linux-kernel - Re: [PATCH v4 bpf-next 1/3] bpf: Add kmem

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Message-ID: <ZwB6SO4ARLMBquku@google.com>
Date: Fri, 4 Oct 2024 16:29:12 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Song Liu <song@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, bpf@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v4 bpf-next 1/3] bpf: Add kmem_cache iterator

On Fri, Oct 04, 2024 at 02:46:43PM -0700, Song Liu wrote:
> On Fri, Oct 4, 2024 at 2:37 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > Hi Song,
> >
> > On Fri, Oct 04, 2024 at 01:33:19PM -0700, Song Liu wrote:
> [...]
> > > > +
> > > > +static void *kmem_cache_iter_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> > > > +{
> > > > +       struct kmem_cache *s = v;
> > > > +       struct kmem_cache *next = NULL;
> > > > +       bool destroy = false;
> > > > +
> > > > +       ++*pos;
> > > > +
> > > > +       mutex_lock(&slab_mutex);
> > > > +
> > > > +       if (list_last_entry(&slab_caches, struct kmem_cache, list) != s) {
> > > > +               next = list_next_entry(s, list);
> > > > +               if (next->refcount > 0)
> > > > +                       next->refcount++;
> > >
> > > What if next->refcount <=0? Shall we find next of next?
> >
> > The slab_mutex should protect refcount == 0 case so it won't see that.
> > The negative refcount means it's a boot_cache and we shouldn't touch the
> > refcount.
> 
> I see. Thanks for the explanation!
> 
> Please add a comment here, and maybe also add
> 
>   WARN_ON_ONCE(next ->refcount == 0).

Sure, thanks for your review!
Namhyung