linux-kernel - Re: kernel BUG at mm/slub.c:3689!

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Message-ID: <Pine.LNX.4.64.0706111129270.18327@schroedinger.engr.sgi.com>
Date:	Mon, 11 Jun 2007 11:32:33 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Håvard Skinnemoen <hskinnemoen@...il.com>
cc:	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: kernel BUG at mm/slub.c:3689!

On Mon, 11 Jun 2007, Håvard Skinnemoen wrote:

> > Note that I do not get why you would be aligning the objects to 32 bytes.
> > Increasing the smallest cache size wastes a lot of memory. And it is
> > usually advantageous if multiple related objects are in the same cacheline
> > unless you have heavy SMP contention.
> 
> It's not about performance at all, it's about DMA buffers allocated
> using kmalloc() getting corrupted. Imagine this:

Uhhh... How about using a separate slab for the DMA buffers?
 
> Maybe there are other solutions to this problem, but the old SLAB
> allocator did guarantee 32-byte alignment as long as SLAB debugging
> was turned off, so setting ARCH_KMALLOC_MINALIGN seemed like the
> easiest way to get back to the old, known-working behaviour.

SLABs mininum object size is 32 thus you had no problems. I see. SLAB 
does not guarantee 32 byte alignment. It just happened to work. If you 
switch on CONFIG_SLAB_DEBUG you will likely get into trouble.

So I'd suggest to set up a special slab for your DMA buffers.