[arch-projects] [initscrupts][RFC] umount tmpfs and ramfs on shutdown

[Tom Gundersen]

uOn Sun, Apr 24, 2011 at 9:42 PM, Seblu <seblu at seblu.net> wrote:
> On Sat, Apr 23, 2011 at 4:32 PM, Tom Gundersen <teg at jklm.no> wrote:
>>
>> If someone knows of any reason not to unmount tmpfs or ramfs on
>> shutdown, please speak up.
>
> So the question, was, is
> there a reason to unmount ramfs at shutdown?
> I understand the unmount chaining issue, but i don't see which case is
> problematic, do you have a ticket # ?

I assume you already saw FS#19783?

If we agree that tmpfs should be unmounted (due to the chaining), then
ramfs should be too (as anyone might use ramfs in place of tmpfs). We
do not unmount tmpfs if /dev is mounted on tmpfs, but this can (no
longer) happen on ramfs, so we always unmount it.

As Thomas points out in the bug report, it might be that we don't
actually need to unmount the file systems at all (only remount them
ro). However, this would need to be analyzed a bit more closely.
Consider this hypothetical case:

a process X (some userspace filesystem daemon) cannot be killed as
long as a mount Y is mounted
process X has a file opened rw on mount Z (maybe a logfile on /var)
a tmpfs/ramfs is a submount of Y

Remounting everything ro will not work in such a scenario, so we would
need to unmount Y and the tmpfs/ramfs. I don't know if this is
actually happening with any setups we support, it might be worth
looking into what others are doing.

I have been looking at what systemd is doing and it seems quite
robust. Their shutdown binary doesn't seem to work for us though.

> 2 small points about your patch:
> - You should use /bin/grep and not grep, like everywhere else.
> - A comment before the if then else to explain why we do this will be cool.

Good points, I'll fix this up once we have agreed on the final solution.

-t