Re: swapon vs GEOM labels

Warner Losh <imp@bsdimp.com> writes:

> On Mon, Oct 21, 2024 at 3:40 AM Stephane Rochoy 
> <stephane.rochoy@stormshield.eu> wrote:
>
>  Hi Hackers,
>
>  I'm playing with glabel(8) and swapon(8) but found a behavior 
>  that
>  is somewhat puzzling.
>
>  I boot main (b88df1e893c4) with an USB memstick plugged. The
>  memstick is da0 and hold a freebsd-swap partition, da0p1. This
>  partition was given the `swap` label using `glabel label` 
>  (i.e.,
>  an automatic label, stored somewhere into da0p1, AFAIK).
>
>    # glabel status
>          Name  Status  Components
>    label/swap     N/A  da0p1
>
>  Then I give the whole device the `usbmemstick` label using 
>  `glabel
>  create` (i.e., a manual label, not stored anywhere).
>
>    # glabel create usbmemstick da0
>    # glabel status
>                Name  Status  Components
>          label/swap     N/A  da0p1
>    label/usbmemstick    N/A  da0
>
>  What puzzle me is what happen when I enable swap:
>
>    # sysctl vm.nswapdev
>    vm.nswapdev: 0
>    # swapctl -l
>    Device:      1024-blocks     Used:
>    # dumpon -l
>    /dev/null
>    # swapon /dev/label/usbmemstickp1
>    # glabel status
>                Name  Status  Components
>    label/usbmemstick    N/A  da0
>    # swapoff /dev/label/usbmemstickp1
>    # glabel status
>                Name  Status  Components
>    label/usbmemstick    N/A  da0
>          label/swap     N/A  label/usbmemstickp1
>
>  While swap is enabled, the `swap` label is no longer available. 
>  It
>  comes back when swap is disabled.
>
>  Note that I also tried to enable swap using the automatic label
>  (i.e., `swap`). In such a case this is the `usbmemstick` label
>  that vanish. And the label is even not restored on `swapoff`.
>
>  Is it the expected behavior? Am I doing something wrong?
>
> This both makes sense to me, and seems wrong to me. When things
> are nested like this, it kinda makes sense that
> label/usbmemstickp1 shows up. geom doesn't enumerate everything
> and this seems to be a bit of an artifact. I'd expect both
> label/usbmemstickp1 and label/swap to be there. geom does odd
> things with devices that are open to keep others from opening
> them, and this may just be an artifact of this kinda weird
> behavior.

Well, I have to admit I'm a bit disappointed by the fact labels
don't completely mimic devices.

What I'm trying to achieve is to provide some sort of hardware
abstraction. I wrote a command that is responsible for probing
hardware and expose the fact that, e.g., an USB memstick is
available, a read/write SD card is inserted, etc…

My first attempt was to use symlinks in /dev/. The problem with
this approach is that GEOM don't seems to dereference them. For
example, if I have /dev/sdcard → /dev/da0, I cannot do things like 
`geom
disk list sdcard`. IOW, the GEOM API is defeated by symlinks.

That's why I'm playing with labels.

If symlinks and labels are not well suited for that, do you know
of some alternative I could look at?

Regards,
--
Stéphane Rochoy
O: Stormshield