[PATCH v3 0/2] Add DAX ABI for memmap_on_memory

[PATCH v3 0/2] Add DAX ABI for memmap_on_memory

[Vishal Verma]

The DAX drivers were missing sysfs ABI documentation entirely.  Add this
missing documentation for the sysfs ABI for DAX regions and Dax devices
in patch 1. Add a new ABI for toggling memmap_on_memory semantics in
patch 2.

The missing ABI was spotted in [1], this series is a split of the new
ABI additions behind the initial documentation creation.

[1]: https://lore.kernel.org/linux-cxl/651f27b728fef_ae7e7294b3@dwillia2-xfh.jf.intel.com.notmuch/

Changes in v3:
- Fix typo in ABI docs (Zhijian Li)
- Add kernel config and module parameter dependencies to the ABI docs
  entry (David Hildenbrand)
- Ensure kmem isn't active when setting the sysfs attribute (Ying
  Huang)
- Simplify returning from memmap_on_memory_store()
- Link to v2: https://lore.kernel.org/r/20231206-vv-dax_abi-v2-0-f4f4f2336d08@intel.com

Changes in v2:
- Fix CC lists, patch 1/2 didn't get sent correctly in v1
- Link to v1: https://lore.kernel.org/r/20231206-vv-dax_abi-v1-0-474eb88e201c@intel.com

---
Vishal Verma (2):
      Documentatiion/ABI: Add ABI documentation for sys-bus-dax
      dax: add a sysfs knob to control memmap_on_memory behavior

 drivers/dax/bus.c                       |  47 +++++++++
 Documentation/ABI/testing/sysfs-bus-dax | 168 ++++++++++++++++++++++++++++++++
 2 files changed, 215 insertions(+)
---
base-commit: c4e1ccfad42352918810802095a8ace8d1c744c9
change-id: 20231025-vv-dax_abi-17a219c46076

Best regards,
-- 
Vishal Verma <vishal.l.verma@intel.com>

[PATCH v3 1/2] Documentatiion/ABI: Add ABI documentation for sys-bus-dax

[Vishal Verma]

Add the missing sysfs ABI documentation for the device DAX subsystem.
Various ABI attributes under this have been present since v5.1, and more
have been added over time. In preparation for adding a new attribute,
add this file with the historical details.

Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
---
 Documentation/ABI/testing/sysfs-bus-dax | 151 ++++++++++++++++++++++++++++++++
 1 file changed, 151 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-dax b/Documentation/ABI/testing/sysfs-bus-dax
new file mode 100644
index 000000000000..a61a7b186017
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-dax
@@ -0,0 +1,151 @@
+What:		/sys/bus/dax/devices/daxX.Y/align
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RW) Provides a way to specify an alignment for a dax device.
+		Values allowed are constrained by the physical address ranges
+		that back the dax device, and also by arch requirements.
+
+What:		/sys/bus/dax/devices/daxX.Y/mapping
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(WO) Provides a way to allocate a mapping range under a dax
+		device. Specified in the format <start>-<end>.
+
+What:		/sys/bus/dax/devices/daxX.Y/mapping[0..N]/start
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) A dax device may have multiple constituent discontiguous
+		address ranges. These are represented by the different
+		'mappingX' subdirectories. The 'start' attribute indicates the
+		start physical address for the given range.
+
+What:		/sys/bus/dax/devices/daxX.Y/mapping[0..N]/end
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) A dax device may have multiple constituent discontiguous
+		address ranges. These are represented by the different
+		'mappingX' subdirectories. The 'end' attribute indicates the
+		end physical address for the given range.
+
+What:		/sys/bus/dax/devices/daxX.Y/mapping[0..N]/page_offset
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) A dax device may have multiple constituent discontiguous
+		address ranges. These are represented by the different
+		'mappingX' subdirectories. The 'page_offset' attribute indicates the
+		offset of the current range in the dax device.
+
+What:		/sys/bus/dax/devices/daxX.Y/resource
+Date:		June, 2019
+KernelVersion:	v5.3
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The resource attribute indicates the starting physical
+		address of a dax device. In case of a device with multiple
+		constituent ranges, it indicates the starting address of the
+		first range.
+
+What:		/sys/bus/dax/devices/daxX.Y/size
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RW) The size attribute indicates the total size of a dax
+		device. For creating subdivided dax devices, or for resizing
+		an existing device, the new size can be written to this as
+		part of the reconfiguration process.
+
+What:		/sys/bus/dax/devices/daxX.Y/numa_node
+Date:		November, 2019
+KernelVersion:	v5.5
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) If NUMA is enabled and the platform has affinitized the
+		backing device for this dax device, emit the CPU node
+		affinity for this device.
+
+What:		/sys/bus/dax/devices/daxX.Y/target_node
+Date:		February, 2019
+KernelVersion:	v5.1
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The target-node attribute is the Linux numa-node that a
+		device-dax instance may create when it is online. Prior to
+		being online the device's 'numa_node' property reflects the
+		closest online cpu node which is the typical expectation of a
+		device 'numa_node'. Once it is online it becomes its own
+		distinct numa node.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/available_size
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The available_size attribute tracks available dax region
+		capacity. This only applies to volatile hmem devices, not pmem
+		devices, since pmem devices are defined by nvdimm namespace
+		boundaries.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/size
+Date:		July, 2017
+KernelVersion:	v5.1
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The size attribute indicates the size of a given dax region
+		in bytes.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/align
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The align attribute indicates alignment of the dax region.
+		Changes on align may not always be valid, when say certain
+		mappings were created with 2M and then we switch to 1G. This
+		validates all ranges against the new value being attempted, post
+		resizing.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/seed
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The seed device is a concept for dynamic dax regions to be
+		able to split the region amongst multiple sub-instances.  The
+		seed device, similar to libnvdimm seed devices, is a device
+		that starts with zero capacity allocated and unbound to a
+		driver.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/create
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RW) The create interface to the dax region provides a way to
+		create a new unconfigured dax device under the given region, which
+		can then be configured (with a size etc.) and then probed.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/delete
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(WO) The delete interface for a dax region provides for deletion
+		of any 0-sized and idle dax devices.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/id
+Date:		July, 2017
+KernelVersion:	v5.1
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The id attribute indicates the region id of a dax region.

-- 
2.41.0

[PATCH v3 2/2] dax: add a sysfs knob to control memmap_on_memory behavior

[Vishal Verma]

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@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Huang Ying <ying.huang@intel.com>
Tested-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
---
 drivers/dax/bus.c                       | 47 +++++++++++++++++++++++++++++++++
 Documentation/ABI/testing/sysfs-bus-dax | 17 ++++++++++++
 2 files changed, 64 insertions(+)

diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index 1ff1ab5fa105..2871e5188f0d 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -1270,6 +1270,52 @@ 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 device_driver *drv = dev->driver;
+	struct dev_dax *dev_dax = to_dev_dax(dev);
+	struct dax_region *dax_region = dev_dax->region;
+	struct dax_device_driver *dax_drv = to_dax_drv(drv);
+	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) {
+		device_unlock(dax_region->dev);
+		return -ENXIO;
+	}
+
+	if (dax_drv->type == DAXDRV_KMEM_TYPE) {
+		device_unlock(dax_region->dev);
+		return -EBUSY;
+	}
+
+	device_lock(dev);
+	dev_dax->memmap_on_memory = val;
+	device_unlock(dev);
+
+	device_unlock(dax_region->dev);
+	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);
@@ -1296,6 +1342,7 @@ 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..b1fd8bf8a7de 100644
--- a/Documentation/ABI/testing/sysfs-bus-dax
+++ b/Documentation/ABI/testing/sysfs-bus-dax
@@ -149,3 +149,20 @@ KernelVersion:	v5.1
 Contact:	nvdimm@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:		October, 2023
+KernelVersion:	v6.8
+Contact:	nvdimm@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'."

-- 
2.41.0

Re: [PATCH v3 2/2] dax: add a sysfs knob to control memmap_on_memory behavior

[Huang, Ying]

Vishal Verma <vishal.l.verma@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 <david@redhat.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Tested-by: Li Zhijian <lizhijian@fujitsu.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
> ---
>  drivers/dax/bus.c                       | 47 +++++++++++++++++++++++++++++++++
>  Documentation/ABI/testing/sysfs-bus-dax | 17 ++++++++++++
>  2 files changed, 64 insertions(+)
>
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index 1ff1ab5fa105..2871e5188f0d 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -1270,6 +1270,52 @@ 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 device_driver *drv = dev->driver;
> +	struct dev_dax *dev_dax = to_dev_dax(dev);
> +	struct dax_region *dax_region = dev_dax->region;
> +	struct dax_device_driver *dax_drv = to_dax_drv(drv);
> +	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) {
> +		device_unlock(dax_region->dev);
> +		return -ENXIO;
> +	}

I think that it should be OK to write to "memmap_on_memory" if no driver
is bound to the device.  We just need to avoid to write to it when kmem
driver is bound.

