To: a.p.zijlstra@chello.nl
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
In-reply-to: <1204023042.6242.271.camel@lappy> (message from Peter Zijlstra on
	Tue, 26 Feb 2008 11:50:42 +0100)
Subject: Re: [PATCH 00/28] Swap over NFS -v16
References: <20080220144610.548202000@chello.nl>
	 <20080223000620.7fee8ff8.akpm@linux-foundation.org>
	 <18371.43950.150842.429997@notabene.brown> <1204023042.6242.271.camel@lappy>
Message-Id: <E1JU1kk-0001t9-25@pomaz-ex.szeredi.hu>
From: Miklos Szeredi <miklos@szeredi.hu>
Date: Tue, 26 Feb 2008 16:29:58 +0100
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1003
Lines: 25

> > 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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