netdev - Re: [PATCH RFC bpf-next 2/2] bpf: Use kmalloc_nolock() in local storage unconditionally

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Message-ID: <CAADnVQ+2OXNo99B2krjwOb5XeFhi6GUagotcyf36xvLDoHqmjw@mail.gmail.com>
Date: Wed, 12 Nov 2025 11:35:22 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Amery Hung <ameryhung@...il.com>
Cc: 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 RFC bpf-next 2/2] bpf: Use kmalloc_nolock() in local
 storage unconditionally

On Wed, Nov 12, 2025 at 9:59 AM Amery Hung <ameryhung@...il.com> wrote:
>
> @@ -80,23 +80,12 @@ bpf_selem_alloc(struct bpf_local_storage_map *smap, void *owner,
>         if (mem_charge(smap, owner, smap->elem_size))
>                 return NULL;
>
> -       if (smap->bpf_ma) {
> -               selem = bpf_mem_cache_alloc_flags(&smap->selem_ma, gfp_flags);
> -               if (selem)
> -                       /* Keep the original bpf_map_kzalloc behavior
> -                        * before started using the bpf_mem_cache_alloc.
> -                        *
> -                        * No need to use zero_map_value. The bpf_selem_free()
> -                        * only does bpf_mem_cache_free when there is
> -                        * no other bpf prog is using the selem.
> -                        */
> -                       memset(SDATA(selem)->data, 0, smap->map.value_size);
> -       } else {
> -               selem = bpf_map_kzalloc(&smap->map, smap->elem_size,
> -                                       gfp_flags | __GFP_NOWARN);
> -       }
> +       selem = bpf_map_kmalloc_nolock(&smap->map, smap->elem_size, gfp_flags, NUMA_NO_NODE);


Pls enable CONFIG_DEBUG_VM=y then you'll see that the above triggers:
void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node)
{
        gfp_t alloc_gfp = __GFP_NOWARN | __GFP_NOMEMALLOC | gfp_flags;
...
        VM_WARN_ON_ONCE(gfp_flags & ~(__GFP_ACCOUNT | __GFP_ZERO |
                                      __GFP_NO_OBJ_EXT));

and benchmarking numbers have to be redone, since with
unsupported gfp flags kmalloc_nolock() is likely doing something wrong.