[v3 PATCH 05/11] mm: vmscan: use a new flag to indicate shrinker is registered

[Yang Shi <shy828301@xxxxxxxxx>]

Currently registered shrinker is indicated by non-NULL shrinker->nr_deferred.
This approach is fine with nr_deferred at the shrinker level, but the following
patches will move MEMCG_AWARE shrinkers' nr_deferred to memcg level, so their
shrinker->nr_deferred would always be NULL.  This would prevent the shrinkers
from unregistering correctly.

Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
 include/linux/shrinker.h |  7 ++++---
 mm/vmscan.c              | 13 +++++++++----
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 0f80123650e2..1eac79ce57d4 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -79,13 +79,14 @@ struct shrinker {
 #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
 
 /* Flags */
-#define SHRINKER_NUMA_AWARE	(1 << 0)
-#define SHRINKER_MEMCG_AWARE	(1 << 1)
+#define SHRINKER_REGISTERED	(1 << 0)
+#define SHRINKER_NUMA_AWARE	(1 << 1)
+#define SHRINKER_MEMCG_AWARE	(1 << 2)
 /*
  * It just makes sense when the shrinker is also MEMCG_AWARE for now,
  * non-MEMCG_AWARE shrinker should not have this flag set.
  */
-#define SHRINKER_NONSLAB	(1 << 2)
+#define SHRINKER_NONSLAB	(1 << 3)
 
 extern int prealloc_shrinker(struct shrinker *shrinker);
 extern void register_shrinker_prepared(struct shrinker *shrinker);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8da765a85569..9761c7c27412 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -494,6 +494,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
 		idr_replace(&shrinker_idr, shrinker, shrinker->id);
 #endif
+	shrinker->flags |= SHRINKER_REGISTERED;
 	up_write(&shrinker_rwsem);
 }
 
@@ -513,13 +514,17 @@ EXPORT_SYMBOL(register_shrinker);
  */
 void unregister_shrinker(struct shrinker *shrinker)
 {
-	if (!shrinker->nr_deferred)
-		return;
-	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
-		unregister_memcg_shrinker(shrinker);
 	down_write(&shrinker_rwsem);
+	if (!(shrinker->flags & SHRINKER_REGISTERED)) {
+		up_write(&shrinker_rwsem);
+		return;
+	}
 	list_del(&shrinker->list);
+	shrinker->flags &= ~SHRINKER_REGISTERED;
 	up_write(&shrinker_rwsem);
+
+	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
+		unregister_memcg_shrinker(shrinker);
 	kfree(shrinker->nr_deferred);
 	shrinker->nr_deferred = NULL;
 }
-- 
2.26.2