Skip dbuf_evict_one() from dbuf_evict_notify() for kswapd

Avoid kswapd thread hang from hash lock contention under high memory
pressure in this code path:
dbuf_evict_one -> dbuf_destroy -> arc_buf_destroy

Signed-off-by: Kaitlin Hoang <kthoang@amazon.com>

Author: khoang98

module/zfs/dbuf.c

@@ -867,7 +867,21 @@ dbuf_evict_notify(uint64_t size)
 	 */
 	if (size > dbuf_cache_target_bytes()) {
 		if (size > dbuf_cache_hiwater_bytes())
+#if defined(_KERNEL)
+			/*
+			 * We saw a case where kswapd thread was hung
+			 * because it wasn't able to grab the hash lock
+			 * in this call sequence:
+			 * dbuf_evict_one -> dbuf_destroy -> arc_buf_destroy
+			 *
+			 * In order to avoid this, don't have kswapd evict
+			 * from dbuf cache.
+			 */
+			if (!current_is_kswapd())
+				dbuf_evict_one();
+#else
 			dbuf_evict_one();
+#endif
 		cv_signal(&dbuf_evict_cv);
 	}
 }