netdev - Re: [PATCH v2 bpf-next 1/3] bpf: add mmap() support for BPF_MAP_TYPE

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Date:   Tue, 12 Nov 2019 14:03:50 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Rik van Riel <riel@...riel.com>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: add mmap() support for BPF_MAP_TYPE_ARRAY

On Tue, Nov 12, 2019 at 11:17 AM Jakub Kicinski
<jakub.kicinski@...ronome.com> wrote:
>
> On Mon, 11 Nov 2019 18:06:42 -0800, Andrii Nakryiko wrote:
> > So let's say if sizeof(struct bpf_array) is 300, then I'd have to either:
> >
> > - somehow make sure that I allocate 4k (for data) + 300 (for struct
> > bpf_array) in such a way that those 4k of data are 4k-aligned. Is
> > there any way to do that?
> > - assuming there isn't, then another way would be to allocate entire
> > 4k page for struct bpf_array itself, but put it at the end of that
> > page, so that 4k of data is 4k-aligned. While wasteful, the bigger
> > problem is that pointer to bpf_array is not a pointer to allocated
> > memory anymore, so we'd need to remember that and adjust address
> > before calling vfree().
> >
> > Were you suggesting #2 as a solution? Or am I missing some other way to do this?
>
> I am suggesting #2, that's the way to do it in the kernel.

So I'm concerned about this approach, because it feels like a bunch of
unnecessarily wasted memory. While there is no way around doing
round_up(PAGE_SIZE) for data itself, it certainly is not necessary to
waste almost entire page for struct bpf_array. And given this is going
to be used for BPF maps backing global variables, there most probably
will be at least 3 (.data, .bss, .rodata) per each program, and could
be more. Also, while on x86_64 page is 4k, on other architectures it
can be up to 64KB, so this seems wasteful.

What's your concern exactly with the way it's implemented in this patch?

>
> You could make the assumption that if you're allocating memory aligned
> to PAGE_SIZE, the address for vfree() is:
>
>         addr = map;
>         if (map->flags & MMAPABLE)
>                 addr = round_down(addr, PAGE_SIZE);
>         vfree(addr);
>
> Just make a note of the fact that we depend on vmalloc()s alignment in
> bpf_map_area_alloc().

will add comment for that