linux-kernel - Re: [uClinux-dev] RE: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded

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Date:	Fri, 13 Mar 2009 08:20:51 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jamie Lokier <jamie@...reable.org>,
	uClinux development list <uclinux-dev@...inux.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [uClinux-dev] RE: [PATCH] NOMMU: Pages allocated to a ramfs
 inode's pagecache may get wrongly discarded

Hi, Peter.

On Thu, 12 Mar 2009 12:50:08 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, 2009-03-12 at 11:29 +0000, Jamie Lokier wrote:
> > Berkhan, Enrik (GE Infra, Oil & Gas) wrote:
> > > Andrew Morton wrote:
> > > > On Wed, 11 Mar 2009 15:30:35 +0000
> > > > David Howells <dhowells@...hat.com> wrote:
> > > >> From: Enrik Berkhan <Enrik.Berkhan@...com>
> > > >> 
> > > >> The solution is to mark the pages dirty at the point of allocation by
> > > >> the truncation code.
> > > > 
> > > > Was there a specific reason for using the low-level SetPageDirty()?
> > > 
> > > No, no specific reason. It was just my first try of a fix after spotting 
> > > the problem. After a short discussion with David, we decided to wait for 
> > > others' comments on using the low-/high-level approach.
> > 
> > Tangentially related...
> > 
> > Does the vm pageout logic include or skip these "dirty" pages looking
> > for candidates to flush to storage?  What about with MMU?
> 
> Includes them, regular pageout will try to do the writeout to clean them
> and then discard them. 
> 
> The ramfs stuff is rather icky in that it adds the pages to the aging
> list, marks them dirty, but does not provide a writeout method. 
> 
> This will make the paging code scan over them (continuously) trying to
> clean them, failing that (lack of writeout method) and putting them back
> on the list.

It ins't true any more. 
UNEVICTABLE_LRU will move ramfs's page from LRU to unevictable list.
Couldn't we solve this problem if NOMMU can support CONFIG_UNEVICTABLE_LRU ?


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-- 
Kinds Regards
Minchan Kim
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