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

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Date:	Mon, 11 Jun 2007 21:04:32 +0200
From:	"Håvard Skinnemoen" <hskinnemoen@...il.com>
To:	"Christoph Lameter" <clameter@....com>
Cc:	"Haavard Skinnemoen" <hskinnemoen@...el.com>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>,
	"David Brownell" <david-b@...bell.net>
Subject: Re: kernel BUG at mm/slub.c:3689!

On 6/11/07, Christoph Lameter <clameter@....com> wrote:
> 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?

If there were just a few, known drivers that did this, sure. But as
long as Documentation/DMA-mapping.txt includes this paragraph:

    If you acquired your memory via the page allocator
    (i.e. __get_free_page*()) or the generic memory allocators
    (i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
    that memory using the addresses returned from those routines.

I think it's best to ensure that memory returned by kmalloc() actually
can be used for DMA. I used to work around this problem in the SPI
controller driver by using a temporary DMA buffer when possible
misalignment was detected, but David Brownell said it was the wrong
way to do it and pointed at the above paragraph.

But, as I mentioned, perhaps ARCH_KMALLOC_MINALIGN isn't the best way
to solve the problem. I'll look into the flush-caches-from-dma_unmap
approach. However, it looks like other arches set
ARCH_KMALLOC_MINALIGN to various values -- I suspect some of them
might run into the same problem as well?

hskinnemoen@...p-255-175:~/git/linux$ grep -r ARCH_KMALLOC_MINALIGN
include/asm-*
include/asm-mips/mach-generic/kmalloc.h:#define ARCH_KMALLOC_MINALIGN   128
include/asm-mips/mach-ip27/kmalloc.h: * All happy, no need to define
ARCH_KMALLOC_MINALIGN
include/asm-mips/mach-ip32/kmalloc.h:#define ARCH_KMALLOC_MINALIGN      32
include/asm-mips/mach-ip32/kmalloc.h:#define ARCH_KMALLOC_MINALIGN      128
include/asm-s390/cache.h:#define ARCH_KMALLOC_MINALIGN  8
include/asm-sh64/uaccess.h:#define ARCH_KMALLOC_MINALIGN 8

> > 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.

Yeah, that's true. CONFIG_SLAB_DEBUG does indeed cause DMA buffer
corruption on avr32, and so does CONFIG_SLOB. I've been wanting to fix
it, but I never understood how. Now that SLUB seems to offer a
solution that doesn't effectively turn off debugging, I thought I'd
finally found it...

Haavard
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