[PATCH v5 6/9] dax: plumb hotplug online_type through dax
From: Gregory Price
Date: Wed Jun 24 2026 - 10:59:03 EST
There is no way for drivers leveraging dax_kmem to plumb through a
preferred auto-online policy - the system default policy is forced.
Add 'enum mmop' field to DAX device creation path to allow drivers
to specify an auto-online policy when using the kmem driver.
Capturing the system default would otherwise break the ABI, because
the system default can change - but we would be statically assigning
the value at device creation time.
To resolve this we add DAX_ONLINE_DEFAULT, which defaults devices to
the current behavior, while providing a clean way to override it.
No behavioural change for existing callers (still the system default).
Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
---
drivers/dax/bus.c | 3 +++
drivers/dax/bus.h | 9 +++++++++
drivers/dax/cxl.c | 1 +
drivers/dax/dax-private.h | 4 ++++
drivers/dax/hmem/hmem.c | 1 +
drivers/dax/kmem.c | 11 +++++++++--
drivers/dax/pmem.c | 1 +
7 files changed, 28 insertions(+), 2 deletions(-)
diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index 492573b47f66..4a03b323b003 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -1,6 +1,7 @@
// SPDX-License-Identifier: GPL-2.0
/* Copyright(c) 2017-2018 Intel Corporation. All rights reserved. */
#include <linux/memremap.h>
+#include <linux/memory_hotplug.h>
#include <linux/device.h>
#include <linux/mutex.h>
#include <linux/list.h>
@@ -394,6 +395,7 @@ static ssize_t create_store(struct device *dev, struct device_attribute *attr,
.size = 0,
.id = -1,
.memmap_on_memory = false,
+ .online_type = DAX_ONLINE_DEFAULT,
};
struct dev_dax *dev_dax = __devm_create_dev_dax(&data);
@@ -1527,6 +1529,7 @@ static struct dev_dax *__devm_create_dev_dax(struct dev_dax_data *data)
ida_init(&dev_dax->ida);
dev_dax->memmap_on_memory = data->memmap_on_memory;
+ dev_dax->online_type = data->online_type;
inode = dax_inode(dax_dev);
dev->devt = inode->i_rdev;
diff --git a/drivers/dax/bus.h b/drivers/dax/bus.h
index 5909171a4428..f3c9dae5de6b 100644
--- a/drivers/dax/bus.h
+++ b/drivers/dax/bus.h
@@ -3,6 +3,7 @@
#ifndef __DAX_BUS_H__
#define __DAX_BUS_H__
#include <linux/device.h>
+#include <linux/memory_hotplug.h>
#include <linux/platform_device.h>
#include <linux/range.h>
#include <linux/workqueue.h>
@@ -16,6 +17,13 @@ struct dax_region;
#define IORESOURCE_DAX_STATIC BIT(0)
#define IORESOURCE_DAX_KMEM BIT(1)
+/*
+ * online_type sentinel: the device was created without an explicit online
+ * policy, so the system default is resolved when the kmem driver binds,
+ * (not at device-creation time, which would freeze a stale policy).
+ */
+#define DAX_ONLINE_DEFAULT (-1)
+
struct dax_region *alloc_dax_region(struct device *parent, int region_id,
struct range *range, int target_node, unsigned int align,
unsigned long flags);
@@ -26,6 +34,7 @@ struct dev_dax_data {
resource_size_t size;
int id;
bool memmap_on_memory;
+ enum mmop online_type;
};
struct dev_dax *devm_create_dev_dax(struct dev_dax_data *data);
diff --git a/drivers/dax/cxl.c b/drivers/dax/cxl.c
index 3ab39b77843d..1a7ec6212213 100644
--- a/drivers/dax/cxl.c
+++ b/drivers/dax/cxl.c
@@ -27,6 +27,7 @@ static int cxl_dax_region_probe(struct device *dev)
.id = -1,
.size = range_len(&cxlr_dax->hpa_range),
.memmap_on_memory = true,
+ .online_type = DAX_ONLINE_DEFAULT,
};
return PTR_ERR_OR_ZERO(devm_create_dev_dax(&data));
diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
index 81e4af49e39c..ccd77965fe3e 100644
--- a/drivers/dax/dax-private.h
+++ b/drivers/dax/dax-private.h
@@ -8,6 +8,7 @@
#include <linux/device.h>
#include <linux/cdev.h>
#include <linux/idr.h>
+#include <linux/memory_hotplug.h>
/* private routines between core files */
struct dax_device;
@@ -79,6 +80,8 @@ struct dev_dax_range {
* @dev: device core
* @pgmap: pgmap for memmap setup / lifetime (driver owned)
* @memmap_on_memory: allow kmem to put the memmap in the memory
+ * @online_type: MMOP_* online type for memory hotplug, or DAX_ONLINE_DEFAULT
+ * to resolve the system default policy when kmem binds
* @nr_range: size of @ranges
* @ranges: range tuples of memory used
*/
@@ -95,6 +98,7 @@ struct dev_dax {
struct device dev;
struct dev_pagemap *pgmap;
bool memmap_on_memory;
+ enum mmop online_type;
int nr_range;
struct dev_dax_range *ranges;
};
diff --git a/drivers/dax/hmem/hmem.c b/drivers/dax/hmem/hmem.c
index af21f66bf872..2de3bc925172 100644
--- a/drivers/dax/hmem/hmem.c
+++ b/drivers/dax/hmem/hmem.c
@@ -37,6 +37,7 @@ static int dax_hmem_probe(struct platform_device *pdev)
.id = -1,
.size = region_idle ? 0 : range_len(&mri->range),
.memmap_on_memory = false,
+ .online_type = DAX_ONLINE_DEFAULT,
};
return PTR_ERR_OR_ZERO(devm_create_dev_dax(&data));
diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
index 592171ec10f4..0a184c0878dd 100644
--- a/drivers/dax/kmem.c
+++ b/drivers/dax/kmem.c
@@ -72,6 +72,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
int i, rc, mapped = 0;
mhp_t mhp_flags;
int numa_node;
+ int online_type;
int adist = MEMTIER_DEFAULT_DAX_ADISTANCE;
/*
@@ -132,6 +133,11 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
goto err_reg_mgid;
data->mgid = rc;
+ /* Resolve system default at bind time in case it changed */
+ online_type = dev_dax->online_type;
+ if (online_type == DAX_ONLINE_DEFAULT)
+ online_type = mhp_get_default_online_type();
+
for (i = 0; i < dev_dax->nr_range; i++) {
struct resource *res;
struct range range;
@@ -172,8 +178,9 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
* Ensure that future kexec'd kernels will not treat
* this as RAM automatically.
*/
- rc = add_memory_driver_managed(data->mgid, range.start,
- range_len(&range), kmem_name, mhp_flags);
+ rc = __add_memory_driver_managed(data->mgid, range.start,
+ range_len(&range), kmem_name, mhp_flags,
+ online_type);
if (rc) {
dev_warn(dev, "mapping%d: %#llx-%#llx memory add failed\n",
diff --git a/drivers/dax/pmem.c b/drivers/dax/pmem.c
index bee93066a849..e7adace69195 100644
--- a/drivers/dax/pmem.c
+++ b/drivers/dax/pmem.c
@@ -63,6 +63,7 @@ static struct dev_dax *__dax_pmem_probe(struct device *dev)
.pgmap = &pgmap,
.size = range_len(&range),
.memmap_on_memory = false,
+ .online_type = DAX_ONLINE_DEFAULT,
};
return devm_create_dev_dax(&data);
--
2.54.0