netdev - Re: [PATCH 00/28] Swap over NFS -v16

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Date:	Tue, 26 Feb 2008 16:29:58 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	a.p.zijlstra@...llo.nl
CC:	neilb@...e.de, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, netdev@...r.kernel.org,
	trond.myklebust@....uio.no
Subject: Re: [PATCH 00/28] Swap over NFS -v16

> > 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 or in a tmpfs file.

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

Miklos
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