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

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Message-ID: <CAPhsuW4HLM=v=eGyT5F7epEKc_tfh=Y643wvkDOJRLdow-RWpg@mail.gmail.com>
Date: Fri, 4 Oct 2024 13:33:19 -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 Wed, Oct 2, 2024 at 11:09 AM Namhyung Kim <namhyung@...nel.org> wrote:
>
[...]
> +
> +       mutex_lock(&slab_mutex);
> +
> +       /*
> +        * Find an entry at the given position in the slab_caches list instead

Nit: style of multi-line comment: "/* Find ...".

> +        * 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;
> +       return s;
> +}
> +
> +static void kmem_cache_iter_seq_stop(struct seq_file *seq, void *v)
> +{
> +       struct bpf_iter_meta meta;
> +       struct bpf_iter__kmem_cache ctx = {
> +               .meta = &meta,
> +               .s = v,
> +       };
> +       struct bpf_prog *prog;
> +       bool destroy = false;
> +
> +       meta.seq = seq;
> +       prog = bpf_iter_get_info(&meta, true);
> +       if (prog)
> +               bpf_iter_run_prog(prog, &ctx);
> +
> +       if (ctx.s == NULL)
> +               return;
> +
> +       mutex_lock(&slab_mutex);
> +
> +       /* Skip kmem_cache_destroy() for active entries */
> +       if (ctx.s->refcount > 1)
> +               ctx.s->refcount--;
> +       else if (ctx.s->refcount == 1)
> +               destroy = true;
> +
> +       mutex_unlock(&slab_mutex);
> +
> +       if (destroy)
> +               kmem_cache_destroy(ctx.s);
> +}
> +
> +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?

> +       }
> +
> +       /* Skip kmem_cache_destroy() for active entries */
> +       if (s->refcount > 1)
> +               s->refcount--;
> +       else if (s->refcount == 1)
> +               destroy = true;
> +
> +       mutex_unlock(&slab_mutex);
> +
> +       if (destroy)
> +               kmem_cache_destroy(s);
> +
> +       return next;
> +}
[...]