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

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Message-ID: <aab45188-d34c-93c6-cfab-3c0cd1326a53@google.com>
Date: Sun, 11 Aug 2024 17:55:46 -0700 (PDT)
From: David Rientjes <rientjes@...gle.com>
To: Pedro Falcato <pedro.falcato@...il.com>
cc: Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>, 
    Joonsoo Kim <iamjoonsoo.kim@....com>, 
    Andrew Morton <akpm@...ux-foundation.org>, 
    Vlastimil Babka <vbabka@...e.cz>, 
    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 Sun, 11 Aug 2024, Pedro Falcato wrote:

> > > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > > index 40b582a014b..1abe6a577d5 100644
> > > --- a/mm/slab_common.c
> > > +++ b/mm/slab_common.c
> > > @@ -88,6 +88,19 @@ unsigned int kmem_cache_size(struct kmem_cache *s)
> > >  EXPORT_SYMBOL(kmem_cache_size);
> > >
> > >  #ifdef CONFIG_DEBUG_VM
> > > +
> > > +static bool kmem_cache_is_duplicate_name(const char *name)
> > > +{
> > > +     struct kmem_cache *s;
> > > +
> > > +     list_for_each_entry(s, &slab_caches, list) {
> > > +             if (!strcmp(s->name, name))
> > > +                     return true;
> > > +     }
> > > +
> > > +     return false;
> > > +}
> > > +
> > >  static int kmem_cache_sanity_check(const char *name, unsigned int size)
> > >  {
> > >       if (!name || in_interrupt() || size > KMALLOC_MAX_SIZE) {
> > > @@ -95,6 +108,11 @@ static int kmem_cache_sanity_check(const char *name, unsigned int size)
> > >               return -EINVAL;
> > >       }
> > >
> > > +     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?