netdev - Re: [PATCH bpf] bpf: avoid setting bpf insns pages read-only when prog is jited

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Message-ID: <ec8264ad-8806-208a-1375-51e7cad1866e@gmail.com>
Date:   Fri, 29 Nov 2019 17:37:57 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        alexei.starovoitov@...il.com
Cc:     peterz@...radead.org, netdev@...r.kernel.org, bpf@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH bpf] bpf: avoid setting bpf insns pages read-only when
 prog is jited



On 11/29/19 2:29 PM, Daniel Borkmann wrote:
> For the case where the interpreter is compiled out or when the prog is jited
> it is completely unnecessary to set the BPF insn pages as read-only. In fact,
> on frequent churn of BPF programs, it could lead to performance degradation of
> the system over time since it would break the direct map down to 4k pages when
> calling set_memory_ro() for the insn buffer on x86-64 / arm64 and there is no
> reverse operation. Thus, avoid breaking up large pages for data maps, and only
> limit this to the module range used by the JIT where it is necessary to set
> the image read-only and executable.

Interesting... But why the non JIT case would need RO protection ?

Do you have any performance measures to share ?

Thanks.