Re: Hanging at shutdown with mystery "file system full" error

["Ian D. Leroux" <idleroux%fastmail.fm@localhost>]

On Wed, 21 Oct 2015 11:12:31 +0200 "Ian D. Leroux"
<idleroux%fastmail.fm@localhost> wrote:
> Tonight I'll try shutting down with various combinations of tmpfs
> filesystems, to see if I can actually narrow down the problem to a
> specific mount point.

As it turns out, I don't need shutdown to trigger the hang, umount is
enough.  Specifically,

umount -f /dev

Causes an immediate hang with the same kernel error message
(..., on /var: file system full), attributed this time to the login
command.  (Without the -f, umount sensibly complains that /dev is busy).

I can unmount /var/shm and /tmp (the other two tmpfs filesystems on
this machine) without causing an immediate hang (but also without
preventing a subsequent hang at shutdown time), so at least on my
machine the problem seems to be specifically with unmounting /dev.

The options as I currently see them are:
- Unmounting /dev during shutdown (specifically during swap1_stop()) is
expected to work.  Then we have a problem with umount or perhaps with
the exact timing of the call to umount.  Maybe the hang has to do with
unmounting a *busy* tmpfs filesystem?  That would explain why I see
this as a /dev-related problem (because /dev is the only tmpfs
filesystem I have that is still busy at shutdown time) while others see
a similar error without having a tmpfs /dev.

- /dev should never be unmounted, and /dev is allowed to be on tmpfs.
Then swap1_stop() should not forcibly unmount all tmpfs filesystems.
Would it make sense to replace "umount -aft tmpfs" with
"umount -at tmpfs", so as to remove only filesystems that aren't busy?

- /dev is not allowed to be on tmpfs.  Then I'm not seeing a bug, I'm
just using a bogus configuration.  Note, however, that the automatic
creation of a tmpfs-mounted /dev is documented in init(8), and it's the
only way I know of running with a read-only root partition.

I'm out of time for tonight, but the next thing I want to try is to
artificially make /tmp or /var/shm busy at unmount or shutdown time and
see if I can trigger a hang that way.

--
IDL