On Mon, 10 Apr 2017, Lennart Poettering wrote:
On Mon, 10.04.17 19:38, Michael Chapman (m...@very.puzzling.org) wrote:
On Mon, 10 Apr 2017, Lennart Poettering wrote:
On Mon, 10.04.17 18:45, Michael Chapman (m...@very.puzzling.org) wrote:
On Mon, 10 Apr 2017, Lennart Poettering wrote:
On Sun, 09.04.17 10:11, Michael Chapman (m...@very.puzzling.org) wrote:
Don't forget, they've provided an interface for software to use if it needs
more than the guarantees provided by sync. Informally speaking, the FIFREEZE
ioctl is intended to place a filesystem into a "fully consistent" state, not
just a "fully recoverable" state. (Formally it's all a bit hazy: POSIX
really doesn't guarantee anything with sync.)
FIFREEZE does considerably more than what you suggest: it also pauses
all further changes until FITHAW is called. And that's semantics we
really cannot have.
If systemd is just about to call reboot(2), why does it matter?
Well, in the general case we don't actually call reboot(), because we
instead transition back into the initrd, which then eventually calls
that. At least that's what happens on the major general purpose
distros that have an initrd that does that (for example: Fedora/RHEL
with Dracut).
If it's not systemd _inside_ the initrd calling reboot(2), then there's
nothing systemd can do about it.
The initrd usually doesn't run a systemd environment anymore, PID 1 is
usually a shell script of some kind. It might use our "reboot" binary
and call it with "-ff" (which means it's really just a pure reboot()
wrapper), but we don't do a umount/kill/detach spree in that case. Or
in other words: if they do use our reboot utility then they use it in
pure sysvinit compat mode, where it won't do more than sync() +
reboot(), exactly the same way as sysvinit() did.
OK, given that there's really no point in pursuing this from the systemd
end.
I am sorry, but just making all accesses hang is just broken. That
can't work.
I do think we should attempt to remount readonly before doing the FIFREEZE.
I thought systemd did that, but it appears that it does not. A readonly
remount will do what we want so long as no remaining processes have any
files opened for writing on the filesystem. The FIFREEZE would only be
necessary when the remount fails.
We remount everything read-only we can if we cannot unmount
something.
Ah, I see the code for that now. I was looking for something after the
umount call (specifically, if umount failed), not before.
Well, the scheme works like this: we kill, umount, remount, detach in
a loop until nothing changes anymore. It's a primitive but robust way,
to deal with stacked storage, where running processes might pin file
systems, which might in turn pin devices, which might in turn pin
backend userspace services, and so on... Hence, yes, we do the umount
first, and the remount second, but then we'll try another umount again
and another remount, until this stops being fruitful.
But do note that we can't do that in all cases. Most
prominently: consider a process that is running from an executable
that has been updated on disk (specifically: whose binary got deleted
because it was replaced by a newer version). This process will keep
the file pinned, and will block all read-only remounts, as the kernel
wants to mark the file properly deleted first, but it can't since the
process is keeping it pinned.
This is specifically the case that happened for Plymouth: the binary
probably got updated, hence the process in memory references a deleted
file, which blocks the read-only remounting, in which case we can't do
anything, and sync and remount.
OK, so how about this. _After_ the unmount-everything loop we do a freeze +
thaw for each remaining filesystem, one filesystem at a time. That won't
permanently block processes that are still writing to the filesystems (and
why would they be?!), it will ensure that all filesystems' journals are
fully flushed (which will make GRUB and other OSs happy), and it won't block
the kernel from doing any kind of reboot()-time cleanups you were talking
about earlier.
Well, I figure that might work, but it's also fricking ugly: it feels
like booking a plane ticket that includes free airplane food, just
because you are hungry: you get considerably more than just the food,
and you have to sit uncomfortably for too long, end up where you
didn't want to go, and the food is quite awful too. ("Chicken or
pasta?")
I'd prefer if the file system folks would simply provide sane
semantics here, and provide an fsync()-style syscall or ioctl that
does what is needed here.
Perhaps we might be able to convince them to make reboot() a full "unmount
all remaining filesystems" operation. To be honest, I'm a little
surprised it isn't... but I suppose it's got all the same problems with
ordering between filesystems within the kernel itself.
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