libsnapshot: Also use empty space in super for COW
The super partition usually has some empty space even after
the target partitions are created, especially for retrofit
Virtual A/B devices. Use that empty space for COW before taking
up userdata space.
- PartitionCowCreator computes free regions in super partition metadata
and use that space until it is used up. It returns a pair of numbers
(cow_partition_size, cow_file_size) and let SnapshotManager to create
the partition / images with proper sizes.
- A region is considered free iff it is used by NEITHER target NOR
source partitions
- The list is in PartitionCowCreator's return value so that
SnapshotManager can use it as a guide when creating partitions.
- These partitions are created under the group named "cow".
- Avoid mapping COW partitions directly during init first stage
mount. Init only maps them when they are needed by the top-level
device.
- CreateCowImage no longer zero-fills the first 4 bytes of the image.
(See below)
- CreateUpdatePartitions: after creating the snapshot, also maps the COW
devices (via MapCowDevices()) and zero-fills the first 4 bytes of the
top-level COW device.
- Add a new SnapshotManager::MapCowDevices() function, which maps both
the COW partition (in super) and the COW image (through
IImageManager) (if necessary). Then, create a dm-linear target that
concatenates them (if necessary).
- Add a new SnapshotManager::UnmapCowDevices() functions that does the
reverse MapCowDevices().
- DeleteSnapshot also unmaps the top-level COW device and COW partition
before unmapping the COW images (before deleting them).
Test: libsnapshot_test
Change-Id: I0098b7e842ab48b0b4dd2a59142098b098d23d34
7 files changed