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

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Message-Id: <20090311170207.1795cad9.akpm@linux-foundation.org>
Date:	Wed, 11 Mar 2009 17:02:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	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, hannes@...xchg.org
Subject: Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may
 get wrongly discarded

On Wed, 11 Mar 2009 15:03:02 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> > The problem is that the pages are not marked dirty.  Anything that creates data
> > in an MMU-based ramfs will cause the pages holding that data will cause the
> > set_page_dirty() aop to be called.
> > 
> > For the NOMMU-based mmap, set_page_dirty() may be called by write(), but it
> > won't be called by page-writing faults on writable mmaps, and it isn't called
> > by ramfs_nommu_expand_for_mapping() when a file is being truncated from nothing
> > to allocate a contiguous run.
> > 
> > The solution is to mark the pages dirty at the point of allocation by
> > the truncation code.
> 
> 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.

So I guess that for now the proposed patch is suitable.  Longer-term we
should bale early in shrink_page_list(), or not add these pages to the
LRU at all.
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