bpf: Only allocate one bpf_mem_cache for bpf_cpumask_ma

The size of bpf_cpumask is fixed, so there is no need to allocate many
bpf_mem_caches for bpf_cpumask_ma, just one bpf_mem_cache is enough.
Also add comments for bpf_mem_alloc_init() in bpf_mem_alloc.h to prevent
future miuse.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230216024821.2202916-1-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Hou Tao

include/linux/bpf_mem_alloc.h

@@ -14,6 +14,13 @@ struct bpf_mem_alloc {
 	struct work_struct work;
 };
 
+/* 'size != 0' is for bpf_mem_alloc which manages fixed-size objects.
+ * Alloc and free are done with bpf_mem_cache_{alloc,free}().
+ *
+ * 'size = 0' is for bpf_mem_alloc which manages many fixed-size objects.
+ * Alloc and free are done with bpf_mem_{alloc,free}() and the size of
+ * the returned object is given by the size argument of bpf_mem_alloc().
+ */
 int bpf_mem_alloc_init(struct bpf_mem_alloc *ma, int size, bool percpu);
 void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma);
 

kernel/bpf/cpumask.c

@@ -55,7 +55,7 @@ __bpf_kfunc struct bpf_cpumask *bpf_cpumask_create(void)
 	/* cpumask must be the first element so struct bpf_cpumask be cast to struct cpumask. */
 	BUILD_BUG_ON(offsetof(struct bpf_cpumask, cpumask) != 0);
 
-	cpumask = bpf_mem_alloc(&bpf_cpumask_ma, sizeof(*cpumask));
+	cpumask = bpf_mem_cache_alloc(&bpf_cpumask_ma);
 	if (!cpumask)
 		return NULL;
 
@@ -123,7 +123,7 @@ __bpf_kfunc void bpf_cpumask_release(struct bpf_cpumask *cpumask)
 
 	if (refcount_dec_and_test(&cpumask->usage)) {
 		migrate_disable();
-		bpf_mem_free(&bpf_cpumask_ma, cpumask);
+		bpf_mem_cache_free(&bpf_cpumask_ma, cpumask);
 		migrate_enable();
 	}
 }
@@ -468,7 +468,7 @@ static int __init cpumask_kfunc_init(void)
 		},
 	};
 
-	ret = bpf_mem_alloc_init(&bpf_cpumask_ma, 0, false);
+	ret = bpf_mem_alloc_init(&bpf_cpumask_ma, sizeof(struct bpf_cpumask), false);
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &cpumask_kfunc_set);
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &cpumask_kfunc_set);
 	return  ret ?: register_btf_id_dtor_kfuncs(cpumask_dtors,