[PATCH 3/3] io_uring/register: add IORING_REGISTER_RESIZE_RINGS

[Jens Axboe <axboe@kernel.dk>]

Once a ring has been created, the size of the CQ and SQ rings are fixed.
Usually this isn't a problem on the SQ ring side, as it merely controls
the available number of requests that can be submitted in a single
system call, and there's rarely a need to change that.

For the CQ ring, it's a different story. For most efficient use of
io_uring, it's important that the CQ ring never overflows. This means
that applications must size it for the worst case scenario, which can
be wasteful.

Add IORING_REGISTER_RESIZE_RINGS, which allows an application to resize
the existing rings. It takes a struct io_uring_params argument, the same
one which is used to setup the ring initially, and resizes rings
according to the sizes given.

Certain properties are always inherited from the original ring setup,
like SQE128/CQE32 and other setup options. The implementation only
allows flag associated with how the CQ ring is sized and clamped.

Existing unconsumed SQE and CQE entries are copied as part of the
process. Any register op holds ->uring_lock, which prevents new
submissions, and the internal mapping holds the completion lock as well
across moving CQ ring state.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/uapi/linux/io_uring.h |   3 +
 io_uring/register.c           | 161 ++++++++++++++++++++++++++++++++++
 2 files changed, 164 insertions(+)

diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 86cb385fe0b5..c4737892c7cd 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -615,6 +615,9 @@ enum io_uring_register_op {
 	/* send MSG_RING without having a ring */
 	IORING_REGISTER_SEND_MSG_RING		= 31,
 
+	/* resize CQ ring */
+	IORING_REGISTER_RESIZE_RINGS		= 33,
+
 	/* this goes last */
 	IORING_REGISTER_LAST,
 
diff --git a/io_uring/register.c b/io_uring/register.c
index 52b2f9b74af8..8dfe46a1cfe4 100644
--- a/io_uring/register.c
+++ b/io_uring/register.c
@@ -29,6 +29,7 @@
 #include "napi.h"
 #include "eventfd.h"
 #include "msg_ring.h"
+#include "memmap.h"
 
 #define IORING_MAX_RESTRICTIONS	(IORING_RESTRICTION_LAST + \
 				 IORING_REGISTER_LAST + IORING_OP_LAST)
@@ -361,6 +362,160 @@ static int io_register_clock(struct io_ring_ctx *ctx,
 	return 0;
 }
 
+/*
+ * State to maintain until we can swap. Both new and old state, used for
+ * either mapping or freeing.
+ */
+struct io_ring_ctx_rings {
+	unsigned short n_ring_pages;
+	unsigned short n_sqe_pages;
+	struct page **ring_pages;
+	struct page **sqe_pages;
+	struct io_uring_sqe *sq_sqes;
+	struct io_rings *rings;
+};
+
+static void io_register_free_rings(struct io_uring_params *p,
+				   struct io_ring_ctx_rings *r)
+{
+	if (!(p->flags & IORING_SETUP_NO_MMAP)) {
+		io_pages_unmap(r->rings, &r->ring_pages, &r->n_ring_pages,
+				true);
+		io_pages_unmap(r->sq_sqes, &r->sqe_pages, &r->n_sqe_pages,
+				true);
+	} else {
+		io_pages_free(&r->ring_pages, r->n_ring_pages);
+		io_pages_free(&r->sqe_pages, r->n_sqe_pages);
+		vunmap(r->rings);
+		vunmap(r->sq_sqes);
+	}
+}
+
+#define swap_old(ctx, o, n, field)		\
+	do {					\
+		(o).field = (ctx)->field;	\
+		(ctx)->field = (n).field;	\
+	} while (0)
+
+#define RESIZE_FLAGS	(IORING_SETUP_CQSIZE | IORING_SETUP_CLAMP)
+#define COPY_FLAGS	(IORING_SETUP_NO_SQARRAY | IORING_SETUP_SQE128 | \
+			 IORING_SETUP_CQE32 | IORING_SETUP_NO_MMAP)
+
+static int io_register_resize_rings(struct io_ring_ctx *ctx, void __user *arg)
+{
+	struct io_ring_ctx_rings o = { }, n = { };
+	size_t size, sq_array_offset;
+	struct io_uring_params p;
+	unsigned i, tail;
+	void *ptr;
+	int ret;
+
+	if (copy_from_user(&p, arg, sizeof(p)))
+		return -EFAULT;
+	if (p.flags & ~RESIZE_FLAGS)
+		return -EINVAL;
+	/* nothing to do */
+	if (p.sq_entries == ctx->sq_entries && p.cq_entries == ctx->cq_entries)
+