netdev - RE: [PATCH v3 bpf-next 2/3] libbpf: make global data internal arrays mmap()-able, if possible

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Date:   Wed, 13 Nov 2019 12:21:39 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Andrii Nakryiko <andriin@...com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...com, daniel@...earbox.net
Cc:     andrii.nakryiko@...il.com, kernel-team@...com,
        Andrii Nakryiko <andriin@...com>
Subject: RE: [PATCH v3 bpf-next 2/3] libbpf: make global data internal arrays
 mmap()-able, if possible

Andrii Nakryiko wrote:
> Add detection of BPF_F_MMAPABLE flag support for arrays and add it as an extra
> flag to internal global data maps, if supported by kernel. This allows users
> to memory-map global data and use it without BPF map operations, greatly
> simplifying user experience.
> 
> Acked-by: Song Liu <songliubraving@...com>
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---

[...]

>  /*
> @@ -856,8 +858,6 @@ bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
>  		pr_warn("failed to alloc map name\n");
>  		return -ENOMEM;
>  	}
> -	pr_debug("map '%s' (global data): at sec_idx %d, offset %zu.\n",
> -		 map_name, map->sec_idx, map->sec_offset);
>  
>  	def = &map->def;
>  	def->type = BPF_MAP_TYPE_ARRAY;
> @@ -865,6 +865,12 @@ bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
>  	def->value_size = data->d_size;
>  	def->max_entries = 1;
>  	def->map_flags = type == LIBBPF_MAP_RODATA ? BPF_F_RDONLY_PROG : 0;
> +	if (obj->caps.array_mmap)
> +		def->map_flags |= BPF_F_MMAPABLE;
> +
> +	pr_debug("map '%s' (global data): at sec_idx %d, offset %zu, flags %x.\n",
> +		 map_name, map->sec_idx, map->sec_offset, def->map_flags);
> +
>  	if (data_buff) {
>  		*data_buff = malloc(data->d_size);
>  		if (!*data_buff) {
> @@ -2160,6 +2166,27 @@ static int bpf_object__probe_btf_datasec(struct bpf_object *obj)
>  	return 0;
>  }

I was a bit concerned we should fall back to making the call without the
BPF_F_MMAPABLE flag set if it fails but did a quick walk through the call
path and it seems like it shouldn't fail except if vmalloc/vzalloc failures
so seems fine.

Acked-by: John Fastabend <john.fastabend@...il.com>