handling EINTR from bpf_map_lookup_batch

[Yan Zhai <yan@cloudflare.com>]

I am getting EINTR when trying to use bpf_map_lookup_batch on an
array_of_maps. The error happens when there is a "hole" in the array.
For example, say the outer map has max entries of 256, each inner map
is used for a transport protocol, and I only populated key 6 and
17 for TCP and UDP. Then when I do batch lookup, I always get EINTR.
This so far seems to only happen with array of maps. Does it make
sense to allow skipping to the next key for this map type? Something
like:

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index c420edbfb7c8..83915a8059ef 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2027,6 +2027,8 @@ int generic_map_lookup_batch(struct bpf_map *map,
                                         attr->batch.elem_flags);

                if (err == -ENOENT) {
+                       if (IS_FD_ARRAY(map)
+                               goto next_key;
                        if (retry) {
                                retry--;
                                continue;
@@ -2048,6 +2050,7 @@ int generic_map_lookup_batch(struct bpf_map *map,
                        goto free_buf;
                }

+next_key:
                if (!prev_key)
                        prev_key = buf_prevkey;


Also the context about my scenario if anyone is curious: I am trying
to associate each map to a userspace service in a multi tenant
environment. This is an addition to cgroup accounting, in case the
creator cgroup goes away, e.g. systemd service restarts always
recreate cgroups. And we also want to monitor the utilization level of
non-prealloc maps of different tenants. When dealing with inner maps,
it is not always trivial. To connect dots I choose to read these IDs
periodically and link them to the tenant of the outer map, that's
where this EINTR occurred.

best
Yan