+ dax-kmem-add-sysfs-interface-for-atomic-whole-device-hotplug.patch added to mm-new branch

+ dax-kmem-add-sysfs-interface-for-atomic-whole-device-hotplug.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: dax/kmem: add sysfs interface for atomic whole-device hotplug
has been added to the -mm mm-new branch.  Its filename is
     dax-kmem-add-sysfs-interface-for-atomic-whole-device-hotplug.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/dax-kmem-add-sysfs-interface-for-atomic-whole-device-hotplug.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days

------------------------------------------------------
From: Gregory Price <gourry@gourry.net>
Subject: dax/kmem: add sysfs interface for atomic whole-device hotplug
Date: Sun, 12 Jul 2026 11:45:03 -0400

There is no atomic mechanism to offline and remove an entire
multi-block DAX kmem device.  This is presently done in two steps:
    1. offline all
    2. remove all

This creates a race condition where another entity operates directly on
the memory blocks and can cause hot-unplug to fail / unbind to deadlock.

Add a new 'state' sysfs attribute that enables an atomic whole-device
hotplug operation across its entire memory region.

daxX.Y/state mirrors the per-block memoryX/state ABI:
  - [offline, online, online_kernel, online_movable]
  - "unplugged" - is added specifically for dax0.0/state

The valid writable states include:
  - "unplugged":      memory blocks are not present
  - "online":         memory is online, zone chosen by the kernel
  - "online_kernel":  memory is online in ZONE_NORMAL
  - "online_movable": memory is online in ZONE_MOVABLE

Valid transitions:
  - unplugged                -> online[_kernel|_movable]
  - online[_kernel|_movable] -> unplugged
  - offline                  -> unplugged

A device can only be onlined from "unplugged", so it must be returned
there before being onlined into a different state.

For backwards compatibility the memory blocks are always created at probe
- existing tools expect them to be present after kmem binds.

"offline" is therefore a reportable state but is not writable: it only
arises from the legacy auto_online_blocks=offline policy.  Onlining such a
device through this attribute requires unplugging it first in an effort to
get drivers creating DAX devices to set a default.

Unplug is atomic across the whole device: dax_kmem_do_hotremove() collects
every added range and offlines/removes them in one operation.  Either the
operation succeeds or is entirely rolled back.

Unbind Note:
  An offline dax device memory is removed on unbind as before.

  If online at unbind, the resources are leaked (as before), but now
  we prevent deadlock if a memory region is impossible to hotremove.

Link: https://lore.kernel.org/20260712154505.3564379-10-gourry@gourry.net
Signed-off-by: Gregory Price <gourry@gourry.net>
Suggested-by: Hannes Reinecke <hare@suse.de>
Suggested-by: David Hildenbrand <david@kernel.org>
Reviewed-by: Dan Williams <djbw@kernel.org>
Cc: Alison Schofield <alison.schofield@intel.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Pankaj Gupta <pankaj.gupta@amd.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/ABI/testing/sysfs-bus-dax |   24 ++
 drivers/dax/bus.h                       |    2 
 drivers/dax/kmem.c                      |  243 ++++++++++++++++++----
 3 files changed, 233 insertions(+), 36 deletions(-)

--- a/Documentation/ABI/testing/sysfs-bus-dax~dax-kmem-add-sysfs-interface-for-atomic-whole-device-hotplug
+++ a/Documentation/ABI/testing/sysfs-bus-dax
@@ -151,3 +151,27 @@ Description:
 		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'."
+
+What:		/sys/bus/dax/devices/daxX.Y/state
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RW) Controls the state of the memory region.
+		Applies to all memory blocks associated with the device.
+		Only applies to dax_kmem devices.
+
+		Reading returns the current state; the writable states mirror
+		the per-block /sys/devices/system/memory/memoryX/state ABI::
+
+		  "unplugged": memory blocks are not present
+		  "online": memory is online, zone chosen by the kernel
+		  "online_kernel": memory is online in ZONE_NORMAL
+		  "online_movable": memory is online in ZONE_MOVABLE
+
+		"offline" (memory blocks are present but offline) may also be
+		reported - this happens when the device is bound while the
+		auto_online_blocks policy is "offline".  It cannot be written,
+		as it's not useful and creates device destruction races.
+
+		A device can only be onlined from the "unplugged" state, so a
+		device must be returned to "unplugged" before it can be onlined
+		into a different state.
--- a/drivers/dax/bus.h~dax-kmem-add-sysfs-interface-for-atomic-whole-device-hotplug
+++ a/drivers/dax/bus.h
@@ -16,6 +16,8 @@ struct dax_region;
 #define IORESOURCE_DAX_STATIC BIT(0)
 #define IORESOURCE_DAX_KMEM BIT(1)
 
