[PATCH 0/2] btrfs: support cloned-device mount capability

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

Guilherme's previous work [1] aimed at the mounting of cloned devices
using a superblock flag SINGLE_DEV during mkfs.
 [1] https://lore.kernel.org/linux-btrfs/20230831001544.3379273-1-gpiccoli@igalia.com/

Building upon this work, here is in-memory only approach. As it mounts
we determine if the same fsid is already mounted if then we generate a
random temp fsid which shall be used the mount, in-memory only not
written to the disk. We distinguish device by devt.

Mount option / superblock flag:
-------------------------------
 These patches show we don't have to limit the single-device / temp_fsid
capability with a mount option or a superblock flag from the btrfs
internals pov. However, if necessary from the user's perspective,
we can add them later on top of this patch. I've prepared a mount option
-o temp_fsid patch, but I'm not included at this time. As most of the
tests was without it.

Compatible with other features that may be affected:
----------------------------------------------------
 Multi device:
    A btrfs filesytem on a single device can be copied using dd and
    mounted simlutaneously. However, a multi device btrfs copied using
    dd and trying to mount simlutaneously is forced to fail:

      mount: /btrfs1: mount(2) system call failed: File exists.

 Send and receive:
    Quick tests shows send and receive between two single devices with
    the same fsid mounted on the _same_ host works!.
    (Also, the receive-mnt can receive from multiple senders as long as
    conflits are managed externally. ;-).)

 Replace: 
     Works fine.

btrfs-progs:
------------
 btrfs-progs needs to be updated to support the commands such as

	btrfs filesystem show

 when devices are not mounted. So the device list is not based on
 the fisd any more.

Testing:
-------
 This patch has been under testing for some time. The challenge is to get
 the fstests to test this reasonably well.

 As of now, this patch runs fine on a large set of fstests test cases
 using a custom-built mkfs.btrfs with the -U option and a new -P option
 to copy the device FSID and UUID from the TEST_DEV to the SCRATCH_DEV
 at the scratch_mkfs time. For example:

  Config file:

     config_fsid=$(btrfs in dump-super $TEST_DEV | grep -E ^fsid | \
							awk '{print $2}')
     config_uuid=$(btrfs in dump-super $TEST_DEV | \
				grep -E ^dev_item.uuid | awk '{print $2}')
     MKFS_OPTIONS="-U $config_fsid -P $config_uuid"

 This configuration option ensures that both TEST_DEV and SCRATCH_DEV will
 have the same FSID and device UUID while still applying test-specific
 scratch mkfs options.

Mkfs.btrfs:
-----------
 mkfs.btrfs needs to be updated to support the -P option for testing only.

   btrfs-progs: allow duplicate fsid for single device
   btrfs-progs: add mkfs -P option for dev_uuid

Anand Jain (2):
  btrfs: add helper function find_fsid_by_disk
  btrfs: support cloned-device mount capability

 fs/btrfs/disk-io.c |  3 +-
 fs/btrfs/volumes.c | 75 +++++++++++++++++++++++++++++++++++++++++++---
 fs/btrfs/volumes.h |  2 ++
 3 files changed, 75 insertions(+), 5 deletions(-)

-- 
2.38.1