Re: [PATCH 02/24] scsi-multipath: introduce basic SCSI device support

[Benjamin Marzinski <bmarzins@redhat.com>]

On Wed, Feb 25, 2026 at 03:36:05PM +0000, John Garry wrote:
> For a scsi_device to support multipath, introduce structure
> scsi_mpath_device to hold multipath-specific details.
> 
> Like NS structure for NVME, scsi_mpath_device holds the mpath_device
> structure to device management and path selection.
> 
> Two module params are introduced to enable multipath:
> - scsi_multipath
> - scsi_multipath_always
> 
> SCSI multipath will only be available until the following conditions:
> - scsi_multipath enabled and ALUA supported and unique ID available in
>   VPD page 83.
> - scsi_multipath_always enabled and unique ID available in VPD page 83
> 
> The scsi_device structure contains a pointer to scsi_mpath_device, which
> means whether multipath is enabled or disabled for the scsi_device.
> 
> Signed-off-by: John Garry <john.g.garry@oracle.com>
>
> diff --git a/include/scsi/scsi_multipath.h b/include/scsi/scsi_multipath.h
> new file mode 100644
> index 0000000000000..ca00ea10cd5db
> --- /dev/null
> +++ b/include/scsi/scsi_multipath.h
> @@ -0,0 +1,55 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _SCSI_SCSI_MULTIPATH_H
> +#define _SCSI_SCSI_MULTIPATH_H
> +
> +#include <linux/list.h>
> +#include <linux/types.h>
> +#include <linux/rcupdate.h>
> +#include <linux/workqueue.h>
> +#include <linux/mutex.h>
> +#include <linux/blk-mq.h>
> +#include <linux/multipath.h>
> +#include <scsi/scsi.h>
> +#include <scsi/scsi_cmnd.h>
> +#include <scsi/scsi_dbg.h>
> +#include <scsi/scsi_device.h>
> +#include <scsi/scsi_devinfo.h>
> +#include <scsi/scsi_driver.h>
> +
> +#ifdef CONFIG_SCSI_MULTIPATH
> +#define SCSI_MPATH_DEVICE_ID_LEN 40

Is there a reason that this is set to 40? scsi_vpd_lun_id() can return
ids larger than 40 (struct alua_port_group uses 256 bytes to hold the
response), and I don't know of any guarantee that the id will be unique
within the first 40 characters, although it certainly seems like only
pathological devices wouldn't.

-Ben

> +
> +struct scsi_mpath_device {
> +	struct mpath_device	mpath_device;
> +	struct scsi_device 	*sdev;
> +
> +	char			device_id_str[SCSI_MPATH_DEVICE_ID_LEN];
> +};
> +#define to_scsi_mpath_device(d) \
> +	container_of(d, struct scsi_mpath_device, mpath_device)
> +
> +int scsi_mpath_dev_alloc(struct scsi_device *sdev);
> +void scsi_mpath_dev_release(struct scsi_device *sdev);
> +int scsi_multipath_init(void);
> +void scsi_multipath_exit(void);
> +#else /* CONFIG_SCSI_MULTIPATH */
> +
> +struct scsi_mpath_device {
> +};
> +
> +static inline int scsi_mpath_dev_alloc(struct scsi_device *sdev)
> +{
> +	return 0;
> +}
> +static inline void scsi_mpath_dev_release(struct scsi_device *sdev)
> +{
> +}
> +static inline int scsi_multipath_init(void)
> +{
> +	return 0;
> +}
> +static inline void scsi_multipath_exit(void)
> +{
> +}
> +#endif /* CONFIG_SCSI_MULTIPATH */
> +#endif /* _SCSI_SCSI_MULTIPATH_H */
> -- 
> 2.43.5