ARC: Notify dbuf cache about target size reduction

ARC target size might drop significantly under memory pressure,
especially if current ARC size was much smaller than the target.
Since dbuf cache size is a fraction of the target ARC size, it
might need eviction too.  Aside of memory from the dbuf eviction
itself, it might help ARC by making more buffers evictable.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #17314

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)
 {