netdev - Re: [PATCH bpf-next 1/2] libbpf: Introduce bpf_map_

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Date:   Fri, 15 Feb 2019 01:52:11 +0000
From:   Yonghong Song <yhs@...com>
To:     Andrey Ignatov <rdna@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/2] libbpf: Introduce bpf_map__resize



On 2/14/19 3:01 PM, Andrey Ignatov wrote:
> Add bpf_map__resize() to change max_entries for a map.
> 
> Quite often necessary map size is unknown at compile time and can be
> calculated only at run time.
> 
> Currently the following approach is used to do so:
> * bpf_object__open_buffer() to open Elf file from a buffer;
> * bpf_object__find_map_by_name() to find relevant map;
> * bpf_map__def() to get map attributes and create struct
>    bpf_create_map_attr from them;
> * update max_entries in bpf_create_map_attr;
> * bpf_create_map_xattr() to create new map with updated max_entries;
> * bpf_map__reuse_fd() to replace the map in bpf_object with newly
>    created one.
> 
> And after all this bpf_object can finally be loaded. The map will have
> new size.
> 
> It 1) is quite a lot of steps; 2) doesn't take BTF into account.
> 
> For "2)" even more steps should be made and some of them require changes
> to libbpf (e.g. to get struct btf * from bpf_object).
> 
> Instead the whole problem can be solved by introducing simple
> bpf_map__resize() API that checks the map and sets new max_entries if
> the map is not loaded yet.
> 
> So the new steps are:
> * bpf_object__open_buffer() to open Elf file from a buffer;
> * bpf_object__find_map_by_name() to find relevant map;
> * bpf_map__resize() to update max_entries.
> 
> That's much simpler and works with BTF.
> 
> Signed-off-by: Andrey Ignatov <rdna@...com>
Acked-by: Yonghong Song <yhs@...com>