Enable tuning of ZVOL open timeout value

The default timeout for ZVOL opens may not be sufficient for all cases,
so we should enable the value to be more easily tuned to account for
systems where the default value is insufficient.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Matthew Ahrens <[email protected]>
Signed-off-by: Prakash Surya <[email protected]>
Closes #15023

module/os/linux/zfs/zvol_os.c

@@ -54,7 +54,7 @@ static unsigned int zvol_prefetch_bytes = (128 * 1024);
 static unsigned long zvol_max_discard_blocks = 16384;
 
 #ifndef HAVE_BLKDEV_GET_ERESTARTSYS
-static const unsigned int zvol_open_timeout_ms = 1000;
+static unsigned int zvol_open_timeout_ms = 1000;
 #endif
 
 static unsigned int zvol_threads = 0;
@@ -1612,4 +1612,9 @@ MODULE_PARM_DESC(zvol_blk_mq_blocks_per_thread,
     "Process volblocksize blocks per thread");
 #endif
 
+#ifndef HAVE_BLKDEV_GET_ERESTARTSYS
+module_param(zvol_open_timeout_ms, uint, 0644);
+MODULE_PARM_DESC(zvol_open_timeout_ms, "Timeout for ZVOL open retries");
+#endif
+
 /* END CSTYLED */