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

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Message-ID: <471D133E.90105@panasas.com>
Date:	Mon, 22 Oct 2007 23:16:46 +0200
From:	Benny Halevy <bhalevy@...asas.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jens Axboe <jens.axboe@...cle.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	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 Oct. 22, 2007, 22:16 +0200, Alan Cox <alan@...rguk.ukuu.org.uk> 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.
> 
> 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...)

Alternatively, I proposed to check for end of list in sg_next 
by calling it with the next iterator value and number of list elements.
We tried that patch here and it seems like a reasonable alternative.
If folks are interested, I can send the full patch for review.

> 
> Alan
> -
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