From: Andrii Nakryiko <firstname.lastname@example.org>
To: Jakub Kicinski <email@example.com>
Cc: Andrii Nakryiko <firstname.lastname@example.org>, bpf <email@example.com>,
Alexei Starovoitov <firstname.lastname@example.org>,
Daniel Borkmann <email@example.com>,
Kernel Team <firstname.lastname@example.org>, Rik van Riel <email@example.com>,
Johannes Weiner <firstname.lastname@example.org>
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: add mmap() support for BPF_MAP_TYPE_ARRAY
Date: Tue, 12 Nov 2019 19:19:33 -0800 [thread overview]
Message-ID: <CAEf4Bzb5y3eEq4QU_3zdGJts4PeSVGr1YVmmWjTgs=LOf4PLKw@mail.gmail.com> (raw)
On Tue, Nov 12, 2019 at 2:38 PM Jakub Kicinski
> On Tue, 12 Nov 2019 14:03:50 -0800, Andrii Nakryiko wrote:
> > On Tue, Nov 12, 2019 at 11:17 AM Jakub Kicinski 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.
> With the extra mutex and int you grew struct bpf_map from 192B to 256B,
> that's for every map on the system, unconditionally, and array map has
> an extra pointer even if it doesn't need it.
> Increasing "wasted" space when an opt-in feature is selected doesn't
> seem all that terrible to me, especially that the overhead of aligning
> up map size to page size is already necessary.
Well, I've been talking about one more extra page for struct bpf_array
itself, on top of what we already potentially waste for mmap()'ing
array data. But I went ahead and posted v3 with layout we discussed
here, aligning array->value on page boundary. Let's see if you like it
> > What's your concern exactly with the way it's implemented in this patch?
> Judging by other threads we seem to care about performance of BPF
> (rightly so). Doing an extra pointer deref for every static data access
> seems like an obvious waste.
> But then again, it's just an obvious suggestion, take it or leave it..
next prev parent reply other threads:[~2019-11-13 3:19 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-09 8:06 [PATCH v2 bpf-next 0/3] Add support for memory-mapping BPF array maps Andrii Nakryiko
2019-11-09 8:06 ` [PATCH v2 bpf-next 1/3] bpf: add mmap() support for BPF_MAP_TYPE_ARRAY Andrii Nakryiko
2019-11-11 16:40 ` Song Liu
2019-11-11 18:37 ` Jakub Kicinski
2019-11-12 2:06 ` Andrii Nakryiko
2019-11-12 19:17 ` Jakub Kicinski
2019-11-12 22:03 ` Andrii Nakryiko
2019-11-12 22:38 ` Jakub Kicinski
2019-11-13 3:19 ` Andrii Nakryiko [this message]
2019-11-11 18:39 ` Jakub Kicinski
2019-11-12 2:01 ` Andrii Nakryiko
2019-11-09 8:06 ` [PATCH v2 bpf-next 2/3] libbpf: make global data internal arrays mmap()-able, if possible Andrii Nakryiko
2019-11-11 18:40 ` Jakub Kicinski
2019-11-12 2:11 ` Andrii Nakryiko
2019-11-09 8:06 ` [PATCH v2 bpf-next 3/3] selftests/bpf: add BPF_TYPE_MAP_ARRAY mmap() tests Andrii Nakryiko
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