Check_mount() should also work with multi device filesystems.
This patch adds checks that allow to detect if a file is a device
file used by a mounted single or multi device btrfs or if it is a
regular file used by a loopback device that is part of a mounted
single or multi device btrfs.
The single device checks also work for non-btrfs filesystems.
This might be helpful to prevent users from running btrfs programs
(e.g. mkfs.btrfs) accidentally on a filesystem used somewhere else.
Signed-off-by: Andi Drebes <lists-receive@programmierforen.de>
This commit introduces a new kind of back reference for btrfs metadata.
Once a filesystem has been mounted with this commit, IT WILL NO LONGER
BE MOUNTABLE BY OLDER KERNELS.
The new back ref provides information about pointer's key, level and in which
tree the pointer lives. This information allow us to find the pointer by
searching the tree. The shortcoming of the new back ref is that it only works
for pointers in tree blocks referenced by their owner trees.
This is mostly a problem for snapshots, where resolving one of these fuzzy back
references would be O(number_of_snapshots) and quite slow. The solution used
here is to use the fuzzy back references in the common case where a given tree
block is only referenced by one root, and use the full back references when
multiple roots have a reference
