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

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Message-ID: <CAPhsuW7NFHbt0yPxh81gkRo8q_z_6JSrGGGLXtPMqvrbxk6b5w@mail.gmail.com>
Date: Fri, 4 Oct 2024 14:46:43 -0700
From: Song Liu <song@...nel.org>
To: Namhyung Kim <namhyung@...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 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).

Song