I was investigating the journaling layer used in the EXT4 (JBD2) and I added some printk to see the behavior of the ext4_journal_start and ext4_journal_stop functions being called.
This is the procedure:
I first format a given partition using:
sudo mke2fs -t ext4 /dev/vdb
(I am using QEMU to run this experiment)
Then I mount it:
sudo mount /dev/vdb /mnt/mydisk
That is the normal procedure for mounting, but when I mount it, because of my printk's functions in both ext4_journal_start/stop, the dmesg shows a lot of calls to journal_stop without any journal_start.
P.S.: I should guess that it is some background behavior of EXT4 or something, but I have no idea what is it.
Here is the dmesg output:
* Restoring resolver state... [ OK ]
* Stopping System V runlevel compatibility [ OK ]
[ 124.648904] JOURNAL STOP
[ 124.778691] JOURNAL STOP
...
[... ] # it is called maybe more than 40 times
...
[ 129.641895] jbd2_journal_commit_transaction
[ 129.769132] JOURNAL STOP
...
[... ] # it is called maybe more than 40 times
...
[ 134.766164] jbd2_journal_commit_transaction
After 134 seconds, it stops these messages, and then I try to write some file into that mounting point, and it behaves as expected.
[ 624.995549] JOURNAL START
[ 624.996849] JOURNAL STOP
[ 625.000676] JOURNAL START
[ 625.001757] JOURNAL START
[ 625.002822] JOURNAL STOP
[ 625.003773] JOURNAL STOP
[ 631.004110] jbd2_journal_commit_transaction
So, it is strange that after mounting, even that I did absolutely nothing, these functions are being called (journal_stop) several times and, furthermore, after two commits (the function call jbd2_journal_commit_transaction) the dmesg gets stable, and it then follows an expected behavior.
To make it clear, my question is: what causes this several calls without any reason (the ext4_journal_stop)?
