linux-kernel - Slab BUG with DEBUG

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Date:	Sat, 30 Nov 2013 13:42:23 +0200 (EET)
From:	Meelis Roos <mroos@...ux.ee>
To:	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>
cc:	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org
Subject: 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!
              \|/ ____ \|/
              "@'/ .. \`@"
              /_| \__/ |_\
                 \__U_/
swapper(0): Kernel bad sw trap 5 [#1]
CPU: 0 PID: 0 Comm: swapper Not tainted 3.11.0-rc2-00058-g20bafb3-dirty #127
task: 00000000008ac468 ti: 000000000089c000 task.ti: 000000000089c000
TSTATE: 0000004480e01606 TPC: 00000000004f57d4 TNPC: 00000000004f57d8 Y: 00000000    Not tainted
TPC: <__kmem_cache_create+0x374/0x480>
g0: 00000000000000f8 g1: 00000000008bb400 g2: 000000000002780b g3: 00000000008b5120
g4: 00000000008ac468 g5: 0000000000000000 g6: 000000000089c000 g7: 0000000000000000
o0: 0000000000845f08 o1: 0000000000000957 o2: ffffffffffffffe0 o3: 0000000000000000
o4: 0000000000002004 o5: 0000000000000000 sp: 000000000089f301 ret_pc: 00000000004f57cc
RPC: <__kmem_cache_create+0x36c/0x480>
l0: fffff8001e812040 l1: fffff8001e819f80 l2: fffff8001e819fb8 l3: fffff8001e819fd8
l4: 0000000000000001 l5: fffff8001e819fc8 l6: 0000000000845f08 l7: fffff8001e8300a0
i0: fffff8001e831fa0 i1: 0000000080002800 i2: 0000000080000000 i3: 0000000000000034
i4: 0000000000000000 i5: 0000000000002000 i6: 000000000089f3b1 i7: 0000000000907464
I7: <create_boot_cache+0x4c/0x84>
Call Trace:
 [0000000000907464] create_boot_cache+0x4c/0x84
 [00000000009074d0] create_kmalloc_cache+0x34/0x60
 [0000000000907540] create_kmalloc_caches+0x44/0x168
 [0000000000908dfc] kmem_cache_init+0x1d0/0x1e0
 [00000000008fc658] start_kernel+0x18c/0x370
 [0000000000761df4] tlb_fixup_done+0x88/0x94
 [0000000000000000]           (null)
Disabling lock debugging due to kernel taint
Caller[0000000000907464]: create_boot_cache+0x4c/0x84
Caller[00000000009074d0]: create_kmalloc_cache+0x34/0x60
Caller[0000000000907540]: create_kmalloc_caches+0x44/0x168
Caller[0000000000908dfc]: kmem_cache_init+0x1d0/0x1e0
Caller[00000000008fc658]: start_kernel+0x18c/0x370
Caller[0000000000761df4]: tlb_fixup_done+0x88/0x94
Caller[0000000000000000]:           (null)
Instruction DUMP: 92102957  7ffccb35  90122308 <91d02005> 90100018  4009b371  920f20d0  ba922000  02480006 
Kernel panic - not syncing: Attempted to kill the idle task!
Press Stop-A (L1-A) to return to the boot prom

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.

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