[PATCH RFC v3 1/2] KVM: guest_memfd: Convert .free_folio() to .release_folio()

[Elliot Berman <quic_eberman@quicinc.com>]

When guest_memfd becomes a library, a callback will need to be made to
the owner (KVM SEV) to transition pages back to hypervisor-owned/shared
state. This is currently being done as part of .free_folio() address
space op, but this callback shouldn't assume that the mapping still
exists. guest_memfd library will need the mapping to still exist to look
up its operations table.

.release_folio() and .invalidate_folio() address space ops can serve the
same purpose here. The key difference between release_folio() and
free_folio() is whether the mapping is still valid at time of the
callback. This approach was discussed in the link in the footer, but not
taken because free_folio() was easier to implement.

Link: https://lore.kernel.org/kvm/20231016115028.996656-1-michael.roth@amd.com/
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
---
 virt/kvm/guest_memfd.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 47a9f68f7b247f4cba0c958b4c7cd9458e7c46b4..13f83ad8a4c26ba82aca4f2684f22044abb4bc19 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -358,22 +358,35 @@ static int kvm_gmem_error_folio(struct address_space *mapping, struct folio *fol
 }
 
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE
-static void kvm_gmem_free_folio(struct folio *folio)
+static bool kvm_gmem_release_folio(struct folio *folio, gfp_t gfp)
 {
 	struct page *page = folio_page(folio, 0);
 	kvm_pfn_t pfn = page_to_pfn(page);
 	int order = folio_order(folio);
 
 	kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order));
+
+	return true;
+}
+
+static void kvm_gmem_invalidate_folio(struct folio *folio, size_t offset,
+				      size_t len)
+{
+	WARN_ON_ONCE(offset != 0);
+	WARN_ON_ONCE(len != folio_size(folio));
+
+	if (offset == 0 && len == folio_size(folio))
+		filemap_release_folio(folio, 0);
 }
 #endif
 
 static const struct address_space_operations kvm_gmem_aops = {
 	.dirty_folio = noop_dirty_folio,
-	.migrate_folio	= kvm_gmem_migrate_folio,
+	.migrate_folio = kvm_gmem_migrate_folio,
 	.error_remove_folio = kvm_gmem_error_folio,
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE
-	.free_folio = kvm_gmem_free_folio,
+	.release_folio = kvm_gmem_release_folio,
+	.invalidate_folio = kvm_gmem_invalidate_folio,
 #endif
 };
 

-- 
2.34.1