linux-kernel - Re: [PATCH net-next] perf trace: fix MSG_SPLICE

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Date:   Tue, 27 Jun 2023 05:31:35 +0200 (GMT+02:00)
From:   Matthieu Baerts <matthieu.baerts@...sares.net>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Namhyung Kim <namhyung@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        dhowells@...hat.com, acme@...nel.org, adrian.hunter@...el.com,
        alexander.shishkin@...ux.intel.com, bpf@...r.kernel.org,
        davem@...emloft.net, irogers@...gle.com, jolsa@...nel.org,
        linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
        linux-perf-users@...r.kernel.org, mark.rutland@....com,
        mingo@...hat.com, netdev@...r.kernel.org, peterz@...radead.org,
        sfr@...b.auug.org.au
Subject: Re: [PATCH net-next] perf trace: fix MSG_SPLICE_PAGES build error

Hi Jakub, Namhyung,

@Namhyung: thank you for the explanations about the header files in the other thread!

26 Jun 2023 23:49:36 Jakub Kicinski <kuba@...nel.org>:
> On Mon, 26 Jun 2023 14:41:56 -0700 Namhyung Kim wrote:
>
>> But in general you don't need to change the copy of the tools
>> headers together.  It also needs to support old & new kernels
>> so different care should be taken.  Please separate tooling
>> changes and let us handle them.
>
> Ack, I'm not sure what makes this a special case, from Stephen's
> original report:
>
> https://lore.kernel.org/all/20230626112847.2ef3d422@canb.auug.org.au/
> > it sounded like perf won't build without the fix.

The copy of the header file is confusing :)

If I understood properly, here, we need the modification in msg_flags.c from Perf code because this file has been recently modified by David's series in net-next to support a new flag (not present in the system header files yet). Without this patch, Perf doesn't build.

Now regarding the "internal" copy of header files: we don't need to modify it because it is just used by Perf maintainers to monitor the modifications in the original file. If I'm not mistaken, it might even be better to revert the line that has been removed in David's original series in net-next in this header file. But now that Namhyung is aware of that modification, I guess the revert is probably not needed. But in short, it is then better not to modify this header file in the networking tree :-)

Cheers,
Matt
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