[PATCH 4/9] 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 atthe 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.

Introduce a new "state" field to indicate if shrinker is registered or not.
We could use the highest bit of flags, but it may be a little bit complicated to
extract that bit and the flags is accessed frequently by vmscan (every time shrinker
is called).  So add a new field in "struct shrinker", we may waster a little bit
memory, but it should be very few since there should be not too many registered
shrinkers on a normal system.

Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
 include/linux/shrinker.h |  4 ++++
 mm/vmscan.c              | 13 +++++++++----
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 0f80123650e2..0bb5be88e41d 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -35,6 +35,9 @@ struct shrink_control {
 
 #define SHRINK_STOP (~0UL)
 #define SHRINK_EMPTY (~0UL - 1)
+
+#define SHRINKER_REGISTERED	0x1
+
 /*
  * A callback you can register to apply pressure to ageable caches.
  *
@@ -66,6 +69,7 @@ struct shrinker {
 	long batch;	/* reclaim batch size, 0 = default */
 	int seeks;	/* seeks to recreate an obj */
 	unsigned flags;
+	unsigned state;
 
 	/* These are for internal use */
 	struct list_head list;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 457ce04eebf2..0d628299e55c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -378,6 +378,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
 		idr_replace(&shrinker_idr, shrinker, shrinker->id);
 #endif
+	shrinker->state |= SHRINKER_REGISTERED;
 	up_write(&shrinker_rwsem);
 }
 
@@ -397,13 +398,17 @@ 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->state) {
+		up_write(&shrinker_rwsem);
+		return;
+	}
 	list_del(&shrinker->list);
+	shrinker->state &= ~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