Vishal Verma <vishal.l.ve...@intel.com> writes:

> Add a sysfs knob for dax devices to control the memmap_on_memory setting
> if the dax device were to be hotplugged as system memory.
>
> The default memmap_on_memory setting for dax devices originating via
> pmem or hmem is set to 'false' - i.e. no memmap_on_memory semantics, to
> preserve legacy behavior. For dax devices via CXL, the default is on.
> The sysfs control allows the administrator to override the above
> defaults if needed.
>
> Cc: David Hildenbrand <da...@redhat.com>
> Cc: Dan Williams <dan.j.willi...@intel.com>
> Cc: Dave Jiang <dave.ji...@intel.com>
> Cc: Dave Hansen <dave.han...@linux.intel.com>
> Cc: Huang Ying <ying.hu...@intel.com>
> Tested-by: Li Zhijian <lizhij...@fujitsu.com>
> Reviewed-by: Jonathan Cameron <jonathan.came...@huawei.com>
> Reviewed-by: David Hildenbrand <da...@redhat.com>
> Signed-off-by: Vishal Verma <vishal.l.ve...@intel.com>

Looks good to me!  Thanks!

Reviewed-by: "Huang, Ying" <ying.hu...@intel.com>

> ---
>  drivers/dax/bus.c                       | 36 
> +++++++++++++++++++++++++++++++++
>  Documentation/ABI/testing/sysfs-bus-dax | 17 ++++++++++++++++
>  2 files changed, 53 insertions(+)
>
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index 6226de131d17..3622b3d1c0de 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -1245,6 +1245,41 @@ static ssize_t numa_node_show(struct device *dev,
>  }
>  static DEVICE_ATTR_RO(numa_node);
>  
> +static ssize_t memmap_on_memory_show(struct device *dev,
> +                                  struct device_attribute *attr, char *buf)
> +{
> +     struct dev_dax *dev_dax = to_dev_dax(dev);
> +
> +     return sysfs_emit(buf, "%d\n", dev_dax->memmap_on_memory);
> +}
> +
> +static ssize_t memmap_on_memory_store(struct device *dev,
> +                                   struct device_attribute *attr,
> +                                   const char *buf, size_t len)
> +{
> +     struct dev_dax *dev_dax = to_dev_dax(dev);
> +     ssize_t rc;
> +     bool val;
> +
> +     rc = kstrtobool(buf, &val);
> +     if (rc)
> +             return rc;
> +
> +     if (val == true && !mhp_supports_memmap_on_memory()) {
> +             dev_dbg(dev, "memmap_on_memory is not available\n");
> +             return -EOPNOTSUPP;
> +     }
> +
> +     guard(device)(dev);
> +     if (dev_dax->memmap_on_memory != val && dev->driver &&
> +         to_dax_drv(dev->driver)->type == DAXDRV_KMEM_TYPE)
> +             return -EBUSY;
> +     dev_dax->memmap_on_memory = val;
> +
> +     return len;
> +}
> +static DEVICE_ATTR_RW(memmap_on_memory);
> +
>  static umode_t dev_dax_visible(struct kobject *kobj, struct attribute *a, 
> int n)
>  {
>       struct device *dev = container_of(kobj, struct device, kobj);
> @@ -1271,6 +1306,7 @@ static struct attribute *dev_dax_attributes[] = {
>       &dev_attr_align.attr,
>       &dev_attr_resource.attr,
>       &dev_attr_numa_node.attr,
> +     &dev_attr_memmap_on_memory.attr,
>       NULL,
>  };
>  
> diff --git a/Documentation/ABI/testing/sysfs-bus-dax 
> b/Documentation/ABI/testing/sysfs-bus-dax
> index 6359f7bc9bf4..b34266bfae49 100644
> --- a/Documentation/ABI/testing/sysfs-bus-dax
> +++ b/Documentation/ABI/testing/sysfs-bus-dax
> @@ -134,3 +134,20 @@ KernelVersion:   v5.1
>  Contact:     nvd...@lists.linux.dev
>  Description:
>               (RO) The id attribute indicates the region id of a dax region.
> +
> +What:                /sys/bus/dax/devices/daxX.Y/memmap_on_memory
> +Date:                January, 2024
> +KernelVersion:       v6.8
> +Contact:     nvd...@lists.linux.dev
> +Description:
> +             (RW) Control the memmap_on_memory setting if the dax device
> +             were to be hotplugged as system memory. This determines whether
> +             the 'altmap' for the hotplugged memory will be placed on the
> +             device being hotplugged (memmap_on_memory=1) or if it will be
> +             placed on regular memory (memmap_on_memory=0). This attribute
> +             must be set before the device is handed over to the 'kmem'
> +             driver (i.e.  hotplugged into system-ram). Additionally, this
> +             depends on CONFIG_MHP_MEMMAP_ON_MEMORY, and a globally enabled
> +             memmap_on_memory parameter for memory_hotplug. This is
> +             typically set on the kernel command line -
> +             memory_hotplug.memmap_on_memory set to 'true' or 'force'."

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