netdev - Re: [PATCH bpf-next v3 3/6] bpf: allow for map-in-map with dynamic inner array map entries

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Date:   Fri, 9 Oct 2020 23:09:51 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     ast@...nel.org, john.fastabend@...il.com, yhs@...com,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH bpf-next v3 3/6] bpf: allow for map-in-map with dynamic
 inner array map entries

On 10/9/20 11:04 PM, Alexei Starovoitov wrote:
> On Fri, Oct 09, 2020 at 10:42:42PM +0200, Daniel Borkmann wrote:
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index b97bc5abb3b8..593963e40956 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -435,6 +435,11 @@ enum {
>>   
>>   /* Share perf_event among processes */
>>   	BPF_F_PRESERVE_ELEMS	= (1U << 11),
>> +
>> +/* Do not inline (array) map lookups so the array map can be used for
>> + * map in map with dynamic max entries.
>> + */
>> +	BPF_F_NO_INLINE		= (1U << 12),
>>   };
> 
> I'm worried about this one.
> It exposes internal detail into uapi.
> Most users are not even aware of that map_lookup_elem() can be 'inlined'.
> 
> How about renaming the flag into BPF_F_INNER_MAP ?
> This way if we change the implementation later it will still be sane from uapi pov.
> The comment above the flag can say:
> /* Create a map that is suitable to be an inner map with dynamic max entries  */
> 
> Or some other name ?
> May be tomorrow we decide to simply load max_entries value in array_map_gen_lookup().
> The progs will become a bit slower, but it could be fine if we also do another
> optimization at the same time. Like the verifier can detect that 'key' is const
> and optimize it even further. Than slower gen_lookup for inner and all arrays
> will be mitigated by ultra fast lookup when !F_INNER_MAP and key is const.
> For F_INNER_MAP and key is const we could still do better inlining.

Hm, yes, agree. BPF_F_INNER_MAP is more generic and doesn't tell anything about
implementation, and ultimately it's only about using it as inner array. I'll take
it and do a v4.

Thanks,
Daniel