--
Best Regards,
Huang, Ying

> +
> +	if (dax_drv->type == DAXDRV_KMEM_TYPE) {
> +		device_unlock(dax_region->dev);
> +		return -EBUSY;
> +	}
> +
> +	device_lock(dev);
> +	dev_dax->memmap_on_memory = val;
> +	device_unlock(dev);
> +
> +	device_unlock(dax_region->dev);
> +	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);
> @@ -1296,6 +1342,7 @@ 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..b1fd8bf8a7de 100644
> --- a/Documentation/ABI/testing/sysfs-bus-dax
> +++ b/Documentation/ABI/testing/sysfs-bus-dax
> @@ -149,3 +149,20 @@ KernelVersion:	v5.1
>  Contact:	nvdimm@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:		October, 2023
> +KernelVersion:	v6.8
> +Contact:	nvdimm@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'."

Re: [PATCH v3 2/2] dax: add a sysfs knob to control memmap_on_memory behavior

[Verma, Vishal L]

On Tue, 2023-12-12 at 08:30 +0800, Huang, Ying wrote:
> Vishal Verma <vishal.l.verma@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 <david@redhat.com>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > Cc: Dave Jiang <dave.jiang@intel.com>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: Huang Ying <ying.huang@intel.com>
> > Tested-by: Li Zhijian <lizhijian@fujitsu.com>
> > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > Reviewed-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
> > ---
> >  drivers/dax/bus.c                       | 47 +++++++++++++++++++++++++++++++++
> >  Documentation/ABI/testing/sysfs-bus-dax | 17 ++++++++++++
> >  2 files changed, 64 insertions(+)
> > 
> > diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> > index 1ff1ab5fa105..2871e5188f0d 100644
> > --- a/drivers/dax/bus.c
> > +++ b/drivers/dax/bus.c
> > @@ -1270,6 +1270,52 @@ 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 device_driver *drv = dev->driver;
> > +       struct dev_dax *dev_dax = to_dev_dax(dev);
> > +       struct dax_region *dax_region = dev_dax->region;
> > +       struct dax_device_driver *dax_drv = to_dax_drv(drv);
> > +       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) {
> > +               device_unlock(dax_region->dev);
> > +               return -ENXIO;
> > +       }
> 
> I think that it should be OK to write to "memmap_on_memory" if no driver
> is bound to the device.  We just need to avoid to write to it when kmem
> driver is bound.

Oh this is just a check on the region driver, not for a dax driver
being bound to the device. It's the same as what things like
align_store(), size_store() etc. do for dax device reconfiguration.

That said, it might be okay to remove this check, as this operation
doesn't change any attributes of the dax region (the other interfaces I
mentioned above can affect regions, so we want to lock the region
device). If removing the check, we'd drop the region lock acquisition
as well.

Dan, any thoughts on this?

Re: [PATCH v3 2/2] dax: add a sysfs knob to control memmap_on_memory behavior

[Huang, Ying]

"Verma, Vishal L" <vishal.l.verma@intel.com> writes:

