[PATCH v1 14/15] md: Ensure resync is reported after it starts

[Logan Gunthorpe <logang@deltatee.com>]

The 07layouts test in mdadm fails on some systems. The failure
presents itself as the backup file not being removed before the next
layout is grown into:

  mdadm: /dev/md0: cannot create backup file /tmp/md-test-backup:
      File exists

This is because the background mdadm process, which is responsible for
cleaning up this backup file gets into an infinite loop waiting for
the reshape to start. mdadm checks the mdstat file if a reshape is
going and, if it is not, it waits for an event on the file or times
out in 5 seconds. On faster machines, the reshape may complete before
the 5 seconds times out, and thus the background mdadm process loops
waiting for a reshape to start that has already occurred.

mdadm reads the mdstat file to start, but mdstat does not report that the
reshape has begun, even though it has indeed begun. So the mdstat_wait()
call (in mdadm) which polls on the mdstat file won't ever return until
timing out.

The reason mdstat reports the reshape has started is due to an issue
in status_resync(). recovery_active is subtracted from curr_resync which
will result in a value of zero for the first chunk of reshaped data, and
the resulting read will report no reshape in progress.

To fix this, if "resync - recovery_active" is zero: force the value to
be 4 so the code reports a resync in progress.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
---
 drivers/md/md.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 8273ac5eef06..dbac63c8e35c 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -8022,10 +8022,18 @@ static int status_resync(struct seq_file *seq, struct mddev *mddev)
 		if (test_bit(MD_RECOVERY_DONE, &mddev->recovery))
 			/* Still cleaning up */
 			resync = max_sectors;
-	} else if (resync > max_sectors)
+	} else if (resync > max_sectors) {
 		resync = max_sectors;
-	else
+	} else {
 		resync -= atomic_read(&mddev->recovery_active);
+		if (!resync) {
+			/*
+			 * Resync has started, but if it's zero, ensure
+			 * it is still reported, by forcing it to be 4
+			 */
+			resync = 4;
+		}
+	}
 
 	if (resync == 0) {
 		if (test_bit(MD_RESYNCING_REMOTE, &mddev->recovery)) {
-- 
2.30.2