[bpf-next,v3,3/6] bpf: allow for map-in-map with dynamic inner array map entries

Recent work in f4d05259213f ("bpf: Add map_meta_equal map ops") and 134fede4eecf
("bpf: Relax max_entries check for most of the inner map types") added support
for dynamic inner max elements for most map-in-map types. Exceptions were maps
like array or prog array where the map_gen_lookup() callback uses the maps'
max_entries field as a constant when emitting instructions.

We recently implemented Maglev consistent hashing into Cilium's load balancer
which uses map-in-map with an outer map being hash and inner being array holding
the Maglev backend table for each service. This has been designed this way in
order to reduce overall memory consumption given the outer hash map allows to
avoid preallocating a large, flat memory area for all services. Also, the
number of service mappings is not always known a-priori.

The use case for dynamic inner array map entries is to further reduce memory
overhead, for example, some services might just have a small number of back
ends while others could have a large number. Right now the Maglev backend table
for small and large number of backends would need to have the same inner array
map entries which adds a lot of unneeded overhead.

Dynamic inner array map entries can be realized by avoiding the inlined code
generation for their lookup. The lookup will still be efficient since it will
be calling into array_map_lookup_elem() directly and thus avoiding retpoline.
The patch adds a BPF_F_NO_INLINE flag to map creation which therefore skips
inline code generation and relaxes array_map_meta_equal() check to ignore both
maps' max_entries.

Example code generation where inner map is dynamic sized array:

  # bpftool p d x i 125
  int handle__sys_enter(void * ctx):
  ; int handle__sys_enter(void *ctx)
     0: (b4) w1 = 0
  ; int key = 0;
     1: (63) *(u32 *)(r10 -4) = r1
     2: (bf) r2 = r10
  ;
     3: (07) r2 += -4
  ; inner_map = bpf_map_lookup_elem(&outer_arr_dyn, &key);
     4: (18) r1 = map[id:468]
     6: (07) r1 += 272
     7: (61) r0 = *(u32 *)(r2 +0)
     8: (35) if r0 >= 0x3 goto pc+5
     9: (67) r0 <<= 3
    10: (0f) r0 += r1
    11: (79) r0 = *(u64 *)(r0 +0)
    12: (15) if r0 == 0x0 goto pc+1
    13: (05) goto pc+1
    14: (b7) r0 = 0
    15: (b4) w6 = -1
  ; if (!inner_map)
    16: (15) if r0 == 0x0 goto pc+6
    17: (bf) r2 = r10
  ;
    18: (07) r2 += -4
  ; val = bpf_map_lookup_elem(inner_map, &key);
    19: (bf) r1 = r0                               | No inlining but instead
    20: (85) call array_map_lookup_elem#149280     | call to array_map_lookup_elem()
  ; return val ? *val : -1;                        | for inner array lookup.
    21: (15) if r0 == 0x0 goto pc+1
  ; return val ? *val : -1;
    22: (61) r6 = *(u32 *)(r0 +0)
  ; }
    23: (bc) w0 = w6
    24: (95) exit

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
---
 include/linux/bpf.h            |  2 +-
 include/uapi/linux/bpf.h       |  5 +++++
 kernel/bpf/arraymap.c          | 17 +++++++++++------
 kernel/bpf/hashtab.c           |  6 +++---
 kernel/bpf/verifier.c          |  4 +++-
 net/xdp/xskmap.c               |  2 +-
 tools/include/uapi/linux/bpf.h |  5 +++++
 7 files changed, 29 insertions(+), 12 deletions(-)

Message ID	20201009204245.27905-4-daniel@iogearbox.net (mailing list archive)
State	Superseded
Delegated to:	BPF
Headers	show Return-Path: <SRS0=vDm1=DQ=vger.kernel.org=bpf-owner@kernel.org> From: Daniel Borkmann <daniel@iogearbox.net> To: ast@kernel.org Cc: daniel@iogearbox.net, john.fastabend@gmail.com, yhs@fb.com, netdev@vger.kernel.org, bpf@vger.kernel.org, Andrii Nakryiko <andrii.nakryiko@gmail.com> Subject: [PATCH bpf-next v3 3/6] bpf: allow for map-in-map with dynamic inner array map entries Date: Fri, 9 Oct 2020 22:42:42 +0200 Message-Id: <20201009204245.27905-4-daniel@iogearbox.net> In-Reply-To: <20201009204245.27905-1-daniel@iogearbox.net> References: <20201009204245.27905-1-daniel@iogearbox.net> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk
Series	Follow-up BPF helper improvements \| expand [bpf-next,v3,0/6] Follow-up BPF helper improvements [bpf-next,v3,1/6] bpf: improve bpf_redirect_neigh helper description [bpf-next,v3,2/6] bpf: add redirect_peer helper [bpf-next,v3,3/6] bpf: allow for map-in-map with dynamic inner array map entries [bpf-next,v3,4/6] bpf, selftests: add test for different array inner map size [bpf-next,v3,5/6] bpf, selftests: make redirect_neigh test more extensible [bpf-next,v3,6/6] bpf, selftests: add redirect_peer selftest

[bpf-next,v3,3/6] bpf: allow for map-in-map with dynamic inner array map entries

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