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

["David Hildenbrand (Arm)" <david@kernel.org>]

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