Merge tag 'md/3.17-final-fix' of git://neil.brown.name/md

Pull raid5 discard fix from Neil Brown:
 "One fix for raid5 discard issue"

* tag 'md/3.17-final-fix' of git://neil.brown.name/md:
  md/raid5: disable 'DISCARD' by default due to safety concerns.

1 files changed, 17 insertions, 1 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 183588b11fc1..9f0fbecd1eb5 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -64,6 +64,10 @@
 #define cpu_to_group(cpu) cpu_to_node(cpu)
 #define ANY_GROUP NUMA_NO_NODE
 
+static bool devices_handle_discard_safely = false;
+module_param(devices_handle_discard_safely, bool, 0644);
+MODULE_PARM_DESC(devices_handle_discard_safely,
+                 "Set to Y if all devices in each array reliably return zeroes on reads from discarded regions");
 static struct workqueue_struct *raid5_wq;
 /*
  * Stripe cache
@@ -6208,7 +6212,7 @@ static int run(struct mddev *mddev)
                 mddev->queue->limits.discard_granularity = stripe;
                 /*
                  * unaligned part of discard request will be ignored, so can't
-                 * guarantee discard_zerors_data
+                 * guarantee discard_zeroes_data
                  */
                 mddev->queue->limits.discard_zeroes_data = 0;
 
@@ -6233,6 +6237,18 @@ static int run(struct mddev *mddev)
                             !bdev_get_queue(rdev->bdev)->
                                                 limits.discard_zeroes_data)
                                 discard_supported = false;
+                        /* Unfortunately, discard_zeroes_data is not currently
+                         * a guarantee - just a hint.  So we only allow DISCARD
+                         * if the sysadmin has confirmed that only safe devices
+                         * are in use by setting a module parameter.
+                         */
+                        if (!devices_handle_discard_safely) {
+                                if (discard_supported) {
+                                        pr_info("md/raid456: discard support disabled due to uncertainty.\n");
+                                        pr_info("Set raid456.devices_handle_discard_safely=Y to override.\n");
+                                }
+                                discard_supported = false;
+                        }
                 }
 
                 if (discard_supported &&