linux-kernel - Re: [PATCH v4 bpf-next 6/7] bpf: introduce bpf_prog

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Date:   Wed, 19 Jan 2022 20:14:21 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Song Liu <song@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, kernel-team@...com, peterz@...radead.org,
        x86@...nel.org, Song Liu <songliubraving@...com>
Subject: Re: [PATCH v4 bpf-next 6/7] bpf: introduce bpf_prog_pack allocator

On Wed, Jan 19, 2022 at 03:06:19PM -0800, Song Liu wrote:
>  
> +/*
> + * BPF program pack allocator.
> + *
> + * Most BPF programs are pretty small. Allocating a hole page for each
> + * program is sometime a waste. Many small bpf program also adds pressure
> + * to instruction TLB. To solve this issue, we introduce a BPF program pack
> + * allocator. The prog_pack allocator uses HPAGE_PMD_SIZE page (2MB on x86)
> + * to host BPF programs.
> + */
> +#define BPF_PROG_PACK_SIZE	HPAGE_PMD_SIZE
> +#define BPF_PROG_MAX_PACK_PROG_SIZE	HPAGE_PMD_SIZE

We have a synthetic test with 1M bpf instructions. How did it JIT?
Are you saying we were lucky that every BPF insn was JITed to <2 bytes x86?
Did I misread the 2MB limit?