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

[Paul Hargrove <phhargrove@lbl.gov>]

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Just to be clear the "BUG...." line I quoted contains an error of my own.

It should say "kmalloc-8" twice rather than having "kmalloc-96" toward the
end.  This is the result of a cut-and-paste error (and brain already on
vacation).  The module ALSO has a larger struct the cache for which gets
merged with the kmalloc-96 cache.  I grabbed the first BUG line from the
dmesg output and replaced "96" with "8" when composing the email, but
missed the second occurrence.

Merry Christmas,
-Paul


On Mon, Dec 24, 2012 at 4:55 PM, Paul Hargrove <phhargrove@lbl.gov> wrote:

>
> 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
>



-- 
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

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