linux-kernel - Re: [patch 2/3] only allow nonlinear vmas for ram backed filesystems

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Message-Id: <20070325160050.fe7cb284.akpm@linux-foundation.org>
Date:	Sun, 25 Mar 2007 16:00:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	a.p.zijlstra@...llo.nl, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch 2/3] only allow nonlinear vmas for ram backed
 filesystems

On Sat, 24 Mar 2007 23:09:19 +0100 Miklos Szeredi <miklos@...redi.hu> wrote:

> Dirty page accounting/limiting doesn't work for nonlinear mappings,

Doesn't it?  iirc the problem is that we don't correctly re-clean the ptes
while starting writeout.  And the dirty-page accounting is in fact correct
(it'd darn well better be).

> so
> for non-ram backed filesystems emulate with linear mappings.  This
> retains ABI compatibility with previous kernels at minimal code cost.
> 
> All known users of nonlinear mappings actually use tmpfs, so this
> shouldn't have any negative effect.

Unless someone is using remap_file_pages() against an ext3 file, in which
case their application stops working?

That would be a problem.  These guys:
http://www.technovelty.org/code/linux/fremap.html, for example, will be in
for a little surprise.

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