From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Andrii Nakryiko <andriin@fb.com>, bpf <bpf@vger.kernel.org>,
Networking <netdev@vger.kernel.org>,
Alexei Starovoitov <ast@fb.com>,
Daniel Borkmann <daniel@iogearbox.net>,
Kernel Team <kernel-team@fb.com>, Rik van Riel <riel@surriel.com>,
Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: add mmap() support for BPF_MAP_TYPE_ARRAY
Date: Mon, 11 Nov 2019 18:06:42 -0800 [thread overview]
Message-ID: <CAEf4Bzay-sCd5+5Y1+toJuEd6vNh+R7pkosYA7V7wDqTdoDxdw@mail.gmail.com> (raw)
In-Reply-To: <20191111103743.1c3a38a3@cakuba>
On Mon, Nov 11, 2019 at 10:37 AM Jakub Kicinski
<jakub.kicinski@netronome.com> wrote:
>
> On Sat, 9 Nov 2019 00:06:30 -0800, Andrii Nakryiko wrote:
> > With BPF_F_MMAPABLE array allocating data in separate chunk of memory,
> > array_map_gen_lookup has to accomodate these changes. For non-memory-mapped
> > there are no changes and no extra instructions. For BPF_F_MMAPABLE case,
> > pointer to where array data is stored has to be dereferenced first.
> >
> > Generated code for non-memory-mapped array:
> >
> > ; p = bpf_map_lookup_elem(&data_map, &zero);
> > 22: (18) r1 = map[id:19]
> > 24: (07) r1 += 408 /* array->inline_data offset */
> > 25: (61) r0 = *(u32 *)(r2 +0)
> > 26: (35) if r0 >= 0x3 goto pc+3
> > 27: (67) r0 <<= 3
> > 28: (0f) r0 += r1
> > 29: (05) goto pc+1
> > 30: (b7) r0 = 0
> >
> > Generated code for memory-mapped array:
> >
> > ; p = bpf_map_lookup_elem(&data_map, &zero);
> > 22: (18) r1 = map[id:27]
> > 24: (07) r1 += 400 /* array->data offset */
> > 25: (79) r1 = *(u64 *)(r1 +0) /* extra dereference */
> > 26: (61) r0 = *(u32 *)(r2 +0)
> > 27: (35) if r0 >= 0x3 goto pc+3
> > 28: (67) r0 <<= 3
> > 29: (0f) r0 += r1
> > 30: (05) goto pc+1
> > 31: (b7) r0 = 0
>
> Would it not be possible to overallocate the memory and align the start
> of the bpf_map in case of BPF_F_MMAPABLE so that no extra dereference
> is needed?
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?
next prev parent reply other threads:[~2019-11-12 2:06 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 [this message]
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
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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