[PATCH 2/2] mm: introduce CONFIG_NUMA_MIGRATION and simplify CONFIG_MIGRATION

David Hildenbrand (Arm)
2026-03-19T08:20:45Z

CONFIG_MEMORY_HOTREMOVE, CONFIG_COMPACTION and CONFIG_CMA all select
CONFIG_MIGRATION, because they require it to work (users).

Only CONFIG_NUMA_BALANCING and CONFIG_BALLOON_MIGRATION depend on
CONFIG_MIGRATION. CONFIG_BALLOON_MIGRATION is not an actual user, but
an implementation of migration support, so the dependency is correct
(CONFIG_BALLOON_MIGRATION does not make any sense without
CONFIG_MIGRATION).

However, kconfig-language.rst clearly states "In general use select only
for non-visible symbols". So far CONFIG_MIGRATION is user-visible ...
and the dependencies rather confusing.

The whole reason why CONFIG_MIGRATION is user-visible is because of
CONFIG_NUMA: some users might want CONFIG_NUMA but not page migration
support.

Let's clean all that up by introducing a dedicated CONFIG_NUMA_MIGRATION
config option for that purpose only. Make CONFIG_NUMA_BALANCING that so
far depended on CONFIG_NUMA && CONFIG_MIGRATION to depend on
CONFIG_MIGRATION instead. CONFIG_NUMA_MIGRATION will depend on
CONFIG_NUMA && CONFIG_MMU.

CONFIG_NUMA_MIGRATION is user-visible and will default to "y". We
use that default so new configs will automatically enable it, just
like it was the case with CONFIG_MIGRATION. The downside is that
some configs that used to have CONFIG_MIGRATION=n might get it
re-enabled by CONFIG_NUMA_MIGRATION=y, which shouldn't be a problem.

CONFIG_MIGRATION is now a non-visible config option. Any code that
select CONFIG_MIGRATION (as before) must depend directly or indirectly
on CONFIG_MMU.

CONFIG_NUMA_MIGRATION is responsible for any NUMA migration code, which is
mempolicy migration code, memory-tiering code, and move_pages() code in
migrate.c. CONFIG_NUMA_BALANCING uses its functionality.

Note that this implies that with CONFIG_NUMA_MIGRATION=n, move_pages() will
not be available even though CONFIG_MIGRATION=y, which is an expected
change.

In migrate.c, we can remove the CONFIG_NUMA check as both
CONFIG_NUMA_MIGRATION and CONFIG_NUMA_BALANCING depend on it.

With this change, CONFIG_MIGRATION is an internal config, all users of
migration selects CONFIG_MIGRATION, and only CONFIG_BALLOON_MIGRATION
depends on it.

Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
include/linux/memory-tiers.h | 2 +-
init/Kconfig | 2 +-
mm/Kconfig | 26 +++++++++++++-------------
mm/memory-tiers.c | 12 ++++++------
mm/mempolicy.c | 2 +-
mm/migrate.c | 5 ++---
6 files changed, 24 insertions(+), 25 deletions(-)

diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
index 96987d9d95a8..7999c58629ee 100644
--- a/include/linux/memory-tiers.h
+++ b/include/linux/memory-tiers.h
@@ -52,7 +52,7 @@ int mt_perf_to_adistance(struct access_coordinate *perf, int *adist);
struct memory_dev_type *mt_find_alloc_memory_type(int adist,
struct list_head *memory_types);
void mt_put_memory_types(struct list_head *memory_types);
-#ifdef CONFIG_MIGRATION
+#ifdef CONFIG_NUMA_MIGRATION
int next_demotion_node(int node, const nodemask_t *allowed_mask);
void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets);
bool node_is_toptier(int node);
diff --git a/init/Kconfig b/init/Kconfig
index 444ce811ea67..3648e401b78b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -997,7 +997,7 @@ config NUMA_BALANCING
bool "Memory placement aware NUMA scheduler"
depends on ARCH_SUPPORTS_NUMA_BALANCING
depends on !ARCH_WANT_NUMA_VARIABLE_LOCALITY
- depends on SMP && NUMA && MIGRATION && !PREEMPT_RT
+ depends on SMP && NUMA_MIGRATION && !PREEMPT_RT
help
This option adds support for automatic NUMA aware memory/task placement.
The mechanism is quite primitive and is based on migrating memory when
diff --git a/mm/Kconfig b/mm/Kconfig
index b2e21d873d3f..bd283958d675 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -627,20 +627,20 @@ config PAGE_REPORTING
those pages to another entity, such as a hypervisor, so that the
memory can be freed within the host for other uses.

