[PATCH 2/9] mm/memory_hotplug: add __add_memory_driver_managed() with online_type arg

Gregory Price
2026-01-29T21:04:54Z

Enable dax kmem driver to select how to online the memory rather than
implicitly depending on the system default. This will allow users of
dax to plumb through a preferred auto-online policy for their region.

Refactor and new interface:
Add __add_memory_driver_managed() which accepts an explicit online_type
and export mhp_get_default_online_type() so callers can pass it when
they want the default behavior.

Refactor:
Extract __add_memory_resource() to take an explicit online_type parameter,
and update add_memory_resource() to pass the system default.

No functional change for existing users.

Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Gregory Price <gourry@gourry.net>
---
include/linux/memory_hotplug.h | 3 ++
mm/memory_hotplug.c | 91 ++++++++++++++++++++++++----------
2 files changed, 67 insertions(+), 27 deletions(-)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index f2f16cdd73ee..1eb63d1a247d 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -293,6 +293,9 @@ extern int __add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags);
extern int add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags);
extern int add_memory_resource(int nid, struct resource *resource,
mhp_t mhp_flags);
+int __add_memory_driver_managed(int nid, u64 start, u64 size,
+ const char *resource_name, mhp_t mhp_flags,
+ int online_type);
extern int add_memory_driver_managed(int nid, u64 start, u64 size,
const char *resource_name,
mhp_t mhp_flags);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 87796b617d9e..d3ca95b872bd 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -239,6 +239,7 @@ int mhp_get_default_online_type(void)

return mhp_default_online_type;
}
+EXPORT_SYMBOL_GPL(mhp_get_default_online_type);

void mhp_set_default_online_type(int online_type)
{
@@ -1490,7 +1491,8 @@ static int create_altmaps_and_memory_blocks(int nid, struct memory_group *group,
*
* we are OK calling __meminit stuff here - we have CONFIG_MEMORY_HOTPLUG
*/
-int add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
+static int __add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags,
+ int online_type)
{
struct mhp_params params = { .pgprot = pgprot_mhp(PAGE_KERNEL) };
enum memblock_flags memblock_flags = MEMBLOCK_NONE;
@@ -1580,12 +1582,9 @@ int add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
merge_system_ram_resource(res);

/* online pages if requested */
- if (mhp_get_default_online_type() != MMOP_OFFLINE) {
- int online_type = mhp_get_default_online_type();
-
+ if (online_type != MMOP_OFFLINE)
walk_memory_blocks(start, size, &online_type,
online_memory_block);
- }

return ret;
error:
@@ -1601,7 +1600,13 @@ int add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
return ret;
}

-/* requires device_hotplug_lock, see add_memory_resource() */
+int add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
+{
+ return __add_memory_resource(nid, res, mhp_flags,
+ mhp_get_default_online_type());
+}
+
+/* requires device_hotplug_lock, see __add_memory_resource() */
int __add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags)
{
struct resource *res;
@@ -1629,29 +1634,24 @@ int add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags)
}
EXPORT_SYMBOL_GPL(add_memory);

-/*
- * Add special, driver-managed memory to the system as system RAM. Such
- * memory is not exposed via the raw firmware-provided memmap as system
- * RAM, instead, it is detected and added by a driver - during cold boot,
- * after a reboot, and after kexec.
- *
- * Reasons why this memory should not be used for the initial memmap of a
- * kexec kernel or for placing kexec images:
- * - The booting kernel is in charge of determining how this memory will be
- * used (e.g., use persistent memory as system RAM)
- * - Coordination with a hypervisor is required before this memory
- * can be used (e.g., inaccessible parts).
+/**
+ * __add_memory_driver_managed - add driver-managed memory with explicit online_type
+ * @nid: NUMA node ID where the memory will be added
+ * @start: Start physical address of the memory range
+ * @size: Size of the memory range in bytes
+ * @resource_name: Resource name in format "System RAM ($DRIVER)"
+ * @mhp_flags: Memory hotplug flags
+ * @online_type: Online behavior (MMOP_ONLINE, MMOP_ONLINE_KERNEL,
+ * MMOP_ONLINE_MOVABLE, or MMOP_OFFLINE)
*
- * For this memory, no entries in /sys/firmware/memmap ("raw firmware-provided
- * memory map") are created. Also, the created memory resource is flagged
- * with IORESOURCE_SYSRAM_DRIVER_MANAGED, so in-kernel users can special-case
- * this memory as well (esp., not place kexec images onto it).
+ * Add driver-managed memory with explicit online_type specification.
+ * The resource_name must have the format "System RAM ($DRIVER)".
*
- * The resource_name (visible via /proc/iomem) has to have the format
- * "System RAM ($DRIVER)".
+ * Return: 0 on success, negative error code on failure.
*/
-int add_memory_driver_managed(int nid, u64 start, u64 size,
- const char *resource_name, mhp_t mhp_flags)
+int __add_memory_driver_managed(int nid, u64 start, u64 size,
+ const char *resource_name, mhp_t mhp_flags,
+ int online_type)
{
struct resource *res;
int rc;
@@ -1661,6 +1661,9 @@ int add_memory_driver_managed(int nid, u64 start, u64 size,
resource_name[strlen(resource_name) - 1] != ')')
return -EINVAL;

+ if (online_type < 0 || online_type > MMOP_ONLINE_MOVABLE)
+ return -EINVAL;
+
lock_device_hotplug();

res = register_memory_resource(start, size, resource_name);
@@ -1669,7 +1672,7 @@ int add_memory_driver_managed(int nid, u64 start, u64 size,
goto out_unlock;
}

- rc = add_memory_resource(nid, res, mhp_flags);
+ rc = __add_memory_resource(nid, res, mhp_flags, online_type);
if (rc < 0)
release_memory_resource(res);

@@ -1677,6 +1680,40 @@ int add_memory_driver_managed(int nid, u64 start, u64 size,
unlock_device_hotplug();
return rc;
}
+EXPORT_SYMBOL_FOR_MODULES(__add_memory_driver_managed, "kmem");
+
+/*
+ * Add special, driver-managed memory to the system as system RAM. Such
+ * memory is not exposed via the raw firmware-provided memmap as system
+ * RAM, instead, it is detected and added by a driver - during cold boot,
+ * after a reboot, and after kexec.
+ *
+ * Reasons why this memory should not be used for the initial memmap of a
+ * kexec kernel or for placing kexec images:
+ * - The booting kernel is in charge of determining how this memory will be
+ * used (e.g., use persistent memory as system RAM)
+ * - Coordination with a hypervisor is required before this memory
+ * can be used (e.g., inaccessible parts).
+ *
+ * For this memory, no entries in /sys/firmware/memmap ("raw firmware-provided
+ * memory map") are created. Also, the created memory resource is flagged
+ * with IORESOURCE_SYSRAM_DRIVER_MANAGED, so in-kernel users can special-case
+ * this memory as well (esp., not place kexec images onto it).
+ *
+ * The resource_name (visible via /proc/iomem) has to have the format
+ * "System RAM ($DRIVER)".
+ *
+ * Memory will be onlined using the system default online type.
+ *
+ * Returns 0 on success, negative error code on failure.
+ */
+int add_memory_driver_managed(int nid, u64 start, u64 size,
+ const char *resource_name, mhp_t mhp_flags)
+{
+ return __add_memory_driver_managed(nid, start, size, resource_name,
+ mhp_flags,
+ mhp_get_default_online_type());
+}
EXPORT_SYMBOL_GPL(add_memory_driver_managed);

/*
--
2.52.0