Re: [patch] delayed disk block allocation

[Andrew Morton <akpm@zip.com.au>]

Jeff Garzik wrote:
> 
> ...
> >
> > You're not even trying to apply this to swap cache right now are you?
> 
> This is a disagreement akpm and I have, actually :)
> 

Misunderstanding, rather.

Swapfiles aren't interesting, IMO.  And I agree that mkswap or swapon
should just barf if the file has any holes in it.

But what I refer to here is, simply, delayed allocate for swapspace.
So swap_out() sticks the target page into the swapcache, marks it
dirty and takes a space reservation for the page out of the swapcache's
address_space.  But no disk space is allocated at swap_out() time.
Instead, the real disk mapping is created when the VM calls
a_ops->vm_writeback() against the swapcache page's address_space.  

All of which rather implies a ripup-and-rewrite of half the swap
code.  It would certainly require a new allocator. So I just mentioned
the possibility.  Glad you're interested :)

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