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

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Message-ID: <200903120819.08724.rgetz@blackfin.uclinux.org>
Date:	Thu, 12 Mar 2009 08:19:08 -0400
From:	Robin Getz <rgetz@...ckfin.uclinux.org>
To:	"KOSAKI Motohiro" <kosaki.motohiro@...fujitsu.com>
CC:	"Minchan Kim" <minchan.kim@...il.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>, dhowells@...hat.com,
	torvalds@...ux-foundation.org, peterz@...radead.org,
	Enrik.Berkhan@...com, uclinux-dev@...inux.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Johannes Weiner" <hannes@...xchg.org>,
	"Rik van Riel" <riel@...riel.com>,
	"Lee Schermerhorn" <lee.schermerhorn@...com>
Subject: Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may  get wrongly discarded

On Wed 11 Mar 2009 21:04, KOSAKI Motohiro pondered:
> Hi
> 
> > >> Page reclaim shouldn't be even attempting to reclaim or write back
> > >> ramfs pagecache pages - reclaim can't possibly do anything with 
> > >> these pages!
> > >>
> > >> Arguably those pages shouldn't be on the LRU at all, but we haven't
> > >> done that yet.
> > >>
> > >> Now, my problem is that I can't 100% be sure that we _ever_
> > >> implemented this properly. ?I _think_ we did, in which case 
> > >> we later broke it. ?If we've always been (stupidly) trying 
> > >> to pageout these pages then OK, I guess your patch is a 
> > >> suitable 2.6.29 stopgap. 
> > >
> > > OK, I can't find any code anywhere in which we excluded ramfs pages
> > > from consideration by page reclaim. ?How dumb.
> > 
> > The ramfs  considers it in just CONFIG_UNEVICTABLE_LRU case
> > It that case, ramfs_get_inode calls mapping_set_unevictable.
> > So,  page reclaim can exclude ramfs pages by page_evictable.
> > It's problem .
> 
> Currently, CONFIG_UNEVICTABLE_LRU can't use on nommu machine
> because nobody of vmscan folk havbe nommu machine.
> 
> Yes, it is very stupid reason. _very_ welcome to tester! :)

As always - if you (or any kernel developer) would like a noMMU machine to 
test on - please send me a private email.
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