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SubjectRe: Slab BUG with DEBUG_* options
On Sat, 30 Nov 2013, Meelis Roos wrote:

On Sat, 30 Nov 2013, Meelis Roos wrote:

> 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.

Hmmm. With CONFIG_DEBUG_PAGEALLOC *and* DEBUG_SLAB you would get a pretty
strange configuration with massive sizes of slabs.

> kernel BUG at mm/slab.c:2391!

Ok so this means that we are trying to create a cache with off slab
management during bootstrap which should not happen.

> __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

We should not be switching on CFLGS_OFF_SLAB here because the
kmalloc array does not contain the necessary entries yet.

Does this fix it? We may need a more sophisticated fix from someone who
knows how handle CONFIG_DEBUG_PAGEALLOC.



Subject: slab: Do not use off slab metadata for CONFIG_DEBUG_PAGEALLOC

Signed-off-by: Christoph Lameter <cl@linux.com>

Index: linux/mm/slab.c
===================================================================
--- linux.orig/mm/slab.c 2013-12-03 09:44:20.463144282 -0600
+++ linux/mm/slab.c 2013-12-03 09:45:09.321786608 -0600
@@ -2243,13 +2243,15 @@ __kmem_cache_create (struct kmem_cache *
* it too early on. Always use on-slab management when
* SLAB_NOLEAKTRACE to avoid recursive calls into kmemleak)
*/
+#ifndef CONFIG_DEBUG_PAGEALLOC
if ((size >= (PAGE_SIZE >> 3)) && !slab_early_init &&
- !(flags & SLAB_NOLEAKTRACE))
+ !(flags & SLAB_NOLEAKTRACE) )
/*
* Size is large, assume best to place the slab management obj
* off-slab (should allow better packing of objs).
*/
flags |= CFLGS_OFF_SLAB;
+#endif

size = ALIGN(size, cachep->align);


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