[RFC PATCH v2 02/11] KVM: guest_memfd: Handle final folio_put() of guest_memfd pages

[Fuad Tabba <tabba@google.com>]

Before transitioning a guest_memfd folio to unshared, thereby
disallowing access by the host and allowing the hypervisor to
transition its view of the guest page as private, we need to be
sure that the host doesn't have any references to the folio.

This patch introduces a new type for guest_memfd folios, which
isn't activated in this series but is here as a placeholder and
to facilitate the code in the next patch. This will be used in
the future to register a callback that informs the guest_memfd
subsystem when the last reference is dropped, therefore knowing
that the host doesn't have any remaining references.

Signed-off-by: Fuad Tabba <tabba@google.com>
---
 include/linux/page-flags.h | 7 +++++++
 mm/debug.c                 | 1 +
 mm/swap.c                  | 5 +++++
 3 files changed, 13 insertions(+)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 6615f2f59144..bab3cac1f93b 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -942,6 +942,7 @@ enum pagetype {
 	PGTY_slab	= 0xf5,
 	PGTY_zsmalloc	= 0xf6,
 	PGTY_unaccepted	= 0xf7,
+	PGTY_guestmem	= 0xf8,
 
 	PGTY_mapcount_underflow = 0xff
 };
@@ -1091,6 +1092,12 @@ FOLIO_TYPE_OPS(hugetlb, hugetlb)
 FOLIO_TEST_FLAG_FALSE(hugetlb)
 #endif
 
+#ifdef CONFIG_KVM_GMEM_MAPPABLE
+FOLIO_TYPE_OPS(guestmem, guestmem)
+#else
+FOLIO_TEST_FLAG_FALSE(guestmem)
+#endif
+
 PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
 
 /*
diff --git a/mm/debug.c b/mm/debug.c
index 95b6ab809c0e..db93be385ed9 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -56,6 +56,7 @@ static const char *page_type_names[] = {
 	DEF_PAGETYPE_NAME(table),
 	DEF_PAGETYPE_NAME(buddy),
 	DEF_PAGETYPE_NAME(unaccepted),
+	DEF_PAGETYPE_NAME(guestmem),
 };
 
 static const char *page_type_name(unsigned int page_type)
diff --git a/mm/swap.c b/mm/swap.c
index 8a66cd9cb9da..73d61c7f8edd 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -37,6 +37,7 @@
 #include <linux/page_idle.h>
 #include <linux/local_lock.h>
 #include <linux/buffer_head.h>
+#include <linux/kvm_host.h>
 
 #include "internal.h"
 
@@ -101,6 +102,10 @@ static void free_typed_folio(struct folio *folio)
 		if (IS_ENABLED(CONFIG_HUGETLBFS))
 			free_huge_folio(folio);
 		return;
+	case PGTY_guestmem:
+		if (IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM))
+			WARN_ONCE(1, "A placeholder that shouldn't trigger.");
+		return;
 	default:
 		WARN_ON_ONCE(1);
 	}
-- 
2.48.1.262.g85cc9f2d1e-goog