A pettiness question.

[liyu]

Hi, All.
  
   I found there are use double operator ! continuously sometimes in 
kernel.
e.g:

   static inline int is_page_cache_freeable(struct page *page)
   {
       return page_count(page) - !!PagePrivate(page) == 2;
   }

   Who would like tell me why write like above?
  
 
   Thanks in advanced.


Liyu
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