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

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Date:   Thu, 6 Oct 2022 15:20:57 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
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 Mon, Oct 03, 2022 at 11:38:30AM +0200, Rasmus Villemoes wrote:
> 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.

Yeah, you are right ;-) I misread that.

> 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.

Right.

> > 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

Looks good to me,
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@...il.com>

Thanks!

-- 
Thanks,
Hyeonggon