zvol: Make Block IO DIRECT operations synchronous

Signed-off-by: Ameer Hamza <[email protected]>

include/os/linux/kernel/linux/blkdev_compat.h

@@ -500,6 +500,39 @@ bio_is_fua(struct bio *bio)
 #endif
 }
 
+/*
+ * 4.8 API,
+ *   REQ_SYNC flag moved to bio->bi_opf
+ *
+ * 2.6.x - 4.7 API,
+ *   REQ_SYNC
+ */
+static inline boolean_t
+bio_is_sync(struct bio *bio)
+{
+#if defined(HAVE_BIO_BI_OPF)
+	return (!!(bio->bi_opf & REQ_SYNC));
+#elif defined(REQ_SYNC)
+	return (!!(bio->bi_rw & REQ_SYNC));
+#else
+#error	"Allowing the build will cause sync requests to be ignored."
+#endif
+}
+
+/*
+ * 4.10 API,
+ *   REQ_IDLE flag replaced by REQ_NOIDLE
+ */
+static inline boolean_t
+bio_is_idle(struct bio *bio)
+{
+#if defined(HAVE_BIO_BI_OPF) && defined(REQ_IDLE)
+	return (!!(bio->bi_opf & REQ_IDLE));
+#else
+	return (0);
+#endif
+}
+
 /*
  * 4.8 API,
  *   REQ_OP_DISCARD
@@ -778,4 +811,28 @@ io_has_data(struct bio *bio, struct request *rq)
 #endif
 	return (bio_has_data(bio));
 }
+
+static inline int
+io_is_sync(struct bio *bio, struct request *rq)
+{
+#ifdef HAVE_BLK_MQ
+	if (rq != NULL)
+		return (!!(rq->cmd_flags & REQ_SYNC));
+#else
+	ASSERT3P(rq, ==, NULL);
+#endif
+	return (bio_is_sync(bio));
+}
+
+static inline int
+io_is_idle(struct bio *bio, struct request *rq)
+{
+#ifdef HAVE_BLK_MQ
+	if (rq != NULL)
+		return (!!(rq->cmd_flags & REQ_IDLE));
+#else
+	ASSERT3P(rq, ==, NULL);
+#endif
+	return (bio_is_idle(bio));
+}
 #endif /* _ZFS_BLKDEV_H */

module/os/linux/zfs/zvol_os.c

@@ -518,9 +518,22 @@ zvol_request_impl(zvol_state_t *zv, struct bio *bio, struct request *rq,
 	uint64_t offset = io_offset(bio, rq);
 	uint64_t size = io_size(bio, rq);
 	int rw = io_data_dir(bio, rq);
+	int is_sync_io = io_is_sync(bio, rq);
+	int is_idle_io = io_is_idle(bio, rq);
 
-	if (zvol_request_sync)
+	/*
+	 * For Direct IO Writes, the block layer sets both the REQ_IDLE
+	 * and REQ_SYNC flags (see dio_bio_write_op). For Direct IO
+	 * Reads (up to 1MB IO size), REQ_SYNC is set by
+	 * submit_bio_wait(). In both cases, since the caller already
+	 * waits for the bio to complete, we should force these
+	 * operations to be synchronous to avoid additional threading
+	 * overhead.
+	 */
+	if (zvol_request_sync || (rw == READ && is_sync_io) ||
+	    (rw == WRITE && is_sync_io && is_idle_io)) {
 		force_sync = 1;
+	}
 
 	zv_request_t zvr = {
 		.zv = zv,