linux-kernel - Re: Slab BUG with DEBUG

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Date:	Tue, 3 Dec 2013 11:17:35 +0200 (EET)
From:	Meelis Roos <mroos@...ux.ee>
To:	David Rientjes <rientjes@...gle.com>
cc:	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org
Subject: Re: Slab BUG with DEBUG_* options

> > I am debugging a reboot problem on Sun Ultra 5 (sparc64) with 512M RAM 
> > and turned on DEBUG_PAGEALLOC DEBUG_SLAB and DEBUG_SLAB_LEAK (and most 
> > other debug options) and got the following BUG and hang on startup. This 
> > happened originally with 3.11-rc2-00058 where my bisection of 
> > another problem lead, but I retested 3.12 to have the same BUG in the 
> > same place.
> > 
> > kernel BUG at mm/slab.c:2391!
[...

> > The line shows that __kmem_cache_create gets a NULL from kmalloc_slab().
> > 
> > I instrumented the code and found the following:
> > 
> > __kmem_cache_create: starting, size=248, flags=8192
> > __kmem_cache_create: now flags=76800
> > __kmem_cache_create: aligned size to 248 because of redzoning
> > __kmem_cache_create: pagealloc debug, setting size to 8192
> > __kmem_cache_create: aligned size to 8192
> > __kmem_cache_create: num=1, slab_size=64
> > __kmem_cache_create: starting, size=96, flags=8192
> > __kmem_cache_create: now flags=76800
> > __kmem_cache_create: aligned size to 96 because of redzoning
> > __kmem_cache_create: pagealloc debug, setting size to 8192
> > __kmem_cache_create: aligned size to 8192
> > __kmem_cache_create: num=1, slab_size=64
> > __kmem_cache_create: starting, size=192, flags=8192
> > __kmem_cache_create: now flags=76800
> > __kmem_cache_create: aligned size to 192 because of redzoning
> > __kmem_cache_create: pagealloc debug, setting size to 8192
> > __kmem_cache_create: aligned size to 8192
> > __kmem_cache_create: num=1, slab_size=64
> > __kmem_cache_create: starting, size=32, flags=8192
> > __kmem_cache_create: now flags=76800
> > __kmem_cache_create: aligned size to 32 because of redzoning
> > __kmem_cache_create: aligned size to 32
> > __kmem_cache_create: num=226, slab_size=960
> > __kmem_cache_create: starting, size=64, flags=8192
> > __kmem_cache_create: now flags=76800
> > __kmem_cache_create: aligned size to 64 because of redzoning
> > __kmem_cache_create: pagealloc debug, setting size to 8192
> > __kmem_cache_create: turning on CFLGS_OFF_SLAB, size=8192
> > __kmem_cache_create: aligned size to 8192
> > __kmem_cache_create: num=1, slab_size=64
> > __kmem_cache_create: CFLGS_OFF_SLAB, size=8192, slab_size=52
> > __kmem_cache_create: CFLGS_OFF_SLAB, allocating slab 52
> > 
> > With slab size 64, it turns on CFLGS_OFF_SLAB and off slab allocation 
> > with this size fails. I do not know slab internals so I can not tell if 
> > this just happens because of the debug paths, or is it a real problem 
> > without the debug options too.
> > 
> 
> Sounds like a problem with create_kmalloc_caches(), what are the values 
> for KMALLOC_SHIFT_LOW and KMALLOC_SHIFT_HIGH?

KMALLOC_SHIFT_LOW=5, KMALLOC_SHIFT_HIGH=23
 
> It's interesting you report this as starting with 3.11-rc2 because this 
> changed in 3.9, what kernels were tested before 3.11-rc2 if any?

No other kernels were tested with slab debug - but all relsese kernels 
and most rc-s starting with rc2 or rc3 eacg time were tested without 
slab debug.

-- 
Meelis Roos (mroos@...ux.ee)
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