[RFC PATCH v2 28/51] mm: Introduce guestmem_hugetlb to support folio_put() handling of guestmem pages

[Ackerley Tng <ackerleytng@google.com>]

The PGTY_guestmem_hugetlb is introduced so folios can be marked for
further cleanup by guestmem_hugetlb.

guestmem_hugetlb folios can have positive mapcounts, which will
conflict with the installation of a page type. Hence,
PGTY_guestmem_hugetlb will only be installed when a folio is
truncated, after the folio has been unmapped and has a mapcount of 0.

Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>

Change-Id: I635f8929e06f73d7899737bd47090b7cbc7222dc
---
 include/linux/page-flags.h | 17 +++++++++++++++++
 mm/Kconfig                 | 10 ++++++++++
 mm/Makefile                |  1 +
 mm/debug.c                 |  1 +
 mm/guestmem_hugetlb.c      | 14 ++++++++++++++
 mm/guestmem_hugetlb.h      |  9 +++++++++
 mm/swap.c                  |  9 +++++++++
 7 files changed, 61 insertions(+)
 create mode 100644 mm/guestmem_hugetlb.c
 create mode 100644 mm/guestmem_hugetlb.h

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 9dd60fb8c33f..543f6481ca60 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -965,6 +965,7 @@ enum pagetype {
 	PGTY_zsmalloc		= 0xf6,
 	PGTY_unaccepted		= 0xf7,
 	PGTY_large_kmalloc	= 0xf8,
+	PGTY_guestmem_hugetlb	= 0xf9,
 
 	PGTY_mapcount_underflow = 0xff
 };
@@ -1114,6 +1115,22 @@ FOLIO_TYPE_OPS(hugetlb, hugetlb)
 FOLIO_TEST_FLAG_FALSE(hugetlb)
 #endif
 
+/*
+ * PGTY_guestmem_hugetlb, for now, is used to mark a folio as requiring further
+ * cleanup by the guestmem_hugetlb allocator.  This page type is installed only
+ * at truncation time, by guest_memfd, if further cleanup is required.  It is
+ * safe to install this page type at truncation time because by then mapcount
+ * would be 0.
+ *
+ * The plan is to always set this page type for any folios allocated by
+ * guestmem_hugetlb once typed folios can be mapped to userspace cleanly.
+ */
+#ifdef CONFIG_GUESTMEM_HUGETLB
+FOLIO_TYPE_OPS(guestmem_hugetlb, guestmem_hugetlb)
+#else
+FOLIO_TEST_FLAG_FALSE(guestmem_hugetlb)
+#endif
+
 PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
 
 /*
diff --git a/mm/Kconfig b/mm/Kconfig
index e113f713b493..131adc49f58d 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1216,6 +1216,16 @@ config SECRETMEM
 	  memory areas visible only in the context of the owning process and
 	  not mapped to other processes and other kernel page tables.
 
+config GUESTMEM_HUGETLB
+	bool "Enable guestmem_hugetlb allocator for guest_memfd"
+	depends on HUGETLBFS
+	help
+	  Enable this to make HugeTLB folios available to guest_memfd
+	  (KVM virtualization) as backing memory.
+
+	  This feature wraps HugeTLB as a custom allocator that
+	  guest_memfd can use.
+
 config ANON_VMA_NAME
 	bool "Anonymous VMA name support"
 	depends on PROC_FS && ADVISE_SYSCALLS && MMU
diff --git a/mm/Makefile b/mm/Makefile
index e7f6bbf8ae5f..c91c8e8fef71 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -127,6 +127,7 @@ obj-$(CONFIG_PAGE_EXTENSION) += page_ext.o
 obj-$(CONFIG_PAGE_TABLE_CHECK) += page_table_check.o
 obj-$(CONFIG_CMA_DEBUGFS) += cma_debug.o
 obj-$(CONFIG_SECRETMEM) += secretmem.o
+obj-$(CONFIG_GUESTMEM_HUGETLB) += guestmem_hugetlb.o
 obj-$(CONFIG_CMA_SYSFS) += cma_sysfs.o
 obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
 obj-$(CONFIG_IDLE_PAGE_TRACKING) += page_idle.o
diff --git a/mm/debug.c b/mm/debug.c
index db83e381a8ae..439ab128772d 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_hugetlb),
 };
 
 static const char *page_type_name(unsigned int page_type)
diff --git a/mm/guestmem_hugetlb.c b/mm/guestmem_hugetlb.c
new file mode 100644
index 000000000000..51a724ebcc50
--- /dev/null
+++ b/mm/guestmem_hugetlb.c
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * guestmem_hugetlb is an allocator for guest_memfd. guest_memfd wraps HugeTLB
+ * as an allocator for guest_memfd.
+ */
+
+#include <linux/mm_types.h>
+
+#include "guestmem_hugetlb.h"
+
+void guestmem_hugetlb_handle_folio_put(struct folio *folio)
+{
+	WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
+}
diff --git a/mm/guestmem_hugetlb.h b/mm/guestmem_hugetlb.h
new file mode 100644
index 000000000000..5c9452b77252
--- /dev/null
+++ b/mm/guestmem_hugetlb.h
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_MM_GUESTMEM_HUGETLB_H
+#define _LINUX_MM_GUESTMEM_HUGETLB_H
+
+#include <linux/mm_types.h>
+
+void guestmem_hugetlb_handle_folio_put(struct folio *folio);
+
+#endif
diff --git a/mm/swap.c b/mm/swap.c
index d0a5971787c4..2747230ced89 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -40,6 +40,10 @@
 
 #include "internal.h"
 
+#ifdef CONFIG_GUESTMEM_HUGETLB
+#include "guestmem_hugetlb.h"
+#endif
+
 #define CREATE_TRACE_POINTS
 #include <trace/events/pagemap.h>
 
@@ -101,6 +105,11 @@ static void free_typed_folio(struct folio *folio)
 	case PGTY_hugetlb:
 		free_huge_folio(folio);
 		return;
+#endif
+#ifdef CONFIG_GUESTMEM_HUGETLB
+	case PGTY_guestmem_hugetlb:
+		guestmem_hugetlb_handle_folio_put(folio);
+		return;
 #endif
 	default:
 		WARN_ON_ONCE(1);
-- 
2.49.0.1045.g170613ef41-goog