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Date: Tue, 6 Nov 2007 19:02:47 -0800 (PST)
From: Christoph Lameter <clameter@sgi.com>
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To: Rik van Riel <riel@redhat.com>
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Subject: Re: [RFC PATCH 3/10] define page_file_cache
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On Tue, 6 Nov 2007, Rik van Riel wrote:

> > 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.
> 
> The bdi will no longer be accessible by the time a page
> makes it to free_hot_cold_page, which is one place in the
> kernel where this information is needed.

At that point you need only information about which list the page
was put on. Dont we need something like PageLRU -> PageFileLRU
and PageMemLRU?

The page may change its nature I think? What if a page becomes
swap backed?