linux-ext4 - Re: [PATCH -next v2 2/6] ext4: introduce last_check

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Date:   Thu, 7 Oct 2021 14:31:00 +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 2/6] ext4: introduce last_check_time record
 previous check time

On Sat 11-09-21 17:00:55, Ye Bin wrote:
> kmmpd:
> ...
>     diff = jiffies - last_update_time;
>     if (diff > mmp_check_interval * HZ) {
> ...
> As "mmp_check_interval = 2 * mmp_update_interval", 'diff' always little
> than 'mmp_update_interval', so there will never trigger detection.
> Introduce last_check_time record previous check time.
> 
> Signed-off-by: Ye Bin <yebin10@...wei.com>

I think the check is there only for the case where write_mmp_block() +
sleep took longer than mmp_check_interval. I agree that should rarely
happen but on a really busy system it is possible and in that case we would
miss updating mmp block for too long and so another node could have started
using the filesystem. I actually don't see a reason why kmmpd should be
checking the block each mmp_check_interval as you do - mmp_check_interval
is just for ext4_multi_mount_protect() to know how long it should wait
before considering mmp block stale... Am I missing something?

								Honza

> ---
>  fs/ext4/mmp.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
> index 12af6dc8457b..c781b09a78c9 100644
> --- a/fs/ext4/mmp.c
> +++ b/fs/ext4/mmp.c
> @@ -152,6 +152,7 @@ static int kmmpd(void *data)
>  	int mmp_update_interval = le16_to_cpu(es->s_mmp_update_interval);
>  	unsigned mmp_check_interval;
>  	unsigned long last_update_time;
> +	unsigned long last_check_time;
>  	unsigned long diff;
>  	int retval = 0;
>  
> @@ -170,6 +171,7 @@ static int kmmpd(void *data)
>  
>  	memcpy(mmp->mmp_nodename, init_utsname()->nodename,
>  	       sizeof(mmp->mmp_nodename));
> +	last_check_time = jiffies;
>  
>  	while (!kthread_should_stop() && !sb_rdonly(sb)) {
>  		if (!ext4_has_feature_mmp(sb)) {
> @@ -198,17 +200,18 @@ static int kmmpd(void *data)
>  		}
>  
>  		diff = jiffies - last_update_time;
> -		if (diff < mmp_update_interval * HZ)
> +		if (diff < mmp_update_interval * HZ) {
>  			schedule_timeout_interruptible(mmp_update_interval *
>  						       HZ - diff);
> +			diff = jiffies - last_update_time;
> +		}
>  
>  		/*
>  		 * We need to make sure that more than mmp_check_interval
> -		 * seconds have not passed since writing. If that has happened
> -		 * we need to check if the MMP block is as we left it.
> +		 * seconds have not passed since check. If that has happened
> +		 * we need to check if the MMP block is as we write it.
>  		 */
> -		diff = jiffies - last_update_time;
> -		if (diff > mmp_check_interval * HZ) {
> +		if (jiffies - last_check_time > mmp_check_interval * HZ) {
>  			struct buffer_head *bh_check = NULL;
>  			struct mmp_struct *mmp_check;
>  
> @@ -234,6 +237,7 @@ static int kmmpd(void *data)
>  				goto wait_to_exit;
>  			}
>  			put_bh(bh_check);
> +			last_check_time = jiffies;
>  		}
>  
>  		 /*
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR