[RFC]should swap-file opened with O_DIRECT?

[RFC]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? 

							Yingchao Zhou
							yc_zhou@ncic.ac.cn

Re: [RFC]should swap-file opened with O_DIRECT?

[Hugh Dickins]

On Mon, 10 Oct 2005, Yingchao Zhou wrote:
> 
>   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,

No, they are written out by swap_writepage.   The opening in sys_swapon
gets a handle of what is to be used for swap, but it then goes its own way.

> 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?

Caches of swapfile are useful (but yes, can be discarded when memory
is tight).  There's nothing indirect about them: the page that is mapped
into userspace gets written to disk, the page that is read in from disk
gets mapped back into userspace.

Hugh