lkml.org 
[lkml]   [2021]   [Oct]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH -next v2 2/6] ext4: introduce last_check_time record previous check time
From
Date


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@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 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;
...
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.
if (retval)
goto failed;
mmp = (struct mmp_struct *)(bh->b_data);
if (seq != le32_to_cpu(mmp->mmp_seq)) {
dump_mmp_msg(sb, mmp,
"Device is already active on another node.");
goto failed;
}
...
}

\
 
 \ /
  Last update: 2021-10-12 13:47    [W:0.144 / U:0.396 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site