Re: [PATCH V2 0/3] Supporting same fsid mounting through a compat_ro feature

[Josef Bacik <josef@toxicpanda.com>]

On Thu, Aug 03, 2023 at 12:43:38PM -0300, Guilherme G. Piccoli wrote:
> Hi all, this is the 2nd attempt of supporting same fsid mounting
> on btrfs. V1 is here:
> https://lore.kernel.org/linux-btrfs/20230504170708.787361-1-gpiccoli@igalia.com/
> 
> The mechanism used to achieve that in V2 was a mix between the suggestion
> from JohnS (spoofed fsid) and Qu (a single-dev compat_ro flag) - it is
> still based in the metadata_uuid feature, leveraging that infrastructure
> since it prevents lots of corner cases, like sysfs same-fsid crashes.
> 
> The patches are based on kernel v6.5-rc3 with Anand's metadata_uuid refactor
> part 2 on top of it [0]; the btrfs-progs patch is based on "v6.3.3".
> 
> Comments/suggestions and overall feedback is much appreciated - tnx in advance!
> Cheers,
> 
> Guilherme
> 

In general the concept is fine with me, and the implementation seems reasonable.

With new features we want fstests to accompany them so we know they work
correctly, and we don't accidentally break them in the future.

I'd like to see tests that validate all the behaviors you're trying to
accomplish work as advertised, and that all the failure cases do in fact fail
properly.

Ideally a test that creates a single device fs image and mounts it in multiple
places as would be used in the Steam Deck.

Then a test that tries to add a device to it, replace, etc.  All the cases that
you expect to fail, and validate that they actually fail.

Then any other corner cases you can think of that I haven't thought of.

Make sure these new tests skip appropriately if the btrfs-progs support doesn't
exist, I'd likely throw the fstests into our CI before the code is merged to
make sure it's ready to be tested if/when it is merged.

Thanks,

Josef