Re: [PATCH v6 bpf-next 1/5] bpf: Add bloom filter map implementation

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Oct 28, 2021 at 5:15 PM Joanne Koong <joannekoong@fb.com> wrote:
>
> On 10/28/21 11:15 AM, Andrii Nakryiko wrote:
>
> > On Wed, Oct 27, 2021 at 4:45 PM Joanne Koong <joannekoong@fb.com> wrote:
> >> This patch adds the kernel-side changes for the implementation of
> >> a bpf bloom filter map.
> >>
> >> The bloom filter map supports peek (determining whether an element
> >> is present in the map) and push (adding an element to the map)
> >> operations.These operations are exposed to userspace applications
> >> through the already existing syscalls in the following way:
> >>
> >> BPF_MAP_LOOKUP_ELEM -> peek
> >> BPF_MAP_UPDATE_ELEM -> push
> >>
> >> The bloom filter map does not have keys, only values. In light of
> >> this, the bloom filter map's API matches that of queue stack maps:
> >> user applications use BPF_MAP_LOOKUP_ELEM/BPF_MAP_UPDATE_ELEM
> >> which correspond internally to bpf_map_peek_elem/bpf_map_push_elem,
> >> and bpf programs must use the bpf_map_peek_elem and bpf_map_push_elem
> >> APIs to query or add an element to the bloom filter map. When the
> >> bloom filter map is created, it must be created with a key_size of 0.
> >>
> >> For updates, the user will pass in the element to add to the map
> >> as the value, with a NULL key. For lookups, the user will pass in the
> >> element to query in the map as the value, with a NULL key. In the
> >> verifier layer, this requires us to modify the argument type of
> >> a bloom filter's BPF_FUNC_map_peek_elem call to ARG_PTR_TO_MAP_VALUE;
> >> as well, in the syscall layer, we need to copy over the user value
> >> so that in bpf_map_peek_elem, we know which specific value to query.
> >>
> >> A few things to please take note of:
> >>   * If there are any concurrent lookups + updates, the user is
> >> responsible for synchronizing this to ensure no false negative lookups
> >> occur.
> >>   * The number of hashes to use for the bloom filter is configurable from
> >> userspace. If no number is specified, the default used will be 5 hash
> >> functions. The benchmarks later in this patchset can help compare the
> >> performance of using different number of hashes on different entry
> >> sizes. In general, using more hashes decreases both the false positive
> >> rate and the speed of a lookup.
> >>   * Deleting an element in the bloom filter map is not supported.
> >>   * The bloom filter map may be used as an inner map.
> >>   * The "max_entries" size that is specified at map creation time is used
> >> to approximate a reasonable bitmap size for the bloom filter, and is not
> >> otherwise strictly enforced. If the user wishes to insert more entries
> >> into the bloom filter than "max_entries", they may do so but they should
> >> be aware that this may lead to a higher false positive rate.
> >>
> >> Signed-off-by: Joanne Koong <joannekoong@fb.com>
> >> ---
> > Don't forget to keep received Acks between revisions.
> >
> > Acked-by: Andrii Nakryiko <andrii@kernel.org>
> Can you elaborate a little on how to keep received Acks between revisions?
>
> Should I copy and paste the "Acked-by: Andrii Nakryiko <andrii@kernel.org>"

Yes, it's all manual. See [0] for an example.

  [0] https://patchwork.kernel.org/project/netdevbpf/patch/20211028063501.2239335-9-memxor@gmail.com/

> line into the commit message for the patch? Or should this information be
> in the subject line of the email for the patch? Or in the patchset series'
> cover letter? Thanks!
>
> >
> >>   include/linux/bpf.h            |   1 +
> >>   include/linux/bpf_types.h      |   1 +
> >>   include/uapi/linux/bpf.h       |   9 ++
> >>   kernel/bpf/Makefile            |   2 +-
> >>   kernel/bpf/bloom_filter.c      | 195 +++++++++++++++++++++++++++++++++
> >>   kernel/bpf/syscall.c           |  24 +++-
> >>   kernel/bpf/verifier.c          |  19 +++-
> >>   tools/include/uapi/linux/bpf.h |   9 ++
> >>   8 files changed, 253 insertions(+), 7 deletions(-)
> >>   create mode 100644 kernel/bpf/bloom_filter.c
> > [...]