Re: [PATCH 3/3] btrfs: Add parameter to force devices behave as single-dev ones

[Josef Bacik <josef@toxicpanda.com>]

On Thu, Aug 03, 2023 at 12:43:41PM -0300, Guilherme G. Piccoli wrote:
> Devices with the single-dev feature enabled in their superblock are
> allowed to be mounted regardless of their fsid being already present
> in the system - the goal of such feature is to have the device in a
> single mode with no advanced features, like RAID; it is a compat_ro
> feature present since kernel v6.5.
> 
> The thing is that such feature comes in the form of a superblock flag,
> so devices that doesn't have it set, can't use the feature of course.
> The Steam Deck console aims to have block-based updates in its
> RO rootfs, and given its A/B partition nature, both block devices are
> required to be the same for their hash to match, so it's not possible
> to compare two images if one has this feature set in the superblock,
> while the other has not. So if we end-up having two old images, we
> couldn't make use of the single-dev feature to mount both at same time,
> or if we set the flag in one of them to enable the feature, we break
> the block-based hash comparison.
> 
> We propose here a module parameter approach to allow forcing any given
> path (to a device holding a btrfs filesystem) behaving as a single-dev
> device. That would useful for cases like the Steam Deck one, or for
> debug purposes. If the filesystem already has the compat_ro flag set
> in its superblock, the parameter is no-op.
> 

Now this one I'm not a fan of.  For old file systems you can simply btrfstune
them to have your new flag.  Is there a reason why that wouldn't be an option?

If it is indeed required, which is a huge if, I'd rather this be accomplished a
mount option.  I have a strong dislike for new mount options, but I think that's
a cleaner way to accomplish this than a module option.  Thanks,

Josef