linux-kernel - Re: lib/klist.c: bit 0 in pointer can't be used as flag

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Message-ID: <Pine.LNX.4.64.0901140026240.3808@blonde.anvils>
Date:	Wed, 14 Jan 2009 00:33:23 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	David Miller <davem@...emloft.net>
cc:	kosaki.motohiro@...fujitsu.com, Jesper.Nilsson@...s.com,
	tj@...nel.org, gregkh@...e.de, stern@...land.harvard.edu,
	jens.axboe@...cle.com, hinko.kocevar@...rtapot.si,
	linux-kernel@...r.kernel.org
Subject: Re: lib/klist.c: bit 0 in pointer can't be used as flag

On Tue, 13 Jan 2009, David Miller wrote:
> From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Date: Wed, 14 Jan 2009 07:40:19 +0900
> 
> > > It may be that we've worked around the other spots, although I haven't
> > > seen anything like that, we might just have been lucky until now.
> > >
> > > Can you recall another place where this trick is used?
> > 
> > rmap.
> > Don't CRIS use mmu?
> 
> I'm beginning to suspect the issue is only with objects
> in the kernel image itself.  Dynamically allocated memory
> is properly aligned and therefore the "low bit status bits
> in pointer" trick works.

Yes: I don't think we fully realized that when adding the
__attribute__((aligned(sizeof(long)))) to struct address_space
(so the PAGE_MAPPING_ANON bit didn't mess up on CRIS), but the
only instance which actually gave a problem was the peculiar
struct swapper_space declared in mm/swap_state.c.

Hugh
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