linux-kernel - Re: [PATCH v2] slab: Warn on duplicate cache names when DEBUG

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Message-ID: <6e5f3c82-a18a-4c20-858f-058f44e90ced@suse.cz>
Date: Mon, 26 Aug 2024 21:23:10 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Pedro Falcato <pedro.falcato@...il.com>,
 David Rientjes <rientjes@...gle.com>
Cc: Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
 Joonsoo Kim <iamjoonsoo.kim@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] slab: Warn on duplicate cache names when DEBUG_VM=y

On 8/16/24 14:17, Pedro Falcato wrote:
> On Mon, Aug 12, 2024 at 1:55 AM David Rientjes <rientjes@...gle.com> wrote:
>>
>> On Sun, 11 Aug 2024, Pedro Falcato wrote:
>> > > > +     if (kmem_cache_is_duplicate_name(name)) {
>> > > > +             /* Duplicate names will confuse slabtop, et al */
>> > > > +             pr_warn("%s: name %s already exists as a cache\n", __func__, name);
>> > >
>> > >
>> > > Shouldn't this be a full WARN_ON() instead of pr_warn()?  I assume we'll
>> > > be interested in who is adding the cache when the name already exists.
>> >
>> > panic_on_warn? :)
>> >
>>
>> Would get the problem fixed up pretty fast, no? :)
>>
>> > Personally I don't have anything against WARN_ON, but we've seen that
>> > panic_on_warn is a real thing on real systems, and DEBUG_VM is also
>> > set on real prod configs (like Fedora does/used to do). I've sent out
>> > one or two loose patches for problems I did find in my own testing
>> > around, but there may be many more (e.g some drivers may call
>> > kmem_cache_create repeatedly in some sort of callback, like 9pfs was
>> > doing when mounting; this is not greppable). And I'd guess grepping
>> > for cache names tends to be easy enough?
>> >
>>
>> Can we add a dump_stack() to make this way easier instead of hiding who is
>> creating the duplicate name?
> 
> 
> Bah, sorry for the delay.
> 
> I'm fully in favour of adding a dump_stack(), but it seems like hand
> coding WARN_ON a bit. Oh well.

After some pondering I've decided we should just go with WARN_ON and not do
a hand coded workaround for hypothetical people who run CONFIG_DEBUG_VM
together with panic_on_warn, so I've updated the commit in the slab tree
accordingly.

> If y'all agree, please squash this in (praying gmail doesn't mangle
> this diff, in any case it's a trivial change):
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 1abe6a577d52..d183655e4b1b 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -111,6 +111,7 @@ static int kmem_cache_sanity_check(const char
> *name, unsigned int size)
>        if (kmem_cache_is_duplicate_name(name)) {
>                /* Duplicate names will confuse slabtop, et al */
>                pr_warn("%s: name %s already exists as a cache\n",
> __func__, name);
> +               dump_stack_lvl(KERN_WARNING);
>        }
> 
>        WARN_ON(strchr(name, ' '));     /* It confuses parsers */