From my reading, it's checking the maximum index into the circular buffer. That is, when it hits the end of the circular buffer, there's an assertion to check

From my reading, it's checking the maximum index into the circular buffer. That is, when it hits the end of the circular buffer, there's an assertion to check that they're properly wrapping the index back to the start of the buffer, but the assertion has an off-by-one error. I presume you find a lot of circular buffers in SSD firmware, for wear-leveling reasons. Samsung's NILFS and NILFS2 are structured as circular buffer append-only logs, at least partly to avoid trusting the firmware wear-leveling.