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

[Paul Hargrove <phhargrove@xxxxxxx>]

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@xxxxxxx> 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@xxxxxxx

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@xxxxxxx

Future Technologies Group

Computer and Data Sciences Department     Tel: +1-510-495-2352

Lawrence Berkeley National Laboratory     Fax: +1-510-486-6900