linux-kernel - Re: [PATCH v5 bpf-next 7/7] bpf, x86_64: use bpf_prog

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Date:   Fri, 21 Jan 2022 10:29:55 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Song Liu <song@...nel.org>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v5 bpf-next 7/7] bpf, x86_64: use bpf_prog_pack allocator

On Fri, Jan 21, 2022 at 9:53 AM Song Liu <songliubraving@...com> wrote:
> >
> > the header->size could be just below 2MB.
> > I don't think kzalloc() can handle that.
>
> Technically, kzalloc can handle 2MB allocation via:
>   kzalloc() => kmalloc() => kmalloc_large() => kmalloc_order()
>
> But this would fail when the memory is fragmented. I guess we should use
> kvmalloc() instead?

Contiguous 2MB allocation?

> >
> >> +                               if (!tmp_header) {
> >> +                                       bpf_jit_binary_free_pack(header);
> >> +                                       header = NULL;
> >> +                                       prog = orig_prog;
> >> +                                       goto out_addrs;
> >> +                               }
> >> +                               tmp_header->size = header->size;
> >> +                               tmp_image = (void *)tmp_header + ((void *)image - (void *)header);
> >
> > Why is 'tmp_image' needed at all?
> > The above math can be done where necessary.
>
> We pass both image and tmp_image to do_jit(), as it needs both of them.
> I think maintaining a tmp_image variable makes the logic cleaner. We can
> remove it from x64_jit_data, I guess.

I'd remove from x64_jit_data. The recompute is cheap.

Speaking of tmp_header name... would be great to come up
with something more descriptive.
Here both tmp_header/tmp_image and header/image are used at the same time.
My initial confusion with the patch was due to the name 'tmp'.
The "tmp" prefix implies that the tmp_image will be used first
and then it will become an image.
But it's not the case.
Maybe call it 'rw_header' and add a comment that 'header/image'
are not writeable directly ?
Or call it 'poke_header' ?
Other ideas?