Re: [PATCH -next v2 2/6] ext4: introduce last_check_time record previous check time
From: yebin
Date: Wed Oct 13 2021 - 08:33:51 EST
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@xxxxxxxxxx>
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