Man page 9 : blk_start_plug

blk_start_plug - initialize blk_plug and track it inside the task_struct

void blk_start_plug(struct blk_plug *  plug );

plug
    The struct blk_plug that needs to be initialized

Tracking blk_plug inside the task_struct will help with auto-flushing the pending I/O should the task end up blocking between blk_start_plug and blk_finish_plug. This is important from a performance perspective, but also ensures that we donAqt deadlock. For instance, if the task is blocking for a memory allocation, memory reclaim could end up wanting to free a page belonging to that request that is currently residing in our private plug. By flushing the pending I/O when the process goes to sleep, we avoid this kind of deadlock.