Masking swap.target to disable swaps (on old systemd versions)

[Max Gautier]

On Tue, Jan 02, 2024 at 09:56:44PM +0300, Andrei Borzenkov wrote:
> On 02.01.2024 18:47, Max Gautier wrote:
> > Hi,
> > 
> > Is masking `swap.target` a reliable/supported way to disable all swaps
> > (partition, file, whatever) ?
> > 
> > I used that approach in Kubespray[1] (a k8s installer) while refactoring
> > our "disable swap" steps, but it looks likes it does not work on Centos
> > 7 [2] (more specifically, systemd 219) ; and in fact while it seemed
> > obvious to me at the time, I didn't find it documented in
> > systemd.special for instance.
> > 
> > It does work on more recent version on systemd, my problem is that I
> > don't know how recent that is. 255 is fine, 219 is not.
> > 
> > Apparently, the .swap unit generated by systemd-fstab-generator does
> > shows as "inactive" in `systemctl status` (see [2] last comment), but
> > the swap **is activated** regardless.
> > 
> > I've searched issues, PRs and commits grepping for swap.target, but I
> > have not found anything relevant (the only thing was
> > https://github.com/systemd/systemd/commit/90060fa6605446bef7078867423b691e4effa575,
> > but it's way before 219...)
> > 
> > 
> > ● dev-mapper-centos\x2dswap.swap - /dev/mapper/centos-swap
> >     Loaded: loaded (/etc/fstab; bad; vendor preset: disabled)
> >     Active: inactive (dead) since Tue 2024-01-02 02:15:31 EST; 2h 6min ago
> >       What: /dev/mapper/centos-swap
> > 
> > The `bad` part in the Loaded line makes me think maybe something else is
> > creating/activating that swap than systemd-fstab-generator ? From the
> > systemctl man page this conveys that "the unit file is invalid or some
> > other error occurred", but the generated unit looks fine to me...
> > 
> > 
> > If anyone has had some success trying similar things on old systemd
> > versions, please let me know if you can.
> > 
> > 
> > [1]: https://github.com/kubernetes-sigs/kubespray
> > [2]: https://github.com/kubernetes-sigs/kubespray/issues/10741
> > 
> 
> As clearly visible in "systemctl show" output
> 
> WantedBy=dev-mapper-centos\x2dswap.device
> 
> So swap is activated when systemd gets notification that device appeared.
> 

Hum, you're right. I only skimmed over the second "systemctl show",
assuming it to be the same as the first, I was not aware it works on
non-existing units.
I can't find anything related to a Wants dependency from .device to
.swap, maybe that's shipped by Centos 7... Thanks !

(apologies for the duplicate mail)

-- 
Max Gautier