linux-kernel - Re: [PATCH -mm] Revoke core code: fix nommu arch compiling error bug

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Message-Id: <20070326035027.c522cdf5.akpm@linux-foundation.org>
Date:	Mon, 26 Mar 2007 03:50:27 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Howells <dhowells@...hat.com>
Cc:	bryan.wu@...log.com, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: [PATCH -mm] Revoke core code: fix nommu arch compiling error
 bug

On Mon, 26 Mar 2007 12:25:18 +0100 David Howells <dhowells@...hat.com> wrote:

> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > Offending patch is
> > ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.21-rc4/2.6.21-rc4-mm1/broken-out/nommu-hide-vm_mm-in-nommu-mode.patch,
> > which seems rather dumb.  Or at least, its changelog does a good job of
> > making it look dumb.
> 
> vm_mm is always NULL under NOMMU as it currently stands.  As far as I know,
> this has been true since the NOMMU mm stuff was first included (I'm not sure
> the VMAs of the first NOMMU mm code *had* a vm_mm).  Hugh (I think it was)
> suggested that since this was always NULL, then it should be excised from the
> struct in NOMMU-mode.
> 
> The reason is that, at the moment, VMAs are a global *shared* resource in
> NOMMU-mode.  Each process has a list of global VMAs that it subscribes to, but
> that's it.  This (a) slightly reduces the amount of metadata allocated
> (possibly), and (b) makes sharing of executables and libraries much easier.

whoa.  You live and learn.  Logical, I guess.

I agree that in that case, we just don't want vm_mm to exist in NOMMU builds - it's
better to fail at compile time.

I'll touch up the changelog for nommu-hide-vm_mm-in-nommu-mode.patch and then
I'll temporarily drop it so the blackfin guys can test their work, I guess.
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