[PATCH 1/2] mm/vmscan: add sync_shrinkers function

[Christian König]

To be able to switch to a spinlock and reduce lock contention in the TTM
shrinker we don't want to hold a mutex while unmapping and freeing pages
from the pool.

But then we somehow need to prevent a race between (for example) the shrinker
trying to free pages and hotplug trying to remove the device which those pages
belong to.

Taking and releasing the shrinker semaphore on the write side after
unmapping and freeing all pages should make sure that no shrinker is running in
paralell any more.

Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
 include/linux/shrinker.h |  1 +
 mm/vmscan.c              | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 0f80123650e2..6b75dc372fce 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -92,4 +92,5 @@ extern void register_shrinker_prepared(struct shrinker *shrinker);
 extern int register_shrinker(struct shrinker *shrinker);
 extern void unregister_shrinker(struct shrinker *shrinker);
 extern void free_prealloced_shrinker(struct shrinker *shrinker);
+extern void sync_shrinkers(void);
 #endif
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 562e87cbd7a1..46cd9c215d73 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -408,6 +408,16 @@ void unregister_shrinker(struct shrinker *shrinker)
 }
 EXPORT_SYMBOL(unregister_shrinker);
 
+/**
+ * sync_shrinker - Wait for all running shrinkers to complete.
+ */
+void sync_shrinkers(void)
+{
+	down_write(&shrinker_rwsem);
+	up_write(&shrinker_rwsem);
+}
+EXPORT_SYMBOL(sync_shrinkers);
+
 #define SHRINK_BATCH 128
 
 static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
-- 
2.25.1