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Date: Wed, 13 Oct 2021 20:33:45 +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 2021/10/13 17:38, Jan Kara wrote:
> On Tue 12-10-21 19:46:24, yebin wrote:
>> On 2021/10/12 16:47, Jan Kara wrote:
>>> 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
>> int ext4_multi_mount_protect(struct super_block *sb,
>> ext4_fsblk_t mmp_block)
>> {
>> struct ext4_super_block *es = EXT4_SB(sb)->s_es;
>> struct buffer_head *bh = NULL;
>> struct mmp_struct *mmp = NULL;
>> u32 seq;
>> unsigned int mmp_check_interval =
>> le16_to_cpu(es->s_mmp_update_interval);
>> unsigned int wait_time = 0; --> wait_time is
>> equal with zero
>> int retval;
>>
>> if (mmp_block < le32_to_cpu(es->s_first_data_block) ||
>> mmp_block >= ext4_blocks_count(es)) {
>> ext4_warning(sb, "Invalid MMP block in superblock");
>> goto failed;
>> }
>>
>> retval = read_mmp_block(sb, &bh, mmp_block);
>> if (retval)
>> goto failed;
>>
>> mmp = (struct mmp_struct *)(bh->b_data);
>>
>> if (mmp_check_interval < EXT4_MMP_MIN_CHECK_INTERVAL)
>> mmp_check_interval = EXT4_MMP_MIN_CHECK_INTERVAL;
>>
>> /*
>> * If check_interval in MMP block is larger, use that instead of
>> * update_interval from the superblock.
>> */
>> if (le16_to_cpu(mmp->mmp_check_interval) > mmp_check_interval)
>> mmp_check_interval = le16_to_cpu(mmp->mmp_check_interval);
>>
>> seq = le32_to_cpu(mmp->mmp_seq);
>> if (seq == EXT4_MMP_SEQ_CLEAN) --> If hostA and hostB mount the
>> same block device at the same time,
>> --> HostA and hostB maybe get 'seq' with the same value EXT4_MMP_SEQ_CLEAN.
>> goto skip;
> Oh, I see. Thanks for explanation.
>
>> ...
>> skip:
>> /*
>> * write a new random sequence number.
>> */
>> seq = mmp_new_seq();
>> mmp->mmp_seq = cpu_to_le32(seq);
>>
>> retval = write_mmp_block(sb, bh);
>> if (retval)
>> goto failed;
>>
>> /*
>> * wait for MMP interval and check mmp_seq.
>> */
>> if (schedule_timeout_interruptible(HZ * wait_time) != 0) {
>> --> If seq is equal with EXT4_MMP_SEQ_CLEAN, wait_time is zero.
>> ext4_warning(sb, "MMP startup interrupted, failing mount");
>> goto failed;
>> }
>>
>> retval = read_mmp_block(sb, &bh, mmp_block); -->We may get the same
>> data with which we wrote, so we can't detect conflict at here.
> OK, I see. So the race in ext4_multi_mount_protect() goes like:
>
> hostA hostB
>
> read_mmp_block() read_mmp_block()
> - sees EXT4_MMP_SEQ_CLEAN - sees EXT4_MMP_SEQ_CLEAN
> write_mmp_block()
> wait_time == 0 -> no wait
> read_mmp_block()
> - all OK, mount
> write_mmp_block()
> wait_time == 0 -> no wait
> read_mmp_block()
> - all OK, mount
Yes, that's what i mean.
>
> Do I get it right? Actually, if we passed seq we wrote in
> ext4_multi_mount_protect() to kmmpd (probably in sb), then kmmpd would
> notice the conflict on its first invocation but still that would be a bit
> late because there would be a time window where hostA and hostB would be
> both using the fs.
>
> We could reduce the likelyhood of this race by always waiting in
> ext4_multi_mount_protect() between write & read but I guess that is
> undesirable as it would slow down all clean mounts. Ted?
>
> Honza
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