-#
-# support for page migration
-#
-config MIGRATION
- bool "Page migration"
+config NUMA_MIGRATION
+ bool "NUMA page migration"
default y
- depends on (NUMA || MEMORY_HOTREMOVE || COMPACTION || CMA) && MMU
- help
- Allows the migration of the physical location of pages of processes
- while the virtual addresses are not changed. This is useful in
- two situations. The first is on NUMA systems to put pages nearer
- to the processors accessing. The second is when allocating huge
- pages as migration can relocate pages to satisfy a huge page
- allocation instead of reclaiming.
+ depends on NUMA && MMU
+ select MIGRATION
+ help
+ Support the migration of pages to other NUMA nodes, available to
+ user space through interfaces like migrate_pages(), move_pages(),
+ and mbind(). Selecting this option also enables support for page
+ demotion for memory tiering.
+
+config MIGRATION
+ bool
+ depends on MMU

config DEVICE_MIGRATION
def_bool MIGRATION && ZONE_DEVICE
diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index 986f809376eb..54851d8a195b 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -69,7 +69,7 @@ bool folio_use_access_time(struct folio *folio)
}
#endif

-#ifdef CONFIG_MIGRATION
+#ifdef CONFIG_NUMA_MIGRATION
static int top_tier_adistance;
/*
* node_demotion[] examples:
@@ -129,7 +129,7 @@ static int top_tier_adistance;
*
*/
static struct demotion_nodes *node_demotion __read_mostly;
-#endif /* CONFIG_MIGRATION */
+#endif /* CONFIG_NUMA_MIGRATION */

static BLOCKING_NOTIFIER_HEAD(mt_adistance_algorithms);

@@ -273,7 +273,7 @@ static struct memory_tier *__node_get_memory_tier(int node)
lockdep_is_held(&memory_tier_lock));
}

-#ifdef CONFIG_MIGRATION
+#ifdef CONFIG_NUMA_MIGRATION
bool node_is_toptier(int node)
{
bool toptier;
@@ -519,7 +519,7 @@ static void establish_demotion_targets(void)

#else
static inline void establish_demotion_targets(void) {}
-#endif /* CONFIG_MIGRATION */
+#endif /* CONFIG_NUMA_MIGRATION */

static inline void __init_node_memory_type(int node, struct memory_dev_type *memtype)
{
@@ -911,7 +911,7 @@ static int __init memory_tier_init(void)
if (ret)
panic("%s() failed to register memory tier subsystem\n", __func__);

-#ifdef CONFIG_MIGRATION
+#ifdef CONFIG_NUMA_MIGRATION
node_demotion = kzalloc_objs(struct demotion_nodes, nr_node_ids);
WARN_ON(!node_demotion);
#endif
@@ -938,7 +938,7 @@ subsys_initcall(memory_tier_init);

bool numa_demotion_enabled = false;

-#ifdef CONFIG_MIGRATION
+#ifdef CONFIG_NUMA_MIGRATION
#ifdef CONFIG_SYSFS
static ssize_t demotion_enabled_show(struct kobject *kobj,
struct kobj_attribute *attr, char *buf)
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index e5528c35bbb8..fd08771e2057 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1239,7 +1239,7 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
return err;
}

-#ifdef CONFIG_MIGRATION
+#ifdef CONFIG_NUMA_MIGRATION
static bool migrate_folio_add(struct folio *folio, struct list_head *foliolist,
unsigned long flags)
{
diff --git a/mm/migrate.c b/mm/migrate.c
index fdbb20163f66..05cb408846f2 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2224,8 +2224,7 @@ struct folio *alloc_migration_target(struct folio *src, unsigned long private)
return __folio_alloc(gfp_mask, order, nid, mtc->nmask);
}

-#ifdef CONFIG_NUMA
-
+#ifdef CONFIG_NUMA_MIGRATION
static int store_status(int __user *status, int start, int value, int nr)
{
while (nr-- > 0) {
@@ -2624,6 +2623,7 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,
{
return kernel_move_pages(pid, nr_pages, pages, nodes, status, flags);
}
+#endif /* CONFIG_NUMA_MIGRATION */

#ifdef CONFIG_NUMA_BALANCING
/*
@@ -2766,4 +2766,3 @@ int migrate_misplaced_folio(struct folio *folio, int node)
return nr_remaining ? -EAGAIN : 0;
}
#endif /* CONFIG_NUMA_BALANCING */
-#endif /* CONFIG_NUMA */

--
2.43.0