Subject: Re: [PATCH 00/28] Swap over NFS -v16
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: neilb@suse.de, akpm@linux-foundation.org, torvalds@linux-foundation.org,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       netdev@vger.kernel.org, trond.myklebust@fys.uio.no
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	 <1204023042.6242.271.camel@lappy>  <E1JU1kk-0001t9-25@pomaz-ex.szeredi.hu>
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Date: Tue, 26 Feb 2008 16:41:57 +0100
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On Tue, 2008-02-26 at 16:29 +0100, Miklos Szeredi wrote:
> > > mm-page_file_methods.patch
> > > 
> > >     This makes page_offset and others more expensive by adding a
> > >     conditional jump to a function call that is not usually made.
> > > 
> > >     Why do swap pages have a different index to everyone else?
> > 
> > Because the page->index of an anonymous page is related to its (anon)vma
> > so that it satisfies the constraints for vm_normal_page().
> > 
> > The index in the swap file it totally unrelated and quite random. Hence
> > the swap-cache uses page->private to store it in.
> 
> Yeah, and putting the condition into page_offset() will confuse code
> which uses it for finding the offset in the VMA 

Right, do we do that anywhere?

> or in a tmpfs file.

Good point. I really should go read tmpfs some day, its really a blind
spot for me.

> So why not just have a separate page_swap_offset() function, used
> exclusively by swap_in/out()?

That would require duplicating quite a lot of NFS code from what I can
see.

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