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SubjectRe: [PATCH -next v2 2/6] ext4: introduce last_check_time record previous check time
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On 2021/10/8 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@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.
>
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
>>>
>

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