[PATCH v5 02/13] mm: Introduce memfile_notifier

[Chao Peng <chao.p.peng@linux.intel.com>]

This patch introduces memfile_notifier facility so existing memory file
subsystems (e.g. tmpfs/hugetlbfs) can provide memory pages to allow a
third kernel component to make use of memory bookmarked in the memory
file and gets notified when the pages in the memory file become
allocated/invalidated.

It will be used for KVM to use a file descriptor as the guest memory
backing store and KVM will use this memfile_notifier interface to
interact with memory file subsystems. In the future there might be other
consumers (e.g. VFIO with encrypted device memory).

It consists two sets of callbacks:
  - memfile_notifier_ops: callbacks for memory backing store to notify
    KVM when memory gets allocated/invalidated.
  - memfile_pfn_ops: callbacks for KVM to call into memory backing store
    to request memory pages for guest private memory.

Userspace is in charge of guest memory lifecycle: it first allocates
pages in memory backing store and then passes the fd to KVM and lets KVM
register each memory slot to memory backing store via
memfile_register_notifier.

The supported memory backing store should maintain a memfile_notifier list
and provide routine for memfile_notifier to get the list head address and
memfile_pfn_ops callbacks for memfile_register_notifier. It also should call
memfile_notifier_fallocate/memfile_notifier_invalidate when the bookmarked
memory gets allocated/invalidated.

Co-developed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
---
 include/linux/memfile_notifier.h |  64 +++++++++++++++++
 mm/Kconfig                       |   4 ++
 mm/Makefile                      |   1 +
 mm/memfile_notifier.c            | 114 +++++++++++++++++++++++++++++++
 4 files changed, 183 insertions(+)
 create mode 100644 include/linux/memfile_notifier.h
 create mode 100644 mm/memfile_notifier.c

diff --git a/include/linux/memfile_notifier.h b/include/linux/memfile_notifier.h
new file mode 100644
index 000000000000..e8d400558adb
--- /dev/null
+++ b/include/linux/memfile_notifier.h
@@ -0,0 +1,64 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_MEMFILE_NOTIFIER_H
+#define _LINUX_MEMFILE_NOTIFIER_H
+
+#include <linux/rculist.h>
+#include <linux/spinlock.h>
+#include <linux/srcu.h>
+#include <linux/fs.h>
+
+struct memfile_notifier;
+
+struct memfile_notifier_ops {
+	void (*invalidate)(struct memfile_notifier *notifier,
+			   pgoff_t start, pgoff_t end);
+	void (*fallocate)(struct memfile_notifier *notifier,
+			  pgoff_t start, pgoff_t end);
+};
+
+struct memfile_pfn_ops {
+	long (*get_lock_pfn)(struct inode *inode, pgoff_t offset, int *order);
+	void (*put_unlock_pfn)(unsigned long pfn);
+};
+
+struct memfile_notifier {
+	struct list_head list;
+	struct memfile_notifier_ops *ops;
+};
+
+struct memfile_notifier_list {
+	struct list_head head;
+	spinlock_t lock;
+};
+
+struct memfile_backing_store {
+	struct list_head list;
+	struct memfile_pfn_ops pfn_ops;
+	struct memfile_notifier_list* (*get_notifier_list)(struct inode *inode);
+};
+
+#ifdef CONFIG_MEMFILE_NOTIFIER
+/* APIs for backing stores */
+static inline void memfile_notifier_list_init(struct memfile_notifier_list *list)
+{
+	INIT_LIST_HEAD(&list->head);
+	spin_lock_init(&list->lock);
+}
+
+extern void memfile_notifier_invalidate(struct memfile_notifier_list *list,
+					pgoff_t start, pgoff_t end);
+extern void memfile_notifier_fallocate(struct memfile_notifier_list *list,
+				       pgoff_t start, pgoff_t end);
+extern void memfile_register_backing_store(struct memfile_backing_store *bs);
+extern void memfile_unregister_backing_store(struct memfile_backing_store *bs);
+
+/*APIs for notifier consumers */
+extern int memfile_register_notifier(struct inode *inode,
+				     struct memfile_notifier *notifier,
+				     struct memfile_pfn_ops **pfn_ops);
+extern void memfile_unregister_notifier(struct inode *inode,
+					struct memfile_notifier *notifier);
+
+#endif /* CONFIG_MEMFILE_NOTIFIER */
+
+#endif /* _LINUX_MEMFILE_NOTIFIER_H */
diff --git a/mm/Kconfig b/mm/Kconfig
index 3326ee3903f3..7c6b1ad3dade 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -892,6 +892,10 @@ config ANON_VMA_NAME
 	  area from being merged with adjacent virtual memory areas due to the
 	  difference in their name.
 
+config MEMFILE_NOTIFIER
+	bool
+	select SRCU
+
 source "mm/damon/Kconfig"
 
 endmenu
diff --git a/mm/Makefile b/mm/Makefile
index 70d4309c9ce3..f628256dce0d 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -132,3 +132,4 @@ obj-$(CONFIG_PAGE_REPORTING) += page_reporting.o
 obj-$(CONFIG_IO_MAPPING) += io-mapping.o
 obj-$(CONFIG_HAVE_BOOTMEM_INFO_NODE) += bootmem_info.o
 obj-$(CONFIG_GENERIC_IOREMAP) += ioremap.o
+obj-$(CONFIG_MEMFILE_NOTIFIER) += memfile_notifier.o
diff --git a/mm/memfile_notifier.c b/mm/memfile_notifier.c
new file mode 100644
index 000000000000..a405db56fde2
--- /dev/null
+++ b/mm/memfile_notifier.c
@@ -0,0 +1,114 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  linux/mm/memfile_notifier.c
+ *
+ *  Copyright (C) 2022  Intel Corporation.
+ *             Chao Peng <chao.p.peng@linux.intel.com>
+ */
+
+#include <linux/memfile_notifier.h>
+#include <linux/srcu.h>
+
+DEFINE_STATIC_SRCU(srcu);
+static LIST_HEAD(backing_store_list);
+
+void memfile_notifier_invalidate(struct memfile_notifier_list *list,
+				 pgoff_t start, pgoff_t end)
+{
+	struct memfile_notifier *notifier;
+	int id;
+
+	id = srcu_read_lock(&srcu);
+	list_for_each_entry_srcu(notifier, &list->head, list,
+				 srcu_read_lock_held(&srcu)) {
+		if (notifier->ops && notifier->ops->invalidate)
+			notifier->ops->invalidate(notifier, start, end);
+	}
+	srcu_read_unlock(&srcu, id);
+}
+
+void memfile_notifier_fallocate(struct memfile_notifier_list *list,
+				pgoff_t start, pgoff_t end)
+{
+	struct memfile_notifier *notifier;
+	int id;
+
+	id = srcu_read_lock(&srcu);
+	list_for_each_entry_srcu(notifier, &list->head, list,
+				 srcu_read_lock_held(&srcu)) {
+		if (notifier->ops && notifier->ops->fallocate)
+			notifier->ops->fallocate(notifier, start, end);
+	}
+	srcu_read_unlock(&srcu, id);
+}
+
+void memfile_register_backing_store(struct memfile_backing_store *bs)
+{
+	BUG_ON(!bs || !bs->get_notifier_list);
+
+	list_add_tail(&bs->list, &backing_store_list);
+}
+
+void memfile_unregister_backing_store(struct memfile_backing_store *bs)
+{
+	list_del(&bs->list);
+}
+
+static int memfile_get_notifier_info(struct inode *inode,
+				     struct memfile_notifier_list **list,
+				     struct memfile_pfn_ops **ops)
+{
+	struct memfile_backing_store *bs, *iter;
+	struct memfile_notifier_list *tmp;
+
+	list_for_each_entry_safe(bs, iter, &backing_store_list, list) {
+		tmp = bs->get_notifier_list(inode);
+		if (tmp) {
+			*list = tmp;
+			if (ops)
+				*ops = &bs->pfn_ops;
+			return 0;
+		}
+	}
+	return -EOPNOTSUPP;
+}
+
+int memfile_register_notifier(struct inode *inode,
+			      struct memfile_notifier *notifier,
+			      struct memfile_pfn_ops **pfn_ops)
+{
+	struct memfile_notifier_list *list;
+	int ret;
+
+	if (!inode || !notifier | !pfn_ops)
+		return -EINVAL;
+
+	ret = memfile_get_notifier_info(inode, &list, pfn_ops);
+	if (ret)
+		return ret;
+
+	spin_lock(&list->lock);
+	list_add_rcu(&notifier->list, &list->head);
+	spin_unlock(&list->lock);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(memfile_register_notifier);
+
+void memfile_unregister_notifier(struct inode *inode,
+				 struct memfile_notifier *notifier)
+{
+	struct memfile_notifier_list *list;
+
+	if (!inode || !notifier)
+		return;
+
+	BUG_ON(memfile_get_notifier_info(inode, &list, NULL));
+
+	spin_lock(&list->lock);
+	list_del_rcu(&notifier->list);
+	spin_unlock(&list->lock);
+
+	synchronize_srcu(&srcu);
+}
+EXPORT_SYMBOL_GPL(memfile_unregister_notifier);
-- 
2.17.1