Re: [PATCH 0/2] Supporting same fsid filesystems mounting on btrfs

[Anand Jain <anand.jain@oracle.com>]

On 5/5/23 01:07, Guilherme G. Piccoli wrote:
> Hi folks, this is an attempt of supporting same fsid mounting on btrfs.
> Currently, we cannot reliably mount same fsid filesystems even one at
> a time in btrfs, but if users want to mount them at the same time, it's
> pretty much impossible. Other filesystems like ext4 are capable of that.
> 


> The goal is to allow systems with A/B partitioning scheme (like the
> Steam Deck console or various mobile devices) to be able to hold
> the same filesystem image in both partitions; it also allows to have
> block device level check for filesystem integrity - this is used in the
> Steam Deck image installation, to check if the current read-only image
> is pristine. A bit more details are provided in the following ML thread:
> 
> https://lore.kernel.org/linux-btrfs/c702fe27-8da9-505b-6e27-713edacf723a@igalia.com/

Confused about your requirement: 2 identical filesystems mounted 
simultaneously or just one at a time? Latter works. Bugs were fixed.

Have you considered using the btrfs seed device feature to avoid 
sacrificing 50% capacity? Create read-only seed device as golden image, 
add writable device on top. Example:

   $ btrfstune -S1 /dev/rdonly-golden-img
   $ mount /dev/rdonly-golden-img /btrfs
   $ btrfs dev add /dev/rw-dev /btrfs
   $ mount -o remount,rw /dev/rw-dev /btrfs

To switch golden image:

   $ btrfs dev del /dev/rdonly-golden-img /btrfs
   $ umount /btrfs
   $ btrfstune -S1 /dev/rw-dev

Thanks, Anand


> The mechanism used to achieve it is based in the metadata_uuid feature,
> leveraging such code infrastructure for that. The patches are based on
> kernel 6.3 and were tested both in a virtual machine as well as in the
> Steam Deck. Comments, suggestions and overall feedback is greatly
> appreciated - thanks in advance!
> 
> Cheers,
> 
> 
> Guilherme
> 
> 
> Guilherme G. Piccoli (2):
>    btrfs: Introduce the virtual_fsid feature
>    btrfs: Add module parameter to enable non-mount scan skipping
> 
>   fs/btrfs/disk-io.c |  22 +++++++--
>   fs/btrfs/ioctl.c   |  18 ++++++++
>   fs/btrfs/super.c   |  41 ++++++++++++-----
>   fs/btrfs/super.h   |   1 +
>   fs/btrfs/volumes.c | 111 +++++++++++++++++++++++++++++++++++++++------
>   fs/btrfs/volumes.h |  11 ++++-
>   6 files changed, 174 insertions(+), 30 deletions(-)
>