> On Tue, 2023-12-12 at 08:30 +0800, Huang, Ying wrote:
>> Vishal Verma <vishal.l.verma@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 <david@redhat.com>
>> > Cc: Dan Williams <dan.j.williams@intel.com>
>> > Cc: Dave Jiang <dave.jiang@intel.com>
>> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> > Cc: Huang Ying <ying.huang@intel.com>
>> > Tested-by: Li Zhijian <lizhijian@fujitsu.com>
>> > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>> > Reviewed-by: David Hildenbrand <david@redhat.com>
>> > Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
>> > ---
>> >  drivers/dax/bus.c                       | 47 +++++++++++++++++++++++++++++++++
>> >  Documentation/ABI/testing/sysfs-bus-dax | 17 ++++++++++++
>> >  2 files changed, 64 insertions(+)
>> >
>> > diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
>> > index 1ff1ab5fa105..2871e5188f0d 100644
>> > --- a/drivers/dax/bus.c
>> > +++ b/drivers/dax/bus.c
>> > @@ -1270,6 +1270,52 @@ 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 device_driver *drv = dev->driver;
>> > +       struct dev_dax *dev_dax = to_dev_dax(dev);
>> > +       struct dax_region *dax_region = dev_dax->region;
>> > +       struct dax_device_driver *dax_drv = to_dax_drv(drv);
>> > +       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) {
>> > +               device_unlock(dax_region->dev);
>> > +               return -ENXIO;
>> > +       }
>>
>> I think that it should be OK to write to "memmap_on_memory" if no driver
>> is bound to the device.  We just need to avoid to write to it when kmem
>> driver is bound.
>
> Oh this is just a check on the region driver, not for a dax driver
> being bound to the device. It's the same as what things like
> align_store(), size_store() etc. do for dax device reconfiguration.

Sorry, I misunderstood it.

> That said, it might be okay to remove this check, as this operation
> doesn't change any attributes of the dax region (the other interfaces I
> mentioned above can affect regions, so we want to lock the region
> device). If removing the check, we'd drop the region lock acquisition
> as well.

This sounds good to me.

And is it necessary to check driver type with device_lock()?  Can driver
be changed between checking and lock?

--
Best Regards,
Huang, Ying

Re: [PATCH v3 2/2] dax: add a sysfs knob to control memmap_on_memory behavior

[Dan Williams]

Verma, Vishal L wrote:
> On Tue, 2023-12-12 at 08:30 +0800, Huang, Ying wrote:
> > Vishal Verma <vishal.l.verma@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 <david@redhat.com>
> > > Cc: Dan Williams <dan.j.williams@intel.com>
> > > Cc: Dave Jiang <dave.jiang@intel.com>
> > > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > > Cc: Huang Ying <ying.huang@intel.com>
> > > Tested-by: Li Zhijian <lizhijian@fujitsu.com>
> > > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > Reviewed-by: David Hildenbrand <david@redhat.com>
> > > Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
> > > ---
> > >  drivers/dax/bus.c                       | 47 +++++++++++++++++++++++++++++++++
> > >  Documentation/ABI/testing/sysfs-bus-dax | 17 ++++++++++++
> > >  2 files changed, 64 insertions(+)
> > > 
> > > diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> > > index 1ff1ab5fa105..2871e5188f0d 100644
> > > --- a/drivers/dax/bus.c
> > > +++ b/drivers/dax/bus.c
> > > @@ -1270,6 +1270,52 @@ 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 device_driver *drv = dev->driver;
> > > +       struct dev_dax *dev_dax = to_dev_dax(dev);
> > > +       struct dax_region *dax_region = dev_dax->region;
> > > +       struct dax_device_driver *dax_drv = to_dax_drv(drv);
> > > +       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) {
> > > +               device_unlock(dax_region->dev);
> > > +               return -ENXIO;
> > > +       }
> > 
> > I think that it should be OK to write to "memmap_on_memory" if no driver
> > is bound to the device.  We just need to avoid to write to it when kmem
> > driver is bound.
> 
> Oh this is just a check on the region driver, not for a dax driver
> being bound to the device. It's the same as what things like
> align_store(), size_store() etc. do for dax device reconfiguration.
> 
> That said, it might be okay to remove this check, as this operation
> doesn't change any attributes of the dax region (the other interfaces I
> mentioned above can affect regions, so we want to lock the region
> device). If removing the check, we'd drop the region lock acquisition
> as well.
> 
> Dan, any thoughts on this?

Since this is a dev_dax attribute then this would have already been
synchronously shutdown when dax_region->dev->driver transitioned to
NULL. I.e. region unbind causes dev_dax deletion.

However, there's a different issue here as dev->driver was referenced
without the device_lock().

Additionally, I think this function also makes it clear that device lock
flavor of guard() would be useful:

    DEFINE_GUARD(dev, struct device *, device_lock(_T), device_unlock(_T))

...then I would expect something like:

        guard(dev)(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;

...maybe with some temp variables to reduce the derefence chain, buy you
get the idea. Only prevent changes while the device is active under
kmem.

