The dma-buf counter is a metric for mapped memory used by it's clients. It is a shared buffer that is typically used for interprocess communication or process to hardware communication. In android we used to have ION,. but it is now replaced with dma-buf. ION had some overview metrics that was similar.
V1 initial version. Add dma-buf counter
V2 Fix build depencendy error suggested by Matthew Wilcox Extent commit message sugged by Köning
V3 Change variable and function names.
V4 Fix function name in code doc Reported-by: kernel test robot lkp@intel.com
V5 Removed EXPORT_SYMBOL_GPL suggested by Muchun Song
V6 Made it a patch set, Adding a addional patch for printing dma-buf counter in show_mem. Suggested by Michal Hocko.