Re: [PATCH v5 bpf-next 1/5] bpf: Add bloom filter map implementation

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

On Wed, Oct 27, 2021 at 12:14 PM Joanne Koong <joannekoong@fb.com> wrote:
>
> On 10/26/21 8:18 PM, Andrii Nakryiko wrote:
>
> >
> > On 10/22/21 3:02 PM, Joanne Koong 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>
> >> ---
> >
> >
> > Apart from few minor comments below and the stuff that Martin
> > mentioned, LGTM.
> >
> > Acked-by: Andrii Nakryiko <andrii@kernel.org>
> >
> >
> >>   include/linux/bpf.h            |   2 +
> >>   include/linux/bpf_types.h      |   1 +
> >>   include/uapi/linux/bpf.h       |   8 ++
> >>   kernel/bpf/Makefile            |   2 +-
> >>   kernel/bpf/bloom_filter.c      | 198 +++++++++++++++++++++++++++++++++
> >>   kernel/bpf/syscall.c           |  19 +++-
> >>   kernel/bpf/verifier.c          |  19 +++-
> >>   tools/include/uapi/linux/bpf.h |   8 ++
> >>   8 files changed, 250 insertions(+), 7 deletions(-)
> >>   create mode 100644 kernel/bpf/bloom_filter.c
> >>
> >> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> >> index 31421c74ba08..953d23740ecc 100644
> >> --- a/include/linux/bpf.h
> >> +++ b/include/linux/bpf.h
> >> @@ -193,6 +193,8 @@ struct bpf_map {
> >>       struct work_struct work;
> >>       struct mutex freeze_mutex;
> >>       u64 writecnt; /* writable mmap cnt; protected by freeze_mutex */
> >> +
> >> +    u64 map_extra; /* any per-map-type extra fields */
> >
> >
> > It's minor, but given this is a read-only value, it makes more sense
> > to put it after map_flags so that it doesn't share a cache line with a
> > refcounting and mutex fields
> >
> >
> Awesome, I will make this change.
>
> One question I have in general that's semi-related is about
> backwards-compatibility.
> I might be completely misremembering, but I recall hearing something
> about only adding
> fields to the end of structs in some headers under the linux/include
> directory, so that this
> doesn't mess up backwards-compatibility with older kernel versions.
>
> Is this 100% false or is there a subset under linux/include (like
> linux/include/uapi/linux/*)
> that we do need to adhere to this for?

This backwards compatibility applies only to UAPI types. So any header
under include/uapi (i.e., include/uapi/linux/bpf.h which defines
"public interface" to BPF). While in this case you are modifying
internal kernel types. struct bpf_map is not exposed to user-space, so
you can re-shuffle fields if necessary.

>
> >
> > [...]
> >