linux-kernel - Re: [patch 02/41] cpu alloc: The allocator

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Message-Id: <200805301552.25423.rusty@rustcorp.com.au>
Date:	Fri, 30 May 2008 15:52:24 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	Eric Dumazet <dada1@...mosbay.com>, akpm@...ux-foundation.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Travis <travis@....com>
Subject: Re: [patch 02/41] cpu alloc: The allocator

On Friday 30 May 2008 15:20:45 Christoph Lameter wrote:
> On Fri, 30 May 2008, Eric Dumazet wrote:
> > > +static DEFINE_PER_CPU(UNIT_TYPE, area[UNITS]);
> >
> > area[] is not guaranteed to be aligned on anything but 4 bytes.
> >
> > If someone then needs to call cpu_alloc(8, GFP_KERNEL, 8), it might get
> > an non aligned result.
> >
> > Either you should add an __attribute__((__aligned__(PAGE_SIZE))),
> > or take into account the real address of area[] in cpu_alloc() to avoid
> > waste of up to PAGE_SIZE bytes
> > per cpu.
>
> I think cacheline aligning should be sufficient. People should not
> allocate large page aligned objects here.

I vaguely recall there were issues with this in the module code.  They might 
be gone now, but failing to meet alignment contraints without a big warning 
would suck.

But modifying your code to consider the actual alignment is actually pretty 
trivial, AFAICT.

Cheers,
Rusty.
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