linux-kernel - Re: [PATCH v2 2/2] dax: add a sysfs knob to control memmap_on

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Message-ID: <8734wd1j4z.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Fri, 08 Dec 2023 11:13:32 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Vishal Verma <vishal.l.verma@...el.com>
Cc:     Dave Jiang <dave.jiang@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        <linux-kernel@...r.kernel.org>, <nvdimm@...ts.linux.dev>,
        <linux-cxl@...r.kernel.org>, David Hildenbrand <david@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Jonathan Cameron" <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH v2 2/2] dax: add a sysfs knob to control
 memmap_on_memory behavior

Vishal Verma <vishal.l.verma@...el.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 <david@...hat.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Dave Jiang <dave.jiang@...el.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Huang Ying <ying.huang@...el.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Vishal Verma <vishal.l.verma@...el.com>
> ---
>  drivers/dax/bus.c                       | 40 +++++++++++++++++++++++++++++++++
>  Documentation/ABI/testing/sysfs-bus-dax | 13 +++++++++++
>  2 files changed, 53 insertions(+)
>
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index 1ff1ab5fa105..11abb57cc031 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -1270,6 +1270,45 @@ 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 sprintf(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);
> +	struct dax_region *dax_region = dev_dax->region;
> +	ssize_t rc;
> +	bool val;
> +
> +	rc = kstrtobool(buf, &val);
> +	if (rc)
> +		return rc;
> +
> +	if (dev_dax->memmap_on_memory == val)
> +		return len;
> +
> +	device_lock(dax_region->dev);
> +	if (!dax_region->dev->driver) {

This still doesn't look right.  Can we check whether the current driver
is kmem?  And only allow change if it's not kmem?

--
Best Regards,
Huang, Ying

> +		device_unlock(dax_region->dev);
> +		return -ENXIO;
> +	}
> +
> +	device_lock(dev);
> +	dev_dax->memmap_on_memory = val;
> +	device_unlock(dev);
> +
> +	device_unlock(dax_region->dev);
> +	return rc == 0 ? len : rc;
> +}
> +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);
> @@ -1296,6 +1335,7 @@ 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 a61a7b186017..bb063a004e41 100644
> --- a/Documentation/ABI/testing/sysfs-bus-dax
> +++ b/Documentation/ABI/testing/sysfs-bus-dax
> @@ -149,3 +149,16 @@ KernelVersion:	v5.1
>  Contact:	nvdimm@...ts.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:		October, 2023
> +KernelVersion:	v6.8
> +Contact:	nvdimm@...ts.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).