Re: [Buildroot] [PATCH 2/6 v3] package/skeleton-systemd: systemd-ify mounting /var tmpfs with ro rootfs

["Yann E. MORIN" <yann.morin.1998@free.fr>]

Norbert, All,

Thanks for the feedback!

On 2022-11-06 16:56 +0100, Norbert Lange spake thusly:
> Am Di., 18. Okt. 2022 um 21:43 Uhr schrieb <yann.morin@orange.com>:
[--SNIP--]
> > diff --git a/package/skeleton-init-systemd/var.mount b/package/skeleton-init-systemd/var.mount
> > new file mode 100644
> > index 0000000000..6b165dff6d
> > --- /dev/null
> > +++ b/package/skeleton-init-systemd/var.mount
> > @@ -0,0 +1,18 @@
> > +# SPDX-License-Identifier: LGPL-2.1-or-later
> > +# Modelled after systemd's tmp.mount
> > +
> > +[Unit]
> > +Description=Buildroot /var tmpfs
> > +DefaultDependencies=no
> > +Conflicts=umount.target
> > +Before=basic.target local-fs.target umount.target systemd-tmpfiles-setup.service
> 
> Change it to
> Before=local-fs.target umount.target
> 
> All other dependencies are implicit

I looked at bootup(7) (thanks for the pointer!), and it seems that
tmpfiles is on a different synchronisation path (elided for brevity):

            (various low-level  (various mounts and
             services: udevd,    fsck services...)
             tmpfiles, random            |
    .    .   seed, sysctl, ...)          v           .
    .    .      |                 local-fs.target    .
    |    |      |                        |           |
    \____|______|_______________   ______|___________/
                                \ /
                                 v
                          sysinit.target

So, if we want tmpfiles to populate /var, we need to mount /var before
tmpfiles are run, so we need it in the Before section.

I suspect that this should also consistent with the other unit about the
overlayfs; we probably want the same Before for both, no?

> > +After=swap.target
> > +
> > +[Mount]
> > +What=tmpfs
> > +Where=/var
> > +Type=tmpfs
> > +Options=mode=1777,strictatime,nosuid,nodev,size=50%%,nr_inodes=1m
> > +
> > +[Install]
> > +WantedBy=basic.target
> 
> Drop the install section.

Then, how do we ensure the unit is enabled and active?

I guess however that this should be consistent between this unit and the
one about the overlayfs.

> While testing some other overlay solutions I found out that
> a /var mount cant be reasonably disabled.

Do you meant that the following preset:

    disable var.mount

would not be enough to prevent our unit from being acted on?

Also, see below...

> What happens is that basic.target will pull in var.mount,

Ah, so it is implicit that basic.target pulls in mount units? Or is
var.mount somehow special?

Also, why "basic.target", when bootup(7) shows that the first sync point
by which filesystems are supposed to be mounted, is sysinit.target?

> the only option to disable it would be masking var.mount

In this case, people who would mask var.mount would rather need to
override it to mount an actual device arther than a tmpfs, no?

Also, since mount units must be named after their mount point, users who
want to mount their own device would have to provide a unit named
var.mount, so they can't disable it.

Finally, usetrs who have custom dispositions (like an initramfs that
mounts /var) can just remove the unit with a post-build script.

> Guess i cant ack it conditionally, so far:
> 
> Reviewed-by: Norbert Lange <nolange79@gmail.com>

Thanks! I'll add it when I respin with your suggestions.

Regards,
Yann E. MORIN.

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