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Date:   Fri, 15 Oct 2021 17:03:53 +0800
From:   Chengming Zhou <>
Subject: [PATCH] bpf: use count for prealloc hashtab too

We only use count for kmalloc hashtab not for prealloc hashtab, because
__pcpu_freelist_pop() return NULL when no more elem in pcpu freelist.

But the problem is that __pcpu_freelist_pop() will traverse all CPUs and
spin_lock for all CPUs to find there is no more elem at last.

We encountered bad case on big system with 96 CPUs that alloc_htab_elem()
would last for 1ms. This patch use count for prealloc hashtab too,
avoid traverse and spin_lock for all CPUs in this case.

Signed-off-by: Chengming Zhou <>
 kernel/bpf/hashtab.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 32471ba02708..0c432a23aa00 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -855,12 +855,12 @@ static void htab_put_fd_value(struct bpf_htab *htab, struct htab_elem *l)
 static void free_htab_elem(struct bpf_htab *htab, struct htab_elem *l)
 	htab_put_fd_value(htab, l);
+	atomic_dec(&htab->count);
 	if (htab_is_prealloc(htab)) {
 		check_and_free_timer(htab, l);
 		__pcpu_freelist_push(&htab->freelist, &l->fnode);
 	} else {
-		atomic_dec(&htab->count);
 		l->htab = htab;
 		call_rcu(&l->rcu, htab_elem_free_rcu);
@@ -938,6 +938,11 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 		} else {
 			struct pcpu_freelist_node *l;
+			if (atomic_inc_return(&htab->count) > htab->map.max_entries) {
+				l_new = ERR_PTR(-E2BIG);
+				goto dec_count;
+			}
 			l = __pcpu_freelist_pop(&htab->freelist);
 			if (!l)
 				return ERR_PTR(-E2BIG);

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