linux-kernel - Re: [RFC PATCH 3/10] define page_file

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Message-ID: <Pine.LNX.4.64.0711061821010.5249@schroedinger.engr.sgi.com>
Date:	Tue, 6 Nov 2007 18:23:44 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Rik van Riel <riel@...hat.com>
cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 3/10] define page_file_cache

On Sat, 3 Nov 2007, Rik van Riel wrote:

> Define page_file_cache() function to answer the question:
> 	is page backed by a file?

Well its not clear what is meant by a file in the first place.
By file you mean disk space in contrast to ram based filesystems?

I think we could add a flag to the bdi to indicate wheter the backing 
store is a disk file. In fact you can also deduce if if a device has
no writeback capability set in the BDI.

> Unfortunately this needs to use a page flag, since the
> PG_swapbacked state needs to be preserved all the way
> to the point where the page is last removed from the
> LRU.  Trying to derive the status from other info in
> the page resulted in wrong VM statistics in earlier
> split VM patchsets.

The bdi may avoid that extra flag.
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