linux-kernel - Re: [patch] radix-tree: use indirect bit

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Message-ID: <20070807025120.GD17986@wotan.suse.de>
Date:	Tue, 7 Aug 2007 04:51:20 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch] radix-tree: use indirect bit

On Mon, Aug 06, 2007 at 11:40:55AM -0700, Andrew Morton wrote:
> On Thu, 2 Aug 2007 07:24:46 +0200 Nick Piggin <npiggin@...e.de> wrote:
> 
> > Rather than sign direct radix-tree pointers with a special bit, sign
> > the indirect one that hangs off the root. This means that, given a
> > lookup_slot operation, the invalid result will be differentiated from
> > the valid (previously, valid results could have the bit either set or
> > clear).
> > 
> > This does not affect slot lookups which occur under lock -- they
> > can never return an invalid result. Is needed in future for lockless
> > pagecache.
> 
> so.. we added 30 bytes of text to radix-tree.o for no purpose?

I guess no functional purpose at this stage. But I do like this
scheme better anyway, because it means that locked API users are
never exposed to the direct/indirect bit.

It's a bit variable... on powerpc, it _removed_ 22 bytes, and if
I delete the extra BUG_ON that I introduced, then it removes 128
bytes. 

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