linux-kernel - Re: [PATCH] Add nid sanity on alloc_pages

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Message-Id: <20070713013819.0f8bd1ea.akpm@linux-foundation.org>
Date:	Fri, 13 Jul 2007 01:38:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Paul Jackson <pj@....com>
Cc:	joe.jin@...cle.com, bill.irwin@...cle.com,
	linux-kernel@...r.kernel.org, gurudas.pai@...cle.com,
	Zwane Mwaikambo <zwane@....linux.org.uk>
Subject: Re: [PATCH] Add nid sanity on alloc_pages_node

On Fri, 13 Jul 2007 01:29:06 -0700 Paul Jackson <pj@....com> wrote:

> > I'm scratching my head over that min_t in __first_node(), too.   I don't think
> > it's possible for find_first_bit(..., N) to return anything >N _anyway_.  And if
> > it does, we want to know about it.
> > 
> > <looks at Paul>
> 
> I'm not sure I've got this right, but looks like that min_t went in after
> Zwane Mwaikambo, then <zwane@...labs.com>, whom I am presuming is the same
> person as now at <zwane@....linux.org.uk>, found a problem with the i386
> find_next_bit implementation returning > N when merging i386 cpu hotplug.

Ah, Zwane was involved - say no more ;)

> See the thread:
> 
>   http://lkml.org/lkml/2004/7/31/102
>   [PATCH][2.6] first/next_cpu returns values > NR_CPUS
> 
> I apparently lobbied at the time to mandate that find_first_bit(..., N)
> return exactly N on failure to find a set bit, but gave up, after some
> confusions on my part.

iirc, find_first_bit(..., N) _must_ return N on nothing-found.  It'd be
untidy to return some randomly-larger number.

I wonder which was the culpable architecture?  Oh, i386.

Note how the i386 implementation's documentation carefully avoids describing
the return value.  I don't think _any_ of our find_foo_bit()
implementations have return-value docs, and here we see the result.

Sigh.  What crap.  I guess we leave it as-is.


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