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

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Message-ID: <ZwBhNqXbn5R1JN1Y@google.com>
Date: Fri, 4 Oct 2024 14:42:14 -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 01:45:09PM -0700, Song Liu wrote:
> On Wed, Oct 2, 2024 at 11:09 AM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> [...]
> > +
> > +static void *kmem_cache_iter_seq_start(struct seq_file *seq, loff_t *pos)
> > +{
> > +       loff_t cnt = 0;
> > +       bool found = false;
> > +       struct kmem_cache *s;
> > +
> > +       mutex_lock(&slab_mutex);
> > +
> > +       /*
> > +        * Find an entry at the given position in the slab_caches list instead
> > +        * of keeping a reference (of the last visited entry, if any) out of
> > +        * slab_mutex. It might miss something if one is deleted in the middle
> > +        * while it releases the lock.  But it should be rare and there's not
> > +        * much we can do about it.
> > +        */
> > +       list_for_each_entry(s, &slab_caches, list) {
> > +               if (cnt == *pos) {
> > +                       /*
> > +                        * Make sure this entry remains in the list by getting
> > +                        * a new reference count.  Note that boot_cache entries
> > +                        * have a negative refcount, so don't touch them.
> > +                        */
> > +                       if (s->refcount > 0)
> > +                               s->refcount++;
> > +                       found = true;
> > +                       break;
> > +               }
> > +               cnt++;
> > +       }
> > +       mutex_unlock(&slab_mutex);
> > +
> > +       if (!found)
> > +               return NULL;
> > +
> > +       ++*pos;
> 
> This should be
> 
> if (*pos == 0)
>     ++*pos;

Oh, I thought there's check for seq->count after the seq->op->show()
for the ->start().  I need to check this logic again, thanks for
pointing this out.

Thanks,
Namhyung

> 
> > +       return s;
> > +}
> > +
> > +static void kmem_cache_iter_seq_stop(struct seq_file *seq, void *v)
> [...]