[PATCH v6 7/7] mm: Pause migrc mechanism at high memory pressure

[Byungchul Park]

Regression was observed when the system is in high memory pressure with
swap on, where migrc might keep a number of folios in its pending queue,
which possibly makes it worse. So temporarily prevented migrc from
working on that condition.

Signed-off-by: Byungchul Park <byungchul@xxxxxx>
---
 mm/internal.h   | 20 ++++++++++++++++++++
 mm/migrate.c    | 18 +++++++++++++++++-
 mm/page_alloc.c | 13 +++++++++++++
 3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/mm/internal.h b/mm/internal.h
index 5b93e5abf953..7643ccb7c156 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1285,6 +1285,8 @@ static inline void shrinker_debugfs_remove(struct dentry *debugfs_entry,
 #endif /* CONFIG_SHRINKER_DEBUG */
 
 #if defined(CONFIG_MIGRATION) && defined(CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH)
+extern atomic_t migrc_pause_cnt;
+
 /*
  * Reset the indicator indicating there are no writable mappings at the
  * beginning of every rmap traverse for unmap. Migrc can work only when
@@ -1313,6 +1315,21 @@ static inline bool can_migrc_test(void)
 	return current->can_migrc && current->tlb_ubc_ro.flush_required;
 }
 
+static inline void migrc_pause(void)
+{
+	atomic_inc(&migrc_pause_cnt);
+}
+
+static inline void migrc_resume(void)
+{
+	atomic_dec(&migrc_pause_cnt);
+}
+
+static inline bool migrc_paused(void)
+{
+	return !!atomic_read(&migrc_pause_cnt);
+}
+
 /*
  * Return the number of folios pending TLB flush that have yet to get
  * freed in the zone.
@@ -1330,6 +1347,9 @@ bool migrc_flush_free_folios(void);
 static inline void can_migrc_init(void) {}
 static inline void can_migrc_fail(void) {}
 static inline bool can_migrc_test(void) { return false; }
+static inline void migrc_pause(void) {}
+static inline void migrc_resume(void) {}
+static inline bool migrc_paused(void) { return false; }
 static inline int migrc_pending_nr_in_zone(struct zone *z) { return 0; }
 static inline bool migrc_flush_free_folios(void) { return false; }
 #endif
diff --git a/mm/migrate.c b/mm/migrate.c
index da4d43ac2622..b838f279f050 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -62,6 +62,12 @@ static struct tlbflush_unmap_batch migrc_ubc;
 static LIST_HEAD(migrc_folios);
 static DEFINE_SPINLOCK(migrc_lock);
 
+/*
+ * Increase on entry of handling high memory pressure e.g. direct
+ * reclaim, decrease on the exit. See __alloc_pages_slowpath().
+ */
+atomic_t migrc_pause_cnt = ATOMIC_INIT(0);
+
 static void init_tlb_ubc(struct tlbflush_unmap_batch *ubc)
 {
 	arch_tlbbatch_clear(&ubc->arch);
@@ -1922,7 +1928,8 @@ static int migrate_pages_batch(struct list_head *from,
 	 */
 	init_tlb_ubc(&pending_ubc);
 	do_migrc = IS_ENABLED(CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH) &&
-		   (reason == MR_DEMOTION || reason == MR_NUMA_MISPLACED);
+		   (reason == MR_DEMOTION || reason == MR_NUMA_MISPLACED) &&
+		   !migrc_paused();
 
 	for (pass = 0; pass < nr_pass && retry; pass++) {
 		retry = 0;
@@ -1961,6 +1968,15 @@ static int migrate_pages_batch(struct list_head *from,
 				continue;
 			}
 
+			/*
+			 * In case that the system is in high memory
+			 * pressure, give up migrc mechanism this turn.
+			 */
+			if (unlikely(do_migrc && migrc_paused())) {
+				fold_ubc(tlb_ubc, &pending_ubc);
+				do_migrc = false;
+			}
+
 			can_migrc_init();
 			rc = migrate_folio_unmap(get_new_folio, put_new_folio,
 					private, folio, &dst, mode, reason,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6ef0c22b1109..366777afce7f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4072,6 +4072,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	unsigned int cpuset_mems_cookie;
 	unsigned int zonelist_iter_cookie;
 	int reserve_flags;
+	bool migrc_paused = false;
 
 restart:
 	compaction_retries = 0;
@@ -4203,6 +4204,16 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	if (page)
 		goto got_pg;
 
+	/*
+	 * The system is in very high memory pressure. Pause migrc from
+	 * expanding its pending queue temporarily.
+	 */
+	if (!migrc_paused) {
+		migrc_pause();
+		migrc_paused = true;
+		migrc_flush_free_folios();
+	}
+
 	/* Caller is not willing to reclaim, we can't balance anything */
 	if (!can_direct_reclaim)
 		goto nopage;
@@ -4330,6 +4341,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	warn_alloc(gfp_mask, ac->nodemask,
 			"page allocation failure: order:%u", order);
 got_pg:
+	if (migrc_paused)
+		migrc_resume();
 	return page;
 }
 
-- 
2.17.1