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

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Message-Id: <20231206-vv-dax_abi-v2-2-f4f4f2336d08@intel.com>
Date:   Wed, 06 Dec 2023 21:36:15 -0700
From:   Vishal Verma <vishal.l.verma@...el.com>
To:     Dave Jiang <dave.jiang@...el.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-cxl@...r.kernel.org, Vishal Verma <vishal.l.verma@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Huang Ying <ying.huang@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: [PATCH v2 2/2] dax: add a sysfs knob to control memmap_on_memory
 behavior

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) {
+		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).

-- 
2.41.0