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

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

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>

>  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

[...]