linux-kernel - Re: [PATCH bpf-next] bpf: remove is_return in struct bpf_session_run

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Message-ID: <CAPhsuW7THs9G+QV5_g+tMvXTAqVJ7jha-m70f675e9phK1Pryg@mail.gmail.com>
Date: Mon, 22 Sep 2025 16:08:36 +0200
From: Song Liu <song@...nel.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: jolsa@...nel.org, kpsingh@...nel.org, mattbobrowski@...gle.com, 
	ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	eddyz87@...il.com, yonghong.song@...ux.dev, john.fastabend@...il.com, 
	sdf@...ichev.me, haoluo@...gle.com, rostedt@...dmis.org, mhiramat@...nel.org, 
	mathieu.desnoyers@...icios.com, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: remove is_return in struct bpf_session_run_ctx

On Mon, Sep 22, 2025 at 11:57 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> The "data" in struct bpf_session_run_ctx is always 8-bytes aligned.
> Therefore, we can store the "is_return" to the last bit of the "data",
> which can make bpf_session_run_ctx 8-bytes aligned and save memory.

Does this really save anything? AFAICT, bpf_session_run_ctx is
only allocated on the stack. Therefore, we don't save any memory
unless there is potential risk of stack overflow.

OTOH, this last-bit logic is confusing and error prone. I would argue
against this type of optimization.

Thanks,
Song