linux-kernel - [PATCH] dax/kmem: add build config for protected dax memory blocks

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Message-ID: <20260115024222.3486455-1-gourry@gourry.net>
Date: Wed, 14 Jan 2026 21:42:22 -0500
From: Gregory Price <gourry@...rry.net>
To: linux-mm@...ck.org
Cc: linux-cxl@...r.kernel.org,
	nvdimm@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux.dev,
	kernel-team@...a.com,
	dan.j.williams@...el.com,
	vishal.l.verma@...el.com,
	dave.jiang@...el.com,
	david@...nel.org,
	mst@...hat.com,
	jasowang@...hat.com,
	xuanzhuo@...ux.alibaba.com,
	eperezma@...hat.com,
	osalvador@...e.de,
	akpm@...ux-foundation.org
Subject: [PATCH] dax/kmem: add build config for protected dax memory blocks

Since this protection may break userspace tools, it should
be an opt-in until those tools have time to update to the
new daxN.M/hotplug interface instead of memory blocks.

Suggested-by: Dan Williams <dan.j.williams@...el.com>
Signed-off-by: Gregory Price <gourry@...rry.net>
---
 drivers/dax/Kconfig | 18 ++++++++++++++++++
 drivers/dax/kmem.c  | 29 ++++++++++++++++++++---------
 2 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index d656e4c0eb84..cc13c22eb8f8 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -78,4 +78,22 @@ config DEV_DAX_KMEM
 
 	  Say N if unsure.
 
+config DEV_DAX_KMEM_PROTECTED
+	bool "Protect DAX_KMEM memory blocks being changed"
+	depends on DEV_DAX_KMEM
+	default n
+	help
+	  Prevents actions from outside the KMEM DAX driver from changing
+	  DAX KMEM memory block states. For example, the memory block
+	  sysfs functions (online, state) will return -EBUSY, and normal
+	  calls to memory_hotplug functions from other drivers and kernel
+	  sources will fail.
+
+	  This may break existing memory block management patterns that
+	  depend on offlining DAX KMEM blocks from userland before unbinding
+	  the driver.  Use this only if your tools have been updated to use
+	  the daxN.M/hotplug interface.
+
+	  Say N if unsure.
+
 endif
diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
index f3562f65376c..094b8a51099e 100644
--- a/drivers/dax/kmem.c
+++ b/drivers/dax/kmem.c
@@ -184,6 +184,21 @@ static int dax_kmem_memory_notifier_cb(struct notifier_block *nb,
 	return NOTIFY_BAD;
 }
 
+static int dax_kmem_register_notifier(struct dax_kmem_data *data)
+{
+	if (!IS_ENABLED(DEV_DAX_KMEM_PROTECTED))
+		return 0;
+	data->mem_nb.notifier_call = dax_kmem_memory_notifier_cb;
+	return register_memory_notifier(&data->mem_nb);
+}
+
+static void dax_kmem_unregister_notifier(struct dax_kmem_data *data)
+{
+	if (!IS_ENABLED(DEV_DAX_KMEM_PROTECTED))
+		return;
+	unregister_memory_notifier(&data->mem_nb);
+}
+
 /**
  * dax_kmem_do_hotplug - hotplug memory for dax kmem device
  * @dev_dax: the dev_dax instance
@@ -563,13 +578,9 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 	if (rc < 0)
 		goto err_resources;
 
-	/* Register memory notifier to block external operations */
-	data->mem_nb.notifier_call = dax_kmem_memory_notifier_cb;
-	rc = register_memory_notifier(&data->mem_nb);
-	if (rc) {
-		dev_warn(dev, "failed to register memory notifier\n");
+	rc = dax_kmem_register_notifier(data);
+	if (rc)
 		goto err_notifier;
-	}
 
 	/*
 	 * Hotplug using the system default policy - this preserves backwards
@@ -595,7 +606,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 	return 0;
 
 err_hotplug:
-	unregister_memory_notifier(&data->mem_nb);
+	dax_kmem_unregister_notifier(data);
 err_notifier:
 	dax_kmem_cleanup_resources(dev_dax, data);
 err_resources:
@@ -619,7 +630,7 @@ static void dev_dax_kmem_remove(struct dev_dax *dev_dax)
 
 	device_remove_file(dev, &dev_attr_hotplug);
 	dax_kmem_cleanup_resources(dev_dax, data);
-	unregister_memory_notifier(&data->mem_nb);
+	dax_kmem_unregister_notifier(data);
 	memory_group_unregister(data->mgid);
 	kfree(data->res_name);
 	kfree(data);
@@ -640,7 +651,7 @@ static void dev_dax_kmem_remove(struct dev_dax *dev_dax)
 	struct dax_kmem_data *data = dev_get_drvdata(dev);
 
 	device_remove_file(dev, &dev_attr_hotplug);
-	unregister_memory_notifier(&data->mem_nb);
+	dax_kmem_unregister_notifier(data);
 
 	/*
 	 * Without hotremove purposely leak the request_mem_region() for the
-- 
2.52.0