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

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Date:   Tue, 24 May 2022 09:45:33 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Xiao Ni <xni@...hat.com>, Stephen Bates <sbates@...thlin.com>,
        Martin Oliveira <Martin.Oliveira@...eticom.com>,
        David Sloan <David.Sloan@...eticom.com>
Subject: Re: [PATCH v1 14/15] md: Ensure resync is reported after it starts



On 2022-05-21 05:51, Christoph Hellwig wrote:
> 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@...tatee.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?

There are a bunch of existing magic numbers in this code. Just before
this hunk there's a if (resync <= 3) and just after the code there's an
if (resync < 3).

There's a comment in md_do_sync() indicating overloaded values for 1 and
2. I can try and turn this into an enum for v2.

Logan