Users working with userdebug or eng builds expect to be able to remount the system partition as read-write and then add or modify any number of files without reflashing the system image, which is understandably efficient for a development cycle. Limited memory systems that chose to use readonly filesystems like squashfs, or Logical Resizable Android Partitions which land system partition images right-sized, and with filesystem that have been deduped on the block level to compress the content; means that either a remount is not possible directly, or when done offers little or no utility because of remaining space limitations or support logistics.
Overlayfs comes to the rescue for these debug scenarios, and logic will automatically setup backing storage for a writable filesystem as an upper reference, and mount overtop the lower. These actions will be performed in the adb disable-verity and adb remount requests.
The typical action to utilize the remount facility is:
$ adb root $ adb disable-verity $ adb reboot $ adb wait-for-device $ adb root $ adb remount
Followed by one of the following:
$ adb stop $ adb sync $ adb start $ adb reboot
$ adb push <source> <destination> $ adb reboot
Note that the sequence above:
$ adb disable-verity $ adb reboot
can be replaced with:
$ adb reboot -R
which will not reboot if everything is already prepared and ready to go.
None of this changes if overlayfs needs to be engaged. The decisions whether to use traditional direct filesystem remount, or one wrapped by overlayfs is automatically determined based on a probe of the filesystem types and space remaining.
When overlayfs logic is feasible, it will use either the /cache/overlay/ directory for non-A/B devices, or the /mnt/scratch/overlay directory for A/B devices that have access to Logical Resizeable Android Partitions. The backing store is used as soon as possible in the boot process and can occur at first stage init, or at the mount_all init rc commands.
This early as possible attachment of overlayfs means that sepolicy or init itself can also be pushed and used after the exec phases that accompany each stage.