[PATCH 4/8] mm/memory_hotplug: export mhp_get_default_online_type

Gregory Price
2026-03-21T15:04:16Z

Drivers which may pass hotplug policy down to DAX need MMOP_ symbols
and the mhp_get_default_online_type function for hotplug use cases.

Some drivers (cxl) co-mingle their hotplug and devdax use-cases into
the same driver code, and chose the dax_kmem path as the default driver
path - making it difficult to require hotplug as a predicate to building
the overall driver (it may break other non-hotplug use-cases).

Export mhp_get_default_online_type function to allow these drivers to
build when hotplug is disabled and still use the DAX use case.

In the built-out case we simply return MMOP_OFFLINE as it's
non-destructive. The internal function can never return -1 either,
so we choose this to allow for defining the function with 'enum mmop'.

Signed-off-by: Gregory Price <gourry@gourry.net>
---
include/linux/memory_hotplug.h | 29 +++++++++++++++++++++++++++++
mm/memory_hotplug.c | 1 +
2 files changed, 30 insertions(+)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index e77ef3d7ff73..a8bcb36f93b8 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -6,6 +6,7 @@
#include <linux/spinlock.h>
#include <linux/notifier.h>
#include <linux/bug.h>
+#include <linux/errno.h>

struct page;
struct zone;
@@ -28,6 +29,27 @@ enum mmop {
MMOP_ONLINE_MOVABLE,
};

+/**
+ * mmop_to_str - convert memory online type to string
+ * @online_type: the MMOP_* value to convert
+ *
+ * Returns a string representation of the memory online type,
+ * suitable for sysfs output (includes trailing newline).
+ */
+static inline const char *mmop_to_str(enum mmop online_type)
+{
+ switch (online_type) {
+ case MMOP_ONLINE:
+ return "online\n";
+ case MMOP_ONLINE_KERNEL:
+ return "online_kernel\n";
+ case MMOP_ONLINE_MOVABLE:
+ return "online_movable\n";
+ default:
+ return "offline\n";
+ }
+}
+
#ifdef CONFIG_MEMORY_HOTPLUG
struct page *pfn_to_online_page(unsigned long pfn);

@@ -221,6 +243,11 @@ static inline bool mhp_supports_memmap_on_memory(void)
static inline void pgdat_kswapd_lock(pg_data_t *pgdat) {}
static inline void pgdat_kswapd_unlock(pg_data_t *pgdat) {}
static inline void pgdat_kswapd_lock_init(pg_data_t *pgdat) {}
+
+static inline int mhp_online_type_from_str(const char *str)
+{
+ return -EOPNOTSUPP;
+}
#endif /* ! CONFIG_MEMORY_HOTPLUG */

/*
@@ -316,6 +343,8 @@ extern struct zone *zone_for_pfn_range(enum mmop online_type,
extern int arch_create_linear_mapping(int nid, u64 start, u64 size,
struct mhp_params *params);
void arch_remove_linear_mapping(u64 start, u64 size);
+#else
+static inline enum mmop mhp_get_default_online_type(void) { return MMOP_OFFLINE; }
#endif /* CONFIG_MEMORY_HOTPLUG */

#endif /* __LINUX_MEMORY_HOTPLUG_H */
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 282bf3d89613..af9a6cb5a2f9 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -240,6 +240,7 @@ enum mmop 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(enum mmop online_type)
{
--
2.53.0