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

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Message-ID: <20090313104442.GB2119@cmpxchg.org>
Date:	Fri, 13 Mar 2009 11:44:42 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	torvalds@...ux-foundation.org, Enrik.Berkhan@...com,
	uclinux-dev@...inux.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded

On Fri, Mar 13, 2009 at 05:15:44PM +0900, KOSAKI Motohiro wrote:
> > > > > 	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.
> > > > > 
> > > > > Not requiring the pages to be added to the LRU would be a really good idea.
> > > > > They are not discardable, be it in MMU or NOMMU mode, except when the inode
> > > > > itself is discarded.
> > > > 
> > > > Yep, these pages shouldn't be on the LRU at all.  I guess that will
> > > > require some tweaks to core filemap.c code.
> > > 
> > > IMHO, UNEVICTABLE_LRU already does lru isolation.
> > > only rest prblem is, getting rid of "depends on MMU" line in mm/Kconfig.
> > > 
> > > Am I missing anything?
> > 
> > Yes, the need to take something off that shouldn't be there to begin
> > with.
> 
> In past unevictable lru discussion, we discuss the same thing.
> at that time, we found two reason of unevictable lru is better than 
> completely taking off.
> 
> (1) page migration code depend on the page stay on lru.
> (2) "taking off at reclaim time" can avoid adding lock to fastpath.
>     anyway, complely removing from lru need something lock.
>     we disliked it at that time.

Agreed with (1).  NOMMU can't support migration, though.  But keeping
them off the LRU on NOMMU needs adjustment of the page cache
read/write code in mm/filemap.c.

I'm not quite sure I understand (2).  But never adding these pages on
the LRU means we never have to remove them anywhere, no?

	Hannes
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