+#define DAX_KMEM_UNPLUGGED	(-1) /* Do not create memory blocks */
+
 struct dax_region *alloc_dax_region(struct device *parent, int region_id,
 		struct range *range, int target_node, unsigned int align,
 		unsigned long flags);
--- a/drivers/dax/kmem.c~dax-kmem-add-sysfs-interface-for-atomic-whole-device-hotplug
+++ a/drivers/dax/kmem.c
@@ -45,6 +45,8 @@ static int dax_kmem_range(struct dev_dax
 struct dax_kmem_data {
 	const char *res_name;
 	int mgid;
+	int state;
+	struct mutex lock; /* protects hotplug state transitions */
 	struct resource *res[];
 };
 
@@ -63,12 +65,22 @@ static void kmem_put_memory_types(void)
 	mt_put_memory_types(&kmem_memory_types);
 }
 
+/* True for the online states a kmem dax device can hold. */
+static bool dax_kmem_state_is_online(int state)
+{
+	return state == MMOP_ONLINE ||
+	       state == MMOP_ONLINE_KERNEL ||
+	       state == MMOP_ONLINE_MOVABLE;
+}
+
 /**
  * dax_kmem_do_hotplug - hotplug memory for dax kmem device
  * @dev_dax: the dev_dax instance
  * @data: the dax_kmem_data structure with resource tracking
+ * @online_type: the online policy to use for the memory blocks
  *
- * Hotplugs all ranges in the dev_dax region as system memory.
+ * Hotplugs all ranges in the dev_dax region as system memory with the
+ * provided online policy (offline, online, online_movable, online_kernel).
  *
  * Returns the number of successfully mapped ranges, or negative error.
  */
@@ -77,9 +89,15 @@ static int dax_kmem_do_hotplug(struct de
 			       int online_type)
 {
 	struct device *dev = &dev_dax->dev;
-	int i, rc, onlined = 0;
+	int i, rc, added = 0;
 	mhp_t mhp_flags;
 
+	if (dax_kmem_state_is_online(data->state))
+		return -EINVAL;
+
+	if (online_type < MMOP_OFFLINE || online_type > MMOP_ONLINE_MOVABLE)
+		return -EINVAL;
+
 	for (i = 0; i < dev_dax->nr_range; i++) {
 		struct range range;
 
@@ -123,14 +141,14 @@ static int dax_kmem_do_hotplug(struct de
 				kfree(data->res[i]);
 				data->res[i] = NULL;
 			}
-			if (onlined)
+			if (added)
 				continue;
 			return rc;
 		}
-		onlined++;
+		added++;
 	}
 
-	return onlined;
+	return added;
 }
 
 /**
@@ -193,45 +211,64 @@ static int dax_kmem_init_resources(struc
  * @dev_dax: the dev_dax instance
  * @data: the dax_kmem_data structure with resource tracking
  *
- * Removes all ranges in the dev_dax region.
+ * Offlines and removes every currently-added range in the dev_dax region
+ * atomically: either all ranges are offlined and removed, or none are and
+ * the device is returned to its prior state.
  *
- * Returns the number of successfully removed ranges.
+ * Returns 0 on success, or a negative errno on failure.
  */
 static int dax_kmem_do_hotremove(struct dev_dax *dev_dax,
 				 struct dax_kmem_data *data)
 {
 	struct device *dev = &dev_dax->dev;
-	int i, success = 0;
+	struct range *ranges;
+	int i, nr_ranges = 0, rc;
+
+	ranges = kmalloc_objs(*ranges, dev_dax->nr_range);
+	if (!ranges)
+		return -ENOMEM;
 
+	/* Collect the ranges that were actually added during probe. */
 	for (i = 0; i < dev_dax->nr_range; i++) {
 		struct range range;
-		int rc;
 
-		rc = dax_kmem_range(dev_dax, i, &range);
-		if (rc)
+		if (!data->res[i])
 			continue;
-
-		/* range was never added during probe, count as removed */
-		if (!data->res[i]) {
-			success++;
+		if (dax_kmem_range(dev_dax, i, &range))
 			continue;
-		}
+		ranges[nr_ranges++] = range;
+	}
+
+	/* Nothing added means nothing to remove. */
+	if (!nr_ranges) {
+		kfree(ranges);
+		return 0;
+	}
+
+	rc = offline_and_remove_memory_ranges(ranges, nr_ranges);
+	kfree(ranges);
+	if (rc) {
+		/* Recoverable: the ranges rolled back, nothing is leaked yet. */
+		dev_err(dev, "hotremove failed, device left online: %d\n", rc);
+		return rc;
+	}
 
-		rc = remove_memory(range.start, range_len(&range));
-		if (rc == 0) {
-			/* Release the resource for the successfully removed range */
-			remove_resource(data->res[i]);
-			kfree(data->res[i]);
-			data->res[i] = NULL;
-			success++;
+	/* All ranges removed; release the reserved resources. */
+	for (i = 0; i < dev_dax->nr_range; i++) {
+		if (!data->res[i])
 			continue;
-		}
-		any_hotremove_failed = true;
-		dev_err(dev, "mapping%d: %#llx-%#llx hotremove failed\n",
-			i, range.start, range.end);
+		remove_resource(data->res[i]);
+		kfree(data->res[i]);
+		data->res[i] = NULL;
 	}
 
-	return success;
+	return 0;
+}
+#else
+static int dax_kmem_do_hotremove(struct dev_dax *dev_dax,
+				 struct dax_kmem_data *data)
+{
+	return -EBUSY;
 }
 #endif /* CONFIG_MEMORY_HOTREMOVE */
 
@@ -247,6 +284,18 @@ static void dax_kmem_cleanup_resources(s
 {
 	int i;
 
+	/*
+	 * If the device unbind occurs before memory is hotremoved, we can never
+	 * remove the memory (requires reboot).  Attempting an offline operation
+	 * here may cause deadlock and a failure to finish the unbind.
+	 *
+	 * Note: This leaks the resources.
+	 */
+	if (WARN(((data->state != DAX_KMEM_UNPLUGGED) &&
+		  (data->state != MMOP_OFFLINE)),
+		 "Hotplug memory regions stuck online until reboot"))
+		return;
+
 	for (i = 0; i < dev_dax->nr_range; i++) {
 		if (!data->res[i])
 			continue;
@@ -256,6 +305,81 @@ static void dax_kmem_cleanup_resources(s
 	}
 }
 
+static int dax_kmem_parse_state(const char *buf)
+{
+	int online_type;
+
+	/* "unplugged" is kmem-specific - the rest map to MMOP_ */
+	if (sysfs_streq(buf, "unplugged"))
+		return DAX_KMEM_UNPLUGGED;
+
+	online_type = mhp_online_type_from_str(buf);
+	/* Disallow "offline": it's not useful and creates race conditions */
+	if (online_type == MMOP_OFFLINE)
+		return -EINVAL;
+	return online_type;
+}
+
+static ssize_t state_show(struct device *dev,
+			    struct device_attribute *attr, char *buf)
+{
+	struct dax_kmem_data *data = dev_get_drvdata(dev);
+	const char *state_str;
+
+	if (data->state == DAX_KMEM_UNPLUGGED)
+		state_str = "unplugged";
+	else
+		state_str = mhp_online_type_to_str(data->state);
+
+	return sysfs_emit(buf, "%s\n", state_str ?: "unknown");
+}
+
+static ssize_t state_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_kmem_data *data = dev_get_drvdata(dev);
+	int online_type;
+	int rc;
+
+	online_type = dax_kmem_parse_state(buf);
+	if (online_type < DAX_KMEM_UNPLUGGED)
+		return online_type;
+
+	guard(mutex)(&data->lock);
+
+	/* Already in requested state */
+	if (data->state == online_type)
+		return len;
+
+	if (online_type == DAX_KMEM_UNPLUGGED) {
+		rc = dax_kmem_do_hotremove(dev_dax, data);
+		if (rc)
+			return rc;
+		data->state = DAX_KMEM_UNPLUGGED;
+		return len;
+	}
+
+	/* Onlining is only allowed from the unplugged state. */
+	if (data->state != DAX_KMEM_UNPLUGGED)
+		return -EBUSY;
+
+	/* Re-acquire resources if previously unplugged, otherwise no-op */
+	rc = dax_kmem_init_resources(dev_dax, data);
+	if (rc < 0)
+		return rc;
+
+	rc = dax_kmem_do_hotplug(dev_dax, data, online_type);
+	if (rc < 0) {
+		/* Total failure, drop the reservations we took. */
+		dax_kmem_cleanup_resources(dev_dax, data);
+		return rc;
+	}
+
+	data->state = online_type;
+	return len;
+}
+
 static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 {
 	struct device *dev = &dev_dax->dev;
@@ -324,6 +448,8 @@ static int dev_dax_kmem_probe(struct dev
 	if (rc < 0)
 		goto err_reg_mgid;
 	data->mgid = rc;
+	data->state = DAX_KMEM_UNPLUGGED;
+	mutex_init(&data->lock);
 
 	dev_set_drvdata(dev, data);
 
@@ -334,6 +460,7 @@ static int dev_dax_kmem_probe(struct dev
 	rc = dax_kmem_do_hotplug(dev_dax, data, online_type);
 	if (rc < 0)
 		goto err_hotplug;
+	data->state = online_type;
 
 	return 0;
 
@@ -352,26 +479,59 @@ err_dax_kmem_data:
 }
 
 #ifdef CONFIG_MEMORY_HOTREMOVE
+/*
+ * Remove the device's added ranges with remove_memory().
+ * Unlike the sysfs unplug path it never offlines and fails if the blocks are
+ * online (-EBUSY), so it is safe from unbind. Failures leak until reboot.
+ *
+ * Returns 0 only if every added range was removed.
+ */
+static int dax_kmem_remove_ranges(struct dev_dax *dev_dax,
+				  struct dax_kmem_data *data)
+{
+	struct device *dev = &dev_dax->dev;
+	int i, rc = 0;
+
+	for (i = 0; i < dev_dax->nr_range; i++) {
+		struct range range;
+
+		if (!data->res[i] || dax_kmem_range(dev_dax, i, &range))
+			continue;
+		if (remove_memory(range.start, range_len(&range))) {
+			dev_warn(dev, "mapping%d: %#llx-%#llx stuck online until reboot\n",
+				 i, range.start, range.end);
+			rc = -EBUSY;
+			continue;
+		}
+		remove_resource(data->res[i]);
+		kfree(data->res[i]);
+		data->res[i] = NULL;
+	}
+	return rc;
+}
+
 static void dev_dax_kmem_remove(struct dev_dax *dev_dax)
 {
-	int success;
 	int node = dev_dax->target_node;
 	struct device *dev = &dev_dax->dev;
 	struct dax_kmem_data *data = dev_get_drvdata(dev);
 
 	/*
-	 * We have one shot for removing memory, if some memory blocks were not
-	 * offline prior to calling this function remove_memory() will fail, and
-	 * there is no way to hotremove this memory until reboot because device
-	 * unbind will succeed even if we return failure.
+	 * Remove every range that is still added.  dax_kmem_remove_ranges()
+	 * uses remove_memory(), which never offlines: an online block fails
+	 * with -EBUSY rather than deadlocking an uninterruptible unbind.
+	 *
+	 * data->state only tracks daxX.Y/state writes, so it can be stale if
+	 * blocks were toggled via memoryX/state. Do not trust it here and
+	 * attempt simply remove_memory() - which reports the true state of
+	 * each range anyway. Anything left online is leaked until reboot.
 	 */
-	success = dax_kmem_do_hotremove(dev_dax, data);
-	if (success < dev_dax->nr_range) {
+	if (dax_kmem_remove_ranges(dev_dax, data)) {
 		dev_err(dev, "Hotplug regions stuck online until reboot\n");
+		any_hotremove_failed = true;
 		return;
 	}
 
-	dax_kmem_cleanup_resources(dev_dax, data);
 	memory_group_unregister(data->mgid);
 	kfree(data->res_name);
 	kfree(data);
@@ -399,10 +559,21 @@ static void dev_dax_kmem_remove(struct d
 }
 #endif /* CONFIG_MEMORY_HOTREMOVE */
 
+static DEVICE_ATTR_RW(state);
+
+static struct attribute *dev_dax_kmem_attrs[] = {
+	&dev_attr_state.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(dev_dax_kmem);
+
 static struct dax_device_driver device_dax_kmem_driver = {
 	.probe = dev_dax_kmem_probe,
 	.remove = dev_dax_kmem_remove,
 	.type = DAXDRV_KMEM_TYPE,
+	.drv = {
+		.dev_groups = dev_dax_kmem_groups,
+	},
 };
 
 static int __init dax_kmem_init(void)
_

Patches currently in -mm which might be from gourry@gourry.net are

mm-vmstat-fold-stranded-per-cpu-node-stats-when-a-node-comes-online.patch
mm-constify-oom_control-scan_control-and-alloc_context-nodemask.patch
mm-mm_init-handle-alloc_percpu-failure-in-free_area_init_core_hotplug.patch
mm-memory-add-memory_block_aligned_range-helper.patch
mm-memory_hotplug-add-mhp_online_type_to_str-and-export-string-helpers.patch
mm-memory_hotplug-pass-online_type-to-online_memory_block-via-arg.patch
mm-memory_hotplug-export-mhp_get_default_online_type.patch
mm-memory_hotplug-add-__add_memory_driver_managed-with-online_type-arg.patch
mm-memory_hotplug-add-offline_and_remove_memory_ranges.patch
dax-kmem-resolve-default-online-type-at-probe-time.patch
dax-kmem-extract-hotplug-hotremove-helper-functions.patch
dax-kmem-add-sysfs-interface-for-atomic-whole-device-hotplug.patch
selftests-dax-add-dax-kmem-hotplug-sysfs-regression-test.patch