[PATCH v13 85/85] KVM: Don't grab reference on VM_MIXEDMAP pfns that have a "struct page"

Sean Christopherson
2024-10-10T18:27:53Z

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>
---
include/linux/kvm_host.h | 3 --
virt/kvm/kvm_main.c | 75 ++--------------------------------------
2 files changed, 2 insertions(+), 76 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d045f8310a48..02f0206fd2dc 100644
--- 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 396ca14f18f3..b1b10dc408a0 100644
--- 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;

--
2.47.0.rc1.288.g06298d1525-goog