should swap-file opened with O_DIRECT?

[Yingchao Zhou]

  The sys_swapon system call open the swap-file through 
filp_open(..., O_RDWR|O_LARGEFILE,...).
  In this way, I think that the pageout process of anonymous
pages finally will write them out through (swap-file)->f_ops->write,
and it will result in caches of swapfile. However, swapping only 
happens when memory is tight. So why not set O_DIRECT? Is there any
special reason to keep caches of swapfile?
  Is the idea right ? Or something misunstood? 


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