netdev - Re: [PATCH bpf-next 2/3] libbpf: Support configurable pinning of maps from BTF annotations

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Message-ID: <87a79rmx2p.fsf@toke.dk>
Date:   Wed, 23 Oct 2019 15:05:18 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 2/3] libbpf: Support configurable pinning of maps from BTF annotations

> You are worried about the case where an application should be able to
> unpin the map before loading a new one so it doesn't get reused?

No, I'm worried about the opposite: Someone running (the equivalent of)
'ip link set dev eth0 xdp off', and then wondering why all resources
aren't freed.

I do believe that the common case could be solved by a logic similar to
the one I proposed, though:

>> Hmm, but I guess it could do that anyway; so maybe what we need is just
>> a "try to find all pinned maps of this program" function? That could
>> then to something like:
>> 
>> - Get the maps IDs and names of all maps attached to that program from
>>    the kernel.
>> 
>> - Look for each map name in /sys/fs/bpf
>> 
>> - If a pinned map with the same name exists, compare the IDs, and unlink
>>    if they match
>> 
>> I don't suppose it'll be possible to do all that in a race-free manner,
>> but that would go for any use of unlink() unless I'm missing something?