ARC: Notify dbuf cache about target size reduction

include/sys/dbuf.h

@@ -446,6 +446,7 @@ int dbuf_dnode_findbp(dnode_t *dn, uint64_t level, uint64_t blkid,
 
 void dbuf_init(void);
 void dbuf_fini(void);
+void dbuf_cache_reduce_target_size(void);
 
 boolean_t dbuf_is_metadata(dmu_buf_impl_t *db);
 

module/zfs/arc.c

@@ -297,6 +297,7 @@
 #include <sys/dsl_pool.h>
 #include <sys/multilist.h>
 #include <sys/abd.h>
+#include <sys/dbuf.h>
 #include <sys/zil.h>
 #include <sys/fm/fs/zfs.h>
 #include <sys/callb.h>
@@ -4555,6 +4556,13 @@ arc_reduce_target_size(uint64_t to_free)
 		to_free = 0;
 	}
 
+	/*
+	 * Since dbuf cache size is a fraction of target ARC size, we should
+	 * notify dbuf about the reduction, which might be significant,
+	 * especially if current ARC size was much smaller than the target.
+	 */
+	dbuf_cache_reduce_target_size();
+
 	/*
 	 * Whether or not we reduced the target size, request eviction if the
 	 * current size is over it now, since caller obviously wants some RAM.

module/zfs/dbuf.c

@@ -872,6 +872,19 @@ dbuf_evict_notify(uint64_t size)
 	}
 }
 
+/*
+ * Since dbuf cache size is a fraction of target ARC size, ARC calls this when
+ * its target size is reduced due to memory pressure.
+ */
+void
+dbuf_cache_reduce_target_size(void)
+{
+	uint64_t size = zfs_refcount_count(&dbuf_caches[DB_DBUF_CACHE].size);
+
+	if (size > dbuf_cache_target_bytes())
+		cv_signal(&dbuf_evict_cv);
+}
+
 static int
 dbuf_kstat_update(kstat_t *ksp, int rw)
 {