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

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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>
This commit is contained in:
Sean Christopherson 2024-10-10 11:24:27 -07:00 committed by Paolo Bonzini
parent 93b7da404f
commit 8b15c3764c
2 changed files with 2 additions and 76 deletions
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3
include/linux/kvm_host.h
View File

@ -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;

75
virt/kvm/kvm_main.c
View File

@ -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;