how mmap() works?

[Jerry Hong]

Hi, 
  mmap() creates a mmaped memory associated with a
physical file. If a process updates the mmaped memory,
Linux will updates the file "automatically". If this
is the case, why do we need msync()? If this is not
the case, what is the interval between 2 "WRITE" (IO
request operation) request to the physical file
because it really updates the physical file somehow
even without msync().
  Any comments about how mmap() works will be
appreciated.
  Thanks.
  
  Jerry
  

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