[PATCH v2 01/13] libmultipath: Add initial framework

[John Garry]

Add initial framework for libmultipath. libmultipath is a library for
multipath-capable block drivers, such as NVMe. The main function is to
support path management, path selection, and failover handling.

Basic support to add and remove the head structure - mpath_head - is
included.

This main purpose of this structure is to manage available paths and path
selection. It is quite similar to the multipath functionality in
nvme_ns_head. It also manages the multipath gendisk.

Each path is represented by the mpath_device structure. It should hold a
pointer to the per-path gendisk and also a list element for all siblings
of paths. For NVMe, there would be a mpath_device per nvme_ns.

All the libmultipath code is more or less taken from
drivers/nvme/host/multipath.c, which was originally authored by Christoph
Hellwig <hch at lst.de>.

Signed-off-by: John Garry <john.g.garry at oracle.com>
---
 include/linux/multipath.h | 28 +++++++++++++++
 lib/Kconfig               |  6 ++++
 lib/Makefile              |  2 ++
 lib/multipath.c           | 74 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 110 insertions(+)
 create mode 100644 include/linux/multipath.h
 create mode 100644 lib/multipath.c

diff --git a/include/linux/multipath.h b/include/linux/multipath.h
new file mode 100644
index 0000000000000..102dfcf21ffc9
--- /dev/null
+++ b/include/linux/multipath.h
@@ -0,0 +1,28 @@
+
+#ifndef _LIBMULTIPATH_H
+#define _LIBMULTIPATH_H
+
+#include <linux/blkdev.h>
+#include <linux/srcu.h>
+
+struct mpath_device {
+	struct list_head	siblings;
+	struct gendisk		*disk;
+};
+
+struct mpath_head {
+	struct srcu_struct	srcu;
+	struct list_head	dev_list;	/* list of all mpath_devs */
+	struct mutex		lock;
+
+	struct kref		ref;
+
+	void			*drvdata;
+	struct mpath_device __rcu		*current_path[];
+};
+
+int mpath_get_head(struct mpath_head *mpath_head);
+void mpath_put_head(struct mpath_head *mpath_head);
+struct mpath_head *mpath_alloc_head(void);
+
+#endif // _LIBMULTIPATH_H
diff --git a/lib/Kconfig b/lib/Kconfig
index 0f2fb96106476..5c70e6df89740 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -636,3 +636,9 @@ config UNION_FIND
 
 config MIN_HEAP
 	bool
+
+config LIBMULTIPATH
+	bool "MULTIPATH BLOCK DRIVER LIBRARY"
+	depends on BLOCK
+	help
+	  If you say yes here then you get a multipath lib for block drivers
diff --git a/lib/Makefile b/lib/Makefile
index 1b9ee167517f3..98948b18af7f7 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -335,3 +335,5 @@ CONTEXT_ANALYSIS_test_context-analysis.o := y
 obj-$(CONFIG_CONTEXT_ANALYSIS_TEST) += test_context-analysis.o
 
 subdir-$(CONFIG_FORTIFY_SOURCE) += test_fortify
+
+obj-$(CONFIG_LIBMULTIPATH)	+= multipath.o
diff --git a/lib/multipath.c b/lib/multipath.c
new file mode 100644
index 0000000000000..0d5690b5ba4ca
--- /dev/null
+++ b/lib/multipath.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2017-2018 Christoph Hellwig.
+ * Copyright (c) 2026 Oracle and/or its affiliates.
+ */
+#include <linux/module.h>
+#include <linux/multipath.h>
+
+static struct workqueue_struct *mpath_wq;
+
+static void mpath_free_head(struct kref *ref)
+{
+	struct mpath_head *mpath_head =
+		container_of(ref, struct mpath_head, ref);
+
+	cleanup_srcu_struct(&mpath_head->srcu);
+	kfree(mpath_head);
+}
+
+int mpath_get_head(struct mpath_head *mpath_head)
+{
+	if (!kref_get_unless_zero(&mpath_head->ref))
+		return -ENXIO;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mpath_get_head);
+
+void mpath_put_head(struct mpath_head *mpath_head)
+{
+	kref_put(&mpath_head->ref, mpath_free_head);
+}
+EXPORT_SYMBOL_GPL(mpath_put_head);
+
+struct mpath_head *mpath_alloc_head(void)
+{
+	struct mpath_head *mpath_head;
+	int ret;
+
+	mpath_head = kzalloc(struct_size(mpath_head, current_path,
+				num_possible_nodes()), GFP_KERNEL);
+	if (!mpath_head)
+		return ERR_PTR(-ENOMEM);
+	INIT_LIST_HEAD(&mpath_head->dev_list);
+	mutex_init(&mpath_head->lock);
+	kref_init(&mpath_head->ref);
+
+	ret = init_srcu_struct(&mpath_head->srcu);
+	if (ret) {
+		kfree(mpath_head);
+		return ERR_PTR(ret);
+	}
+
+	return mpath_head;
+}
+EXPORT_SYMBOL_GPL(mpath_alloc_head);
+
+static int __init mpath_init(void)
+{
+	mpath_wq = alloc_workqueue("mpath-wq",
+			WQ_UNBOUND | WQ_MEM_RECLAIM | WQ_SYSFS, 0);
+	if (!mpath_wq)
+		return -ENOMEM;
+	return 0;
+}
+
+static void __exit mpath_exit(void)
+{
+	destroy_workqueue(mpath_wq);
+}
+
+module_init(mpath_init);
+module_exit(mpath_exit);
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("libmultipath");
-- 
2.43.5