Re: BUG: slub creates kmalloc slabs with refcount=0

[JoonSoo Kim <js1304@gmail.com>]

Hello, Paul.

2012/12/25 Paul Hargrove <phhargrove@lbl.gov>:
>
> I have a 3.7.1 kernel on x86-86
> It is configured with
>   CONFIG_SLUB=y
>   CONFIG_SLUB_DEBUG=y
>
> I have an out-of-tree module calling KMEM_CACHE for an 8-byte struct:
>         cr_pdata_cachep = KMEM_CACHE(cr_pdata_s,0);
>         if (!cr_pdata_cachep) goto no_pdata_cachep;
>         printk(KERN_ERR "@ refcount = %d name = '%s'\n",
> cr_pdata_cachep->refcount, cr_pdata_cachep->name);
>
> The output of the printk, below, shows that the request has been merged with
> the built-in 8-byte kmalloc pool, BUT the resulting refcount is 1, rather
> than 2 (or more):
>     @ refcount = 1 name = 'kmalloc-8'
>
> This results in a very unhappy kernel when the module calls
>     kmem_cache_destroy(cr_pdata_cachep);
> at rmmod time, resulting is messages like
>     BUG kmalloc-8 (Tainted: G           O): Objects remaining in kmalloc-96
> on kmem_cache_close()
>
> A quick look through mm/slub.c appears to confirm my suspicion that
> "s->refcount" is never incremented for the built-in kmalloc-* caches.
> However, I leave it to the experts to determine where the increment belongs.
>
> FWIW: I am currently passing SLAB_POISON for the flags argument to
> KMEM_CACHE() as a work-around (it prevents merging and, if I understand
> correctly, has no overhead in a non-debug build).
>
> -Paul
>
> --
> Paul H. Hargrove                          PHHargrove@lbl.gov
> Future Technologies Group
> Computer and Data Sciences Department     Tel: +1-510-495-2352
> Lawrence Berkeley National Laboratory     Fax: +1-510-486-6900

My e-mail client's 'Reply to message ID' is not working properly.
I sent a patch('slub:assign refcount for kmalloc_caches') for fixing
this and Cc'ed you.

Thanks.

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