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

[Christoph Hellwig <hch@infradead.org>]

On Thu, May 19, 2022 at 01:13:10PM -0600, Logan Gunthorpe wrote:
> 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;

Where does this magic 4 come from?