netdev - Re: [PATCH v2 bpf-next 4/4] bpf: Replace bpf memory allocator with kmalloc

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Message-ID: <ebbf8915-1404-4d4f-9b5a-b2f3924ec43a@linux.dev>
Date: Mon, 17 Nov 2025 11:21:57 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Amery Hung <ameryhung@...il.com>, bpf <bpf@...r.kernel.org>,
 Network Development <netdev@...r.kernel.org>,
 Andrii Nakryiko <andrii@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...nel.org>,
 Kumar Kartikeya Dwivedi <memxor@...il.com>, KP Singh <kpsingh@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>, Song Liu <song@...nel.org>,
 Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH v2 bpf-next 4/4] bpf: Replace bpf memory allocator with
 kmalloc_nolock() in local storage

On 11/14/25 6:01 PM, Alexei Starovoitov wrote:
> On Fri, Nov 14, 2025 at 12:13 PM Amery Hung <ameryhung@...il.com> wrote:
>>
>>
>> -       if (smap->bpf_ma) {
>> +       if (smap->use_kmalloc_nolock) {
>>                  rcu_barrier_tasks_trace();
>> -               if (!rcu_trace_implies_rcu_gp())
>> -                       rcu_barrier();
>> -               bpf_mem_alloc_destroy(&smap->selem_ma);
>> -               bpf_mem_alloc_destroy(&smap->storage_ma);
>> +               rcu_barrier();
> 
> Why unconditional rcu_barrier() ?
> It's implied in rcu_barrier_tasks_trace().
> What am I missing?

Amery probably can confirm. I think the bpf_obj_free_fields() may only need to
wait for a rcu gp without going through a rcu_tasks_trace gp and the tasks_trace
cb, so it needs to ensure all rcu callbacks has finished.

@@ -247,18 +231,11 @@ void bpf_selem_free(struct bpf_local_storage_elem *selem,
  	}
  
  	if (reuse_now) {
-		/* reuse_now == true only happens when the storage owner
-		 * (e.g. task_struct) is being destructed or the map itself
-		 * is being destructed (ie map_free). In both cases,
-		 * no bpf prog can have a hold on the selem. It is
-		 * safe to unpin the uptrs and free the selem now.
-		 */
-		bpf_obj_free_fields(smap->map.record, SDATA(selem)->data);
-		/* Instead of using the vanilla call_rcu(),
-		 * bpf_mem_cache_free will be able to reuse selem
-		 * immediately.
+		/*
+		 * While it is okay to call bpf_obj_free_fields() that unpins uptr when
+		 * reuse_now == true, keep it in bpf_selem_free_rcu() for simplicity.
  		 */
-		bpf_mem_cache_free(&smap->selem_ma, selem);
+		call_rcu(&selem->rcu, bpf_selem_free_rcu);
  		return;
  	}


Others lgtm also,

Reviewed-by: Martin KaFai Lau <martin.lau@...nel.org>