[PATCH v9 02/29] libmultipath: Add initial framework

[John Garry <john.garry@linux.dev>]

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@lst.de>.

Reviewed-by: Hannes Reinecke <hare@kernel.org>
Signed-off-by: John Garry <john.garry@linux.dev>
---
 include/linux/multipath.h | 28 ++++++++++++++++
 lib/Kconfig               |  6 ++++
 lib/Makefile              |  2 ++
 lib/multipath.c           | 68 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 104 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 000000000000..407d985cd31f
--- /dev/null
+++ b/include/linux/multipath.h
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#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;
+
+	refcount_t		refcount;
+
+	struct mpath_device __rcu 		*current_path[MAX_NUMNODES];
+};
+
+int mpath_get_head(struct mpath_head *mpath_head);
+void mpath_put_head(struct mpath_head *mpath_head);
+int mpath_head_init(struct mpath_head *mpath_head);
+void mpath_head_uninit(struct mpath_head *mpath_head);
+
+#endif // _LIBMULTIPATH_H
diff --git a/lib/Kconfig b/lib/Kconfig
index 1e42f5167c68..3580bab7c2b1 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -630,3 +630,9 @@ 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 a2afe954805c..b4c0da268b67 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -350,3 +350,5 @@ 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 000000000000..31ef98980ba4
--- /dev/null
+++ b/lib/multipath.c
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2017-2018 Christoph Hellwig.
+ *
+ * Base on NVMe multipath support in drivers/nvme/host/multipath.c
+ */
+#include <linux/module.h>
+#include <linux/multipath.h>
+#include <linux/wait_bit.h>
+
+static struct workqueue_struct *mpath_wq;
+
+int mpath_get_head(struct mpath_head *mpath_head)
+{
+	if (!refcount_inc_not_zero(&mpath_head->refcount))
+		return -ENXIO;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mpath_get_head);
+
+void mpath_put_head(struct mpath_head *mpath_head)
+{
+	refcount_t *refcount = &mpath_head->refcount;
+
+	if (refcount_dec_and_test(&mpath_head->refcount))
+		wake_up_var(refcount);
+}
+EXPORT_SYMBOL_GPL(mpath_put_head);
+
+void mpath_head_uninit(struct mpath_head *mpath_head)
+{
+	refcount_t *refcount = &mpath_head->refcount;
+
+	if (!refcount_dec_and_test(refcount))
+		wait_var_event(refcount, !refcount_read(refcount));
+	cleanup_srcu_struct(&mpath_head->srcu);
+}
+EXPORT_SYMBOL_GPL(mpath_head_uninit);
+
+int mpath_head_init(struct mpath_head *mpath_head)
+{
+	memset(mpath_head, 0, sizeof(*mpath_head));
+	INIT_LIST_HEAD(&mpath_head->dev_list);
+	mutex_init(&mpath_head->lock);
+	refcount_set(&mpath_head->refcount, 1);
+
+	return init_srcu_struct(&mpath_head->srcu);
+}
+EXPORT_SYMBOL_GPL(mpath_head_init);
+
+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");
-- 
2.43.0