Re: [PATCH v4 01/28] libmultipath: Add initial framework

[Hannes Reinecke <hare@suse.de>]

On 7/23/26 11:35 AM, John Garry wrote:
> 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>.
> 
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>   include/linux/multipath.h | 28 +++++++++++++++++
>   lib/Kconfig               |  6 ++++
>   lib/Makefile              |  2 ++
>   lib/multipath.c           | 66 +++++++++++++++++++++++++++++++++++++++
>   4 files changed, 102 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..e98b4b241020a
> --- /dev/null
> +++ b/include/linux/multipath.h
> @@ -0,0 +1,28 @@

Please add an SPDX tag.
And possibly a copyright.

> +
> +#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 55748b68714e0..d0258bef374a1 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 7f75cc6edf94a..7ba5e13be4171 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -334,3 +334,5 @@ 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..9cc398d266adf
> --- /dev/null
> +++ b/lib/multipath.c
> @@ -0,0 +1,66 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (c) 2017-2018 Christoph Hellwig.
A bit unusual, creating a new file and add a copyright
from _another_ person ...

> + * Copyright (c) 2026 Oracle and/or its affiliates.
> + */
> +#include <linux/module.h>
> +#include <linux/multipath.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");

Otherwise:
Reviewed-by: Hannes Reinecke <hare@kernel.org>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
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