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Subject[PATCH 5.5 040/120] bpf: Improve bucket_log calculation logic
From: Martin KaFai Lau <>

commit 88d6f130e5632bbf419a2e184ec7adcbe241260b upstream.

It was reported that the max_t, ilog2, and roundup_pow_of_two macros have
exponential effects on the number of states in the sparse checker.

This patch breaks them up by calculating the "nbuckets" first so that the
"bucket_log" only needs to take ilog2().

In addition, Linus mentioned:

Patch looks good, but I'd like to point out that it's not just sparse.

You can see it with a simple

make net/core/bpf_sk_storage.i
grep 'smap->bucket_log = ' net/core/bpf_sk_storage.i | wc

and see the end result:

1 365071 2686974

That's one line (the assignment line) that is 2,686,974 characters in

Now, sparse does happen to react particularly badly to that (I didn't
look to why, but I suspect it's just that evaluating all the types
that don't actually ever end up getting used ends up being much more
expensive than it should be), but I bet it's not good for gcc either.

Fixes: 6ac99e8f23d4 ("bpf: Introduce bpf sk local storage")
Reported-by: Randy Dunlap <>
Reported-by: Luc Van Oostenryck <>
Suggested-by: Linus Torvalds <>
Signed-off-by: Martin KaFai Lau <>
Signed-off-by: Daniel Borkmann <>
Reviewed-by: Luc Van Oostenryck <>
Signed-off-by: Greg Kroah-Hartman <>

net/core/bpf_sk_storage.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- a/net/core/bpf_sk_storage.c
+++ b/net/core/bpf_sk_storage.c
@@ -643,9 +643,10 @@ static struct bpf_map *bpf_sk_storage_ma
return ERR_PTR(-ENOMEM);
bpf_map_init_from_attr(&smap->map, attr);

+ nbuckets = roundup_pow_of_two(num_possible_cpus());
/* Use at least 2 buckets, select_bucket() is undefined behavior with 1 bucket */
- smap->bucket_log = max_t(u32, 1, ilog2(roundup_pow_of_two(num_possible_cpus())));
- nbuckets = 1U << smap->bucket_log;
+ nbuckets = max_t(u32, 2, nbuckets);
+ smap->bucket_log = ilog2(nbuckets);
cost = sizeof(*smap->buckets) * nbuckets + sizeof(*smap);

ret = bpf_map_charge_init(&smap->map.memory, cost);

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