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

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Date:   Fri, 8 Oct 2021 10:38:31 +0800
From:   yebin <yebin10@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <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 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 fisrt and wait 
'wait_time * HZ'  seconds,
read mmp data do check.Most of the time, If 'wait_time' is zero, it can 
pass check.
>>> ---
>>>   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
>>>
>