linux-kernel - Re: [PATCHv6] mm: slab: free kmem_cache

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Date:	Fri, 5 Feb 2016 19:44:33 +0300
From:	Dmitry Safonov <dsafonov@...tuozzo.com>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
CC:	<akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <0x7f454c46@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCHv6] mm: slab: free kmem_cache_node after destroy sysfs file

On 02/05/2016 07:11 PM, Vladimir Davydov wrote:
> On Fri, Feb 05, 2016 at 06:45:36PM +0300, Dmitry Safonov wrote:
>> With enabled slub_debug alloc_calls_show will try to track location and
>> user of slab object on each online node, kmem_cache_node structure
>> shouldn't be freed till there is the last reference to sysfs file.
>>
>> Fixes the following panic:
>> [43963.463055] BUG: unable to handle kernel
>> [43963.463090] NULL pointer dereference at 0000000000000020
>> [43963.463146] IP: [<ffffffff811c6959>] list_locations+0x169/0x4e0
>> [43963.463185] PGD 257304067 PUD 438456067 PMD 0
>> [43963.463220] Oops: 0000 [#1] SMP
>> [43963.463850] CPU: 3 PID: 973074 Comm: cat ve: 0 Not tainted 3.10.0-229.7.2.ovz.9.30-00007-japdoll-dirty #2 9.30
>> [43963.463913] Hardware name: DEPO Computers To Be Filled By O.E.M./H67DE3, BIOS L1.60c 07/14/2011
>> [43963.463976] task: ffff88042a5dc5b0 ti: ffff88037f8d8000 task.ti: ffff88037f8d8000
>> [43963.464036] RIP: 0010:[<ffffffff811c6959>]  [<ffffffff811c6959>] list_locations+0x169/0x4e0
>> [43963.464725] Call Trace:
>> [43963.464756]  [<ffffffff811c6d1d>] alloc_calls_show+0x1d/0x30
>> [43963.464793]  [<ffffffff811c15ab>] slab_attr_show+0x1b/0x30
>> [43963.464829]  [<ffffffff8125d27a>] sysfs_read_file+0x9a/0x1a0
>> [43963.464865]  [<ffffffff811e3c6c>] vfs_read+0x9c/0x170
>> [43963.464900]  [<ffffffff811e4798>] SyS_read+0x58/0xb0
>> [43963.464936]  [<ffffffff81612d49>] system_call_fastpath+0x16/0x1b
>> [43963.464970] Code: 5e 07 12 00 b9 00 04 00 00 3d 00 04 00 00 0f 4f c1 3d 00 04 00 00 89 45 b0 0f 84 c3 00 00 00 48 63 45 b0 49 8b 9c c4 f8 00 00 00 <48> 8b 43 20 48 85 c0 74 b6 48 89 df e8 46 37 44 00 48 8b 53 10
>> [43963.465119] RIP  [<ffffffff811c6959>] list_locations+0x169/0x4e0
>> [43963.465155]  RSP <ffff88037f8dbe28>
>> [43963.465185] CR2: 0000000000000020
>>
>> Separated nodes structures freeing into __kmem_cache_free_nodes and use
>> it at kmem_cache_release.
> Well, that's better indeed, although still not perfect.
>
> ...
>> diff --git a/mm/slab.c b/mm/slab.c
>> index 6ecc697..3ccdf3c 100644
>> --- a/mm/slab.c
>> +++ b/mm/slab.c
>> @@ -2276,6 +2276,7 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
>>   	err = setup_cpu_cache(cachep, gfp);
>>   	if (err) {
>>   		__kmem_cache_shutdown(cachep);
>> +		__kmem_cache_free_nodes(cachep);
> You don't need to call the whole shutdown procedure here - the cache
> hasn't been used yet. __kmem_cache_release (about it below) would be
> enough.
Ok,
>
>>   		return err;
>>   	}
>>   
>> @@ -2414,8 +2415,6 @@ int __kmem_cache_shrink(struct kmem_cache *cachep, bool deactivate)
>>   
>>   int __kmem_cache_shutdown(struct kmem_cache *cachep)
>>   {
>> -	int i;
>> -	struct kmem_cache_node *n;
>>   	int rc = __kmem_cache_shrink(cachep, false);
>>   
>>   	if (rc)
>> @@ -2423,6 +2422,14 @@ int __kmem_cache_shutdown(struct kmem_cache *cachep)
>>   
>>   	free_percpu(cachep->cpu_cache);
> And how come ->cpu_cache (and ->cpu_slab in case of SLUB) is special?
> Can't sysfs access it either? I propose to introduce a method called
> __kmem_cache_release (instead of __kmem_cache_free_nodes), which would
> do all freeing, both per-cpu and per-node.
AFAICS, they aren't used by this sysfs.
Anyway, seems reasonable, will do.
>
> Thanks,
> Vladimir
>
>>   
>> +	return 0;
>> +}
>> +
>> +void __kmem_cache_free_nodes(struct kmem_cache *cachep)
>> +{
>> +	int i;
>> +	struct kmem_cache_node *n;
>> +
>>   	/* NUMA: free the node structures */
>>   	for_each_kmem_cache_node(cachep, i, n) {
>>   		kfree(n->shared);


-- 
Regards,
Dmitry Safonov