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

[Nilay Shroff <nilay@linux.ibm.com>]

Hi John,

On 2/25/26 9:02 PM, 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. However a separate structure will introduced after to manage
> 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           | 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..18cd133b7ca21
> --- /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;
> +
> +	struct mpath_device __rcu 		*current_path[MAX_NUMNODES];
> +	void			*drvdata;
> +};

Can we use current_path[] as last element and flex array (same as what
we have today under struct nvme_ns_head) so that we don't need to 
allocate array as big as MAX_NUMANODES? With flex array we can use 
num_possible_nodes() which may be much smaller than MAX_NUMANODES.

Thanks,
--Nilay