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

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Date:   Tue, 12 Oct 2021 10:47:27 +0200
From:   Jan Kara <jack@...e.cz>
To:     yebin <yebin10@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v2 2/6] ext4: introduce last_check_time record
 previous check time

On Fri 08-10-21 10:38:31, yebin wrote:
> On 2021/10/8 9:56, yebin wrote:
> > On 2021/10/7 20:31, Jan Kara wrote:
> > > 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
> > I'm sorry, I didn't understand the detection mechanism here before. Now
> > I understand
> > the detection mechanism here.
> > As you said, it's just an abnormal protection. There's really no problem.
> > 
> Yeah, i did test as following steps
> hostA                        hostB
>    mount
>      ext4_multi_mount_protect  -> seq == EXT4_MMP_SEQ_CLEAN
>         delay 5s after label "skip" so hostB will see seq is
> EXT4_MMP_SEQ_CLEAN
>                        mount
>                        ext4_multi_mount_protect -> seq == EXT4_MMP_SEQ_CLEAN
>                                run  kmmpd
>     run kmmpd
> 
> Actually，in this  situation kmmpd will not detect  confliction.
> In ext4_multi_mount_protect function we write mmp data first and wait
> 'wait_time * HZ'  seconds,
> read mmp data do check. Most of the time, If 'wait_time' is zero, it can pass
> check.

But how can be wait_time zero? As far as I'm reading the code, wait_time
must be at least EXT4_MMP_MIN_CHECK_INTERVAL...

								Honza

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