expose required memory for the ring

Signed-off-by: Dylan Yudaken <dylany@fb.com>

src/include/liburing.h

diff --git a/src/include/liburing.h b/src/include/liburing.h
index b3f3cfe..1f9ceff 100644
--- a/src/include/liburing.h
+++ b/src/include/liburing.h
@@ -112,6 +112,8 @@
 	return (p->ops[op].flags & IO_URING_OP_SUPPORTED) != 0;
 }
 
+int io_uring_queue_required_mem(unsigned entries,
+				const struct io_uring_params *p, size_t *res);
 int io_uring_queue_init_mem(unsigned entries, struct io_uring *ring,
 				struct io_uring_params *p,
 				void *buf, size_t buf_size);

src/setup.c

diff --git a/src/setup.c b/src/setup.c
index 662f31c..3ca8506 100644
--- a/src/setup.c
+++ b/src/setup.c
@@ -23,7 +23,7 @@
 	return 1UL << __fls(depth - 1);
 }
 
-static int get_sq_cq_entries(unsigned entries, struct io_uring_params *p,
+static int get_sq_cq_entries(unsigned entries, const struct io_uring_params *p,
 			     unsigned *sq, unsigned *cq)
 {
 	unsigned cq_entries;
@@ -188,6 +188,52 @@
 	return 0;
 }
 
+int io_uring_calc_required_mem(unsigned entries,
+			const struct io_uring_params *p, size_t *sqes_mem,
+			size_t *ring_mem,
+			int *p_mem_used)
+{
+	unsigned sq_entries, cq_entries;
+	int ret;
+	unsigned long page_size = get_page_size();
+
+	ret = get_sq_cq_entries(entries, p, &sq_entries, &cq_entries);
+	if (ret)
+		return ret;
+
+
+	*sqes_mem = sq_entries * sizeof(struct io_uring_sqe);
+	*sqes_mem = (*sqes_mem + page_size - 1) & ~(page_size - 1);
+	*ring_mem = cq_entries * sizeof(struct io_uring_cqe);
+	*ring_mem += sq_entries * sizeof(unsigned);
+
+	if (p_mem_used) {
+		int mem_used = *sqes_mem;
+		mem_used += sq_entries * sizeof(unsigned int);
+		mem_used += cq_entries * sizeof(unsigned int);
+		/* round to full page */
+		mem_used = (mem_used + page_size - 1) & ~(page_size - 1);
+		*p_mem_used = mem_used;
+	}
+
+	return 0;
+}
+
+int io_uring_queue_required_mem(unsigned entries,
+				const struct io_uring_params *p, size_t *res)
+{
+	size_t sqe, ring;
+	int ret;
+
+	ret = io_uring_calc_required_mem(entries, p, &sqe, &ring, NULL);
+	if (ret)
+		return ret;
+
+	*res = sqe + ring;
+
+	return 0;
+}
+
 /* FIXME */
 static int huge_page_size = 2 * 1024 * 1024;
 
@@ -198,21 +244,15 @@
 			       struct io_uring_sq *sq, struct io_uring_cq *cq,
 			       void *buf, size_t buf_size)
 {
-	unsigned long page_size = get_page_size();
-	unsigned sq_entries, cq_entries;
-	size_t ring_mem, sqes_mem;
-	int ret, mem_used = 0;
+	size_t sqes_mem, ring_mem;
+	int ret, mem_used;
 	void *ptr;
 
-	ret = get_sq_cq_entries(entries, p, &sq_entries, &cq_entries);
+	ret = io_uring_calc_required_mem(entries, p,
+					 &sqes_mem, &ring_mem, &mem_used);
 	if (ret)
 		return ret;
 
-	sqes_mem = sq_entries * sizeof(struct io_uring_sqe);
-	sqes_mem = (sqes_mem + page_size - 1) & ~(page_size - 1);
-	ring_mem = cq_entries * sizeof(struct io_uring_cqe);
-	ring_mem += sq_entries * sizeof(unsigned);
-
 	if (buf) {
 		if (sqes_mem + ring_mem > buf_size)
 			return -ENOMEM;
@@ -247,13 +287,6 @@
 		cq->ring_sz = 0;
 	}
 
-	/* add up memory used */
-	mem_used += sqes_mem;
-	mem_used += sq_entries * sizeof(unsigned int);
-	mem_used += cq_entries * sizeof(unsigned int);
-	/* round to full page */
-	mem_used = (mem_used + page_size - 1) & ~(page_size - 1);
-
 	cq->ring_ptr = (void *) sq->ring_ptr;
 	p->sq_off.user_addr = (unsigned long) sq->sqes;
 	p->cq_off.user_addr = (unsigned long) sq->ring_ptr;