Re: [PATCH v3 2/2] dax: add a sysfs knob to control memmap_on_memory behavior

[Verma, Vishal L]

On Tue, 2023-12-12 at 08:56 +0800, Huang, Ying wrote:
> "Verma, Vishal L" <vishal.l.verma@intel.com> writes:
> 
> > On Tue, 2023-12-12 at 08:30 +0800, Huang, Ying wrote:
> > > 
> > > > +
> > > > +static ssize_t memmap_on_memory_store(struct device *dev,
> > > > +                                     struct device_attribute *attr,
> > > > +                                     const char *buf, size_t len)
> > > > +{
> > > > +       struct device_driver *drv = dev->driver;
> > > > +       struct dev_dax *dev_dax = to_dev_dax(dev);
> > > > +       struct dax_region *dax_region = dev_dax->region;
> > > > +       struct dax_device_driver *dax_drv = to_dax_drv(drv);
> > > > +       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) {
> > > > +               device_unlock(dax_region->dev);
> > > > +               return -ENXIO;
> > > > +       }
> > > 
> > > I think that it should be OK to write to "memmap_on_memory" if no driver
> > > is bound to the device.  We just need to avoid to write to it when kmem
> > > driver is bound.
> > 
> > Oh this is just a check on the region driver, not for a dax driver
> > being bound to the device. It's the same as what things like
> > align_store(), size_store() etc. do for dax device reconfiguration.
> 
> Sorry, I misunderstood it.
> 
> > That said, it might be okay to remove this check, as this operation
> > doesn't change any attributes of the dax region (the other interfaces I
> > mentioned above can affect regions, so we want to lock the region
> > device). If removing the check, we'd drop the region lock acquisition
> > as well.
> 
> This sounds good to me.
> 
> And is it necessary to check driver type with device_lock()?  Can driver
> be changed between checking and lock?
> 
Oh, good point, the type check should happen with the device lock held.
I'll make that change.

Re: [PATCH v3 2/2] dax: add a sysfs knob to control memmap_on_memory behavior

[David Hildenbrand]

On 11.12.23 23:52, Vishal Verma wrote:
> 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@redhat.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Tested-by: Li Zhijian <lizhijian@fujitsu.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
> ---
>   drivers/dax/bus.c                       | 47 +++++++++++++++++++++++++++++++++
>   Documentation/ABI/testing/sysfs-bus-dax | 17 ++++++++++++
>   2 files changed, 64 insertions(+)
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index 1ff1ab5fa105..2871e5188f0d 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -1270,6 +1270,52 @@ 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 device_driver *drv = dev->driver;
> +	struct dev_dax *dev_dax = to_dev_dax(dev);
> +	struct dax_region *dax_region = dev_dax->region;
> +	struct dax_device_driver *dax_drv = to_dax_drv(drv);
> +	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) {
> +		device_unlock(dax_region->dev);
> +		return -ENXIO;
> +	}
> +
> +	if (dax_drv->type == DAXDRV_KMEM_TYPE) {
> +		device_unlock(dax_region->dev);
> +		return -EBUSY;
> +	}
> +
> +	device_lock(dev);
> +	dev_dax->memmap_on_memory = val;
> +	device_unlock(dev);
> +
> +	device_unlock(dax_region->dev);
> +	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);
> @@ -1296,6 +1342,7 @@ 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..b1fd8bf8a7de 100644
> --- a/Documentation/ABI/testing/sysfs-bus-dax
> +++ b/Documentation/ABI/testing/sysfs-bus-dax
> @@ -149,3 +149,20 @@ KernelVersion:	v5.1
>   Contact:	nvdimm@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:		October, 2023
> +KernelVersion:	v6.8
> +Contact:	nvdimm@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'."
> 

Thinking about it, I wonder if we could disallow setting that property 
to "true" if the current configuration does not allow it.

That is:

1) Removing the "size" parameter from mhp_supports_memmap_on_memory(), 
it doesn't make any sense anymore.

2) Exporting mhp_supports_memmap_on_memory() to modules.

3) When setting memmap_on_memory, check whether 
mhp_supports_memmap_on_memory() == true.

Then, the user really gets an error when trying to set it to "true".

-- 
Cheers,

David / dhildenb