linux-kernel - Re: [PATCH 09/10] Change table chaining layout

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Message-ID: <20071022203848.GI17536@waste.org>
Date:	Mon, 22 Oct 2007 15:38:48 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	mingo@...e.hu, Linux/m68k <linux-m68k@...r.kernel.org>
Subject: Re: [PATCH 09/10] Change table chaining layout

On Mon, Oct 22, 2007 at 09:16:17PM +0100, Alan Cox wrote:
> On Mon, 22 Oct 2007 12:49:40 -0700 (PDT)
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > 
> > 
> > On Mon, 22 Oct 2007, Geert Uytterhoeven wrote:
> > >
> > > Better safe than sorry...
> > > 
> > > Is it possible that a chain entry pointer has bit 1 set on architectures
> > > (e.g. m68k) where the natural alignment of 32-bit quantities is _2_ bytes,
> > > not 4?
> > 
> > Better make sure that such alignment never happens... But no, I don't 
> > think it will, since these things would generally always have to be 
> > allocated with an allocator, and the *allocator* won't return 2-byte 
> > aligned data structures.
> 
> No - but a structure which has other objects in it before the object
> being written out may well be 2 byte aligned on M68K and some of the
> other externally 16bit platforms - ditto local dynamic objects.

Also, the current version of SLOB will return objects aligned at 2 bytes if the
architecture allows it.
 
> Why can't we just make the list one item longer than the entry count and
> stick a NULL on the end of it like normal people ? Then you need one bit
> which ought to be safe for everyone (and if the bit is a macro any CPU
> warped enough to have byte alignment is surely going to have top bits
> spare...)

I'm guessing the extra entry makes slab-like allocators unhappy.

-- 
Mathematics is the supreme nostalgia of our time.
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