#ifndef _LINUX_SWAPFILE_H
  #define _LINUX_SWAPFILE_H
  
  /*
   * these were static in swapfile.c but frontswap.c needs them and we don't
   * want to expose them to the dozens of source files that include swap.h
   */
  extern spinlock_t swap_lock;
  extern struct plist_head swap_active_head;
  extern struct swap_info_struct *swap_info[];
  extern int try_to_unuse(unsigned int, bool, unsigned long);
  
  #endif /* _LINUX_SWAPFILE_H */