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

[Seblu]

On Sun, Apr 24, 2011 at 11:55 PM, Tom Gundersen <teg at jklm.no> wrote:
> 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.

Tricky case. Unmouting tmpfs will not solve the case.
Mount Z cannot be unmounted until X process stop and process cannot
unmount until it stop to write his log.
It's a bad designed mounting.

I think in this case, lazy unmount offer a solution. no?

> 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.
And what's your conclusion?

Another question why we don't unmount _netdev marked fs? Network fs
should not be unmounted?

-- 
Sébastien Luttringer
www.seblu.net