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Date:	Sun, 27 Sep 2015 19:14:58 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [4.3-rc1, regression] ext2 vs ext3/ext4 fs probing issues

On Sat, Sep 26, 2015 at 09:51:26AM +1000, Dave Chinner wrote:
> Ping?

Sorry, I somehow missed this the first time you posted this.

> > Which tells me that there's a problem with fstype probe ordering
> > regressions w.r.t ext2 and ext3 as a result of removing the ext3
> > module. It also doesn't fail fsck checks now, so boots successfully
> > every time. I suspect the "boot hang" problem is that e2fsck sees a
> > dirty journal, fixes everything and then asks for a reboot, which
> > fails.

The original probe ordering was: ext3, ext2, ext4.  As you've
correctly pointed out, this allowed us to preferentially use ext3 over
ext2 even if the file system did not need a file system replay.  We
put ext4 at the end so that if there was an ext2-only root file
system, we would use ext2 in preference over ext4.  This was useful
for backwards compatibility for certain ancient enterprise
distributions, and/or when ext4 was still under development and we
wanted to make sure for an ext2-only root, we would use ext2 if
possible.

When ext3 was removed, we were now left with the boot order: ext2,
ext4.  We could swap this around, but in that case, ext4 would
*always* be used in preference to ext2, which not necessarily the best
thing.  Especially since in most cases most people will be using
distro initrd's, which don't use the brute-force probe order in
init/do_mounts.c.  The main people who still use the kernel boot order
tend to be us died-in-the stick developers who don't believe in
initrd's (and probably wouldn't be using systemd unless our distro was
forcing us to).

Something which I'm thinking about doing --- but which is an awful
hack --- is to switch the order so we probe ext4 first, but also add
code in ext4's mount function which checks to see if (a) the pid is 1
in the root pid namespace, (b) the file system feature set is one that
can be supported by the ext2 driver, and (c) the ext2 driver is
available.  In that case, we could fail the mount, so that in the case
where we are doing the initial boot time probing, and the root file
system is an ext2 file system, we properly use the ext2 file system if
it's available.

This should do what we want in all circumstances, but the question is
whether I'd respect myself in the morning.....  :-)

	    	    	      	  		- Ted

P.S.  Regarding the problem which triggered your investigation of the
boot order:

> > One the first cold boot of a new kernel, the boot appears to hang.
> > What i've discovered (which took a long time thanks to the shitpile
> > that is systemd) is that it appears to be doing a e2fsck on the root
> > device, and that is failing resulting in systemd outputing:
> >
> > [FAILED] Failed to start File System Check on Root Device.

Clearly both you and I don't have the same refined tastes as Lennart
Poettering.  :-)

After all, instead of grepping through shell scripts, Lennart clearly
prefers people to have to go diving through C source code to figure
out what is going on....

I could be wrong, since the systemd sources is a twisty maze of C
code, all different, but I *believe* that error is caused by the fact
that for some reason, systemd wasn't able to start the executable
/lib/systemd/systemd-fsck (The string "File System Check on Root
Device" comes from the file
/lib/systemd/system/systemd-fsck-root.service, and I'm pretty sure
this means it wasn't able to start the ExecStart program,
/lib/systemd/systemd-fsck).

If /lib/systemd/systemd-fsck (a C program, why use a shell script when
you can force hard-working programmres to have to comprehend someone
else's C code) had managed to start fsck.ext[234] and it returned an
error, you should have seen an explicit message about fsck failing
with a specific error code and/or signal:

        if (status.si_code != CLD_EXITED || (status.si_status & ~1)) {

                if (status.si_code == CLD_KILLED || status.si_code == CLD_DUMPED)
                        log_error("fsck terminated by signal %s.", signal_to_string(status.si_status));
                else if (status.si_code == CLD_EXITED)
                        log_error("fsck failed with error code %i.", status.si_status);
                else
                        log_error("fsck failed due to unknown reason.");

                if (status.si_code == CLD_EXITED && (status.si_status & 2) && root_directory)
                        /* System should be rebooted. */
                        start_target(SPECIAL_REBOOT_TARGET);
                else if (status.si_code == CLD_EXITED && (status.si_status & 6))
                        /* Some other problem */
                        start_target(SPECIAL_EMERGENCY_TARGET);
                else {
                        r = EXIT_SUCCESS;
                        log_warning("Ignoring error.");
                }

And it *does* appear that if we had modified the root file system and
had requested a reboot (at least from the version of systemd sources I
examined) , it does appear that systemd should have immediately
started rebooting the system, having printed the exit code (probably
3, i.e. FSCK_NONDESTRUCT | FSCK_REBOOT).

So I think something else was going on here, but given the inability
for systemd to save logs in this instance, I'm not sure we'll be able
to figure out what was going on.
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