linux-kernel - Re: [PATCH] mm: slub: remove dead and buggy code from sysfs_slab

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Message-ID: <dfa55d7d-4063-c249-f6b8-e7b7d2efc8cc@rasmusvillemoes.dk>
Date:   Mon, 3 Oct 2022 11:38:30 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: slub: remove dead and buggy code from
 sysfs_slab_add()

On 03/10/2022 09.02, Hyeonggon Yoo wrote:
> On Fri, Sep 30, 2022 at 10:47:42AM +0200, Rasmus Villemoes wrote:
>> The function sysfs_slab_add() has two callers:
>>
>> One is slab_sysfs_init(), which first initializes slab_kset, and only
>> when that succeeds sets slab_state to FULL, and then proceeds to call
>> sysfs_slab_add() for all previously created slabs.
>>
>> The other is __kmem_cache_create(), but only after a
>>
>> 	if (slab_state <= UP)
>> 		return 0;
>>
>> check.
>>
>> So in other words, sysfs_slab_add() is never called without
>> slab_kset (aka the return value of cache_kset()) being non-NULL.
>>
>> And this is just as well, because if we ever did take this path and
>> called kobject_init(&s->kobj), and then later when called again from
>> slab_sysfs_init() would end up calling kobject_init_and_add(), we
>> would hit
>>
>> 	if (kobj->state_initialized) {
>> 		/* do not error out as sometimes we can recover */
>> 		pr_err("kobject (%p): tried to init an initialized object, something is seriously wrong.\n",
>> 		dump_stack();
>> 	}
>>
>> in kobject.c.
>>
>> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
>> ---
>>  mm/slub.c | 5 -----
>>  1 file changed, 5 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 4b98dff9be8e..04a7f75a7b1f 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -5937,11 +5937,6 @@ static int sysfs_slab_add(struct kmem_cache *s)
>>  	struct kset *kset = cache_kset(s);
>>  	int unmergeable = slab_unmergeable(s);
>>  
>> -	if (!kset) {
>> -		kobject_init(&s->kobj, &slab_ktype);
>> -		return 0;
>> -	}
>> -
>>  	if (!unmergeable && disable_higher_order_debug &&
>>  			(slub_debug & DEBUG_METADATA_FLAGS))
>>  		unmergeable = 1;
>> -- 
>> 2.37.2
> 
> I assumed that it's hit when SLUB failed to initialize slab_kset in
> slab_sysfs_init(). (Yeah, it is too unlikely, though....)

No, it is not, because if the creation of slab_kset fails,
slab_sysfs_init() returns early, and hence slab_state never transitions
to FULL. I don't see anywhere else where slab_state could become FULL
(of course in slab.c and slob.c, but those are not built when slub.c
is), so I do believe my analysis in the commit log is correct.

> And obviously it's a bug if sysfs_slab_add() is called early than
> slab_sysfs_init().

Yes, and that's already what the existing slab_state check guards.

Rasmus