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

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Message-ID: <20211007131207.GJ12712@quack2.suse.cz>
Date:   Thu, 7 Oct 2021 15:12:07 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ye Bin <yebin10@...wei.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        jack@...e.cz
Subject: Re: [PATCH -next v2 6/6] ext4: fix possible store wrong check
 interval value in disk when umount

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@...wei.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

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR