mm/percpu.c: introduce pcpu_alloc_size()

Introduce pcpu_alloc_size() to get the size of the dynamic per-cpu
area. It will be used by bpf memory allocator in the following patches.
BPF memory allocator maintains per-cpu area caches for multiple area
sizes and its free API only has the to-be-freed per-cpu pointer, so it
needs the size of dynamic per-cpu area to select the corresponding cache
when bpf program frees the dynamic per-cpu pointer.

Acked-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20231020133202.4043247-3-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/percpu.h

@@ -132,6 +132,7 @@ extern void __init setup_per_cpu_areas(void);
 extern void __percpu *__alloc_percpu_gfp(size_t size, size_t align, gfp_t gfp) __alloc_size(1);
 extern void __percpu *__alloc_percpu(size_t size, size_t align) __alloc_size(1);
 extern void free_percpu(void __percpu *__pdata);
+extern size_t pcpu_alloc_size(void __percpu *__pdata);
 
 DEFINE_FREE(free_percpu, void __percpu *, free_percpu(_T))
 

mm/percpu.c

@@ -2244,6 +2244,37 @@ static void pcpu_balance_workfn(struct work_struct *work)
 	mutex_unlock(&pcpu_alloc_mutex);
 }
 
+/**
+ * pcpu_alloc_size - the size of the dynamic percpu area
+ * @ptr: pointer to the dynamic percpu area
+ *
+ * Returns the size of the @ptr allocation.  This is undefined for statically
+ * defined percpu variables as there is no corresponding chunk->bound_map.
+ *
+ * RETURNS:
+ * The size of the dynamic percpu area.
+ *
+ * CONTEXT:
+ * Can be called from atomic context.
+ */
+size_t pcpu_alloc_size(void __percpu *ptr)
+{
+	struct pcpu_chunk *chunk;
+	unsigned long bit_off, end;
+	void *addr;
+
+	if (!ptr)
+		return 0;
+
+	addr = __pcpu_ptr_to_addr(ptr);
+	/* No pcpu_lock here: ptr has not been freed, so chunk is still alive */
+	chunk = pcpu_chunk_addr_search(addr);
+	bit_off = (addr - chunk->base_addr) / PCPU_MIN_ALLOC_SIZE;
+	end = find_next_bit(chunk->bound_map, pcpu_chunk_map_bits(chunk),
+			    bit_off + 1);
+	return (end - bit_off) * PCPU_MIN_ALLOC_SIZE;
+}
+
 /**
  * free_percpu - free percpu area
  * @ptr: pointer to area to free