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SubjectRe: [PATCH -next v2 2/6] ext4: introduce last_check_time record previous check time
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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@huawei.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.

>> ---
>> 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
>>

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