drivers/dax/Kconfig | 18 ++++++++++++++++++ drivers/dax/kmem.c | 29 ++++++++++++++++++++--------- 2 files changed, 38 insertions(+), 9 deletions(-)
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@intel.com>
Signed-off-by: Gregory Price <gourry@gourry.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
--
2.52.0On Wed, 14 Jan 2026 21:42:22 -0500 Gregory Price <gourry@gourry.net> wrote: > 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. > > --- 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 Users must rebuild and redeploy kernels after having updated a userspace tool. They won't thank us for this ;) Isn't there something we can do to make this feature backward-compatible?
On Tue, Jan 27, 2026 at 01:34:31PM -0800, Andrew Morton wrote: > On Wed, 14 Jan 2026 21:42:22 -0500 Gregory Price <gourry@gourry.net> wrote: > > > 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. > > > > --- 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 > > Users must rebuild and redeploy kernels after having updated a > userspace tool. They won't thank us for this ;) > > Isn't there something we can do to make this feature > backward-compatible? > This feature is likely getting dropped in favor of pushing such policy to a driver if it cares that much to prevent users toggling memory blocks. I will likely re-spin this series in a week or so when other non-mm changes flesh out a little clearer. This will be removed and some of the mm/memory-hotplug.c changes will be changed to prevent the modification of an already extern'd function. ~Gregory
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