io_uring: impose max limit on apoll cache

Caches like this tend to grow to the peak size, and then never get any
smaller. Impose a max limit on the size, to prevent it from growing too
big.

A somewhat randomly chosen 512 is the max size we'll allow the cache
to get. If a batch of frees come in and would bring it over that, we
simply start kfree'ing the surplus.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring_types.h

@@ -160,6 +160,7 @@ struct io_ev_fd {
 
 struct io_alloc_cache {
 	struct hlist_head	list;
+	unsigned int		nr_cached;
 };
 
 struct io_ring_ctx {

io_uring/alloc_cache.h

 #ifndef IOU_ALLOC_CACHE_H
 #define IOU_ALLOC_CACHE_H
 
+/*
+ * Don't allow the cache to grow beyond this size.
+ */
+#define IO_ALLOC_CACHE_MAX	512
+
 struct io_cache_entry {
 	struct hlist_node	node;
 };
 
-static inline void io_alloc_cache_put(struct io_alloc_cache *cache,
+static inline bool io_alloc_cache_put(struct io_alloc_cache *cache,
 				      struct io_cache_entry *entry)
 {
-	hlist_add_head(&entry->node, &cache->list);
+	if (cache->nr_cached < IO_ALLOC_CACHE_MAX) {
+		cache->nr_cached++;
+		hlist_add_head(&entry->node, &cache->list);
+		return true;
+	}
+	return false;
 }
 
 static inline struct io_cache_entry *io_alloc_cache_get(struct io_alloc_cache *cache)
@@ -26,6 +36,7 @@ static inline struct io_cache_entry *io_alloc_cache_get(struct io_alloc_cache *c
 static inline void io_alloc_cache_init(struct io_alloc_cache *cache)
 {
 	INIT_HLIST_HEAD(&cache->list);
+	cache->nr_cached = 0;
 }
 
 static inline void io_alloc_cache_free(struct io_alloc_cache *cache,
@@ -37,5 +48,6 @@ static inline void io_alloc_cache_free(struct io_alloc_cache *cache,
 		hlist_del(node);
 		free(container_of(node, struct io_cache_entry, node));
 	}
+	cache->nr_cached = 0;
 }
 #endif

io_uring/io_uring.c

@@ -1181,7 +1181,8 @@ void io_free_batch_list(struct io_ring_ctx *ctx, struct io_wq_work_node *node)
 
 				if (apoll->double_poll)
 					kfree(apoll->double_poll);
-				io_alloc_cache_put(&ctx->apoll_cache, &apoll->cache);
+				if (!io_alloc_cache_put(&ctx->apoll_cache, &apoll->cache))
+					kfree(apoll);
 				req->flags &= ~REQ_F_POLLED;
 			}
 			if (req->flags & IO_REQ_LINK_FLAGS)