Re: [PATCH -next v2 6/6] ext4: fix possible store wrong check interval value in disk when umount

[yebin <yebin10@huawei.com>]

On 2021/10/7 21:12, Jan Kara wrote:
> On Sat 11-09-21 17:00:59, Ye Bin wrote:
>> Test follow steps:
>> 1. mkfs.ext4 /dev/sda -O mmp
>> 2. mount /dev/sda  /mnt
>> 3. wait for about 1 minute
>> 4. umount mnt
>> 5. debugfs /dev/sda
>> 6. dump_mmp
>> 7. fsck.ext4 /dev/sda
>>
>> I found 'check_interval' is range in [5, 10]. And sometime run fsck
>> print "MMP interval is 10 seconds and total wait time is 42 seconds.
>> Please wait...".
>> kmmpd:
>> ...
>> 	if (diff < mmp_update_interval * HZ)
>> 		schedule_timeout_interruptible(mmp_update_interval * HZ - diff);
>> 	 diff = jiffies - last_update_time;
>> ...
>> 	mmp_check_interval = max(min(EXT4_MMP_CHECK_MULT * diff / HZ,
>> 				EXT4_MMP_MAX_CHECK_INTERVAL),
>> 			        EXT4_MMP_MIN_CHECK_INTERVAL);
>> 	mmp->mmp_check_interval = cpu_to_le16(mmp_check_interval);
>> ...
>> We will call ext4_stop_mmpd to stop kmmpd kthread when umount, and
>> schedule_timeout_interruptible will be interrupted, so 'diff' maybe
>> little than mmp_update_interval. Then mmp_check_interval will range
>> in [EXT4_MMP_MAX_CHECK_INTERVAL, EXT4_MMP_CHECK_MULT * diff / HZ].
>> To solve this issue, if 'diff' little then mmp_update_interval * HZ
>> just break loop, don't update check interval.
>>
>> Signed-off-by: Ye Bin <yebin10@huawei.com>
>> ---
>>   fs/ext4/mmp.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
>> index a0d47a906faa..f39e1fa0c6db 100644
>> --- a/fs/ext4/mmp.c
>> +++ b/fs/ext4/mmp.c
>> @@ -205,6 +205,14 @@ static int kmmpd(void *data)
>>   			schedule_timeout_interruptible(mmp_update_interval *
>>   						       HZ - diff);
>>   			diff = jiffies - last_update_time;
>> +			/* If 'diff' little 'than mmp_update_interval * HZ', it
>> +			 * means someone call ext4_stop_mmpd to stop kmmpd
>> +			 * kthread. We don't need to update mmp_check_interval
>> +			 * any more, as 'diff' is not exact value.
>> +			 */
>> +			if (unlikely(diff < mmp_update_interval * HZ &&
>> +			    kthread_should_stop()))
>> +				break;
>>   		}
> So in this case, mmp_check_interval would be EXT4_MMP_MIN_CHECK_INTERVAL. I
> don't quite understand what the practical problem is - the fsck message?
> That will happen anytime mmp_check_interval is >= 10 AFAICT and I don't
> quite see how that is connected to this condition... Can you explain a bit
> more please?
>
> 								Honza
I just think 'mmp_check_interval' is not reflect real check interval, 
and also sometime run fsck
print "MMP interval is 10 seconds and total wait time is 42 seconds. 
Please wait...", but
sometime not.