KVM: guest_memfd: Stop returning struct page from PFN lookup

KVM currently expects guest_memfd PFN lookups to return a refcounted struct
page, which callers hold across fault handling.

Drop the page's refcount before returning from kvm_gmem_get_pfn() to
prepare for the in-place conversions series.

CoCo shared-to-private conversion handling must inspect folio refcounts to
ensure exclusive ownership by guest_memfd. A concurrent guest page fault
taking a temporary reference on the folio causes conversions to fail due to
an elevated refcount.

While this refcount is also taken on host userspace page faults, that
refcount is taken on behalf of the host userspace page tables. This
refcount will be dropped when conversions unmaps the page. Either way, once
there's an mmap() or userspace mapping, the pages are open to way more
refcounts, transient or not. This patch focuses on just dropping refcounts
before handing KVM a page.

guest_memfd already notifies KVM of page invalidations, so callers within
KVM only need to respect the MMU invalidation protocol to safely rely on
guest_memfd for page presence.

Since the page refcounts are dropped, don't return the struct page pointer.

Not returning the struct page from the guest_memfd PFN lookup moves KVM
closer toward supporting memory backends that are not backed by struct
page.

Here are some notes on the cleanup in the callers of kvm_gmem_get_pfn():

kvm_release_faultin_page() in ARM's gmem_abort() originally also serves to
set the page dirty and accessed under some conditions. The dirty and
accessed flags don't matter for guest_memfd anyway, so it is safe to just
drop the call to kvm_release_faultin_page().

For ARM's kvm_translate_vncr(), the local page pointer must be initialized
to NULL so that the shared cleanup path that releases faulted-in pages
safely no-ops for guest_memfd.

For x86, no additional changes are required in the MMU fault path because
the page fault tracking structure is zero-initialized at the start of page
fault handling, ensuring the refcounted page pointer is already NULL.

Reported-by: Yan Zhao <yan.y.zhao@intel.com>
Closes: https://lore.kernel.org/all/anZ4W9o5pTWIEgMY@yzhao56-desk.sh.intel.com/
Co-developed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Michael Roth <michael.roth@amd.com>
Tested-by: Michael Roth <michael.roth@amd.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Tested-by: Fuad Tabba <fuad.tabba@linux.dev>
Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Link: https://patch.msgid.link/20260826-gmem-no-return-page-v4-5-3bb9c1ddb4e3@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions