On 10/7/21 7:20 AM, Toke Høiland-Jørgensen wrote:
Joanne Koong <joannekoong@xxxxxx> writes:
This patch adds the kernel-side changes for the implementation of
a bitset map with bloom filter capabilities.
The bitset map does not have keys, only values since it is a
non-associative data type. When the bitset map is created, it must
be created with a key_size of 0, and the max_entries value should be the
desired size of the bitset, in number of bits.
The bitset map supports peek (determining whether a bit is set in the
map), push (setting a bit in the map), and pop (clearing a bit in the
map) operations. These operations are exposed to userspace applications
through the already existing syscalls in the following way:
BPF_MAP_UPDATE_ELEM -> bpf_map_push_elem
BPF_MAP_LOOKUP_ELEM -> bpf_map_peek_elem
BPF_MAP_LOOKUP_AND_DELETE_ELEM -> bpf_map_pop_elem
For updates, the user will pass in a NULL key and the index of the
bit to set in the bitmap as the value. For lookups, the user will pass
in the index of the bit to check as the value. If the bit is set, 0
will be returned, else -ENOENT. For clearing the bit, the user will pass
in the index of the bit to clear as the value.
This is interesting, and I can see other uses of such a data structure.
However, a couple of questions (talking mostly about the 'raw' bitmap
without the bloom filter enabled):
- How are you envisioning synchronisation to work? The code is using the
atomic set_bit() operation, but there's no test_and_{set,clear}_bit().
Any thoughts on how users would do this?
I was thinking for users who are doing concurrent lookups + updates,
they are responsible for synchronizing the operations through mutexes.
Do you think this makes sense / is reasonable?
- It would be useful to expose the "find first set (ffs)" operation of
the bitmap as well. This can be added later, but thinking about the
API from the start would be good to avoid having to add a whole
separate helper for this. My immediate thought is to reserve peek(-1)
for this use - WDYT?
I think using peek(-1) for "find first set" sounds like a great idea!
- Any thoughts on inlining the lookups? This should at least be feasible
for the non-bloom-filter type, but I'm not quite sure if the use of
map_extra allows the verifier to distinguish between the map types
(I'm a little fuzzy on how the inlining actually works)? And can
peek()/push()/pop() be inlined at all?
I am not too familiar with how bpf instructions and inlining works, but
from a first glance, this looks doable for both the non-bloom filter
and bloom filter cases. From my cursory understanding of how it works,
it seems like we could have something like "bitset_map_gen_lookup" where
we parse the bpf_map->map_extra to see if the bloom filter is enabled;
if it is, we could call the hash function directly to compute which bit
to look up,
and then use the same insn logic for looking up the bit in both cases
(the bitmap w/ and w/out the bloom filter).
I don't think there is support yet in the verifier for inlining
peek()/push()/pop(), but it seems like this should be doable as well.
I think these changes would maybe warrant a separate patchset
on top of this one. What are your thoughts?
-Toke
