm_pfn_to_refcounted_page(pfn);
-                       if (page && !get_page_unless_zero(page))
-                               return KVM_PFN_ERR_FAULT;
-               }
-       } else {
+       else
                pfn = page_to_pfn(page);
-       }
 
        *kfp->refcounted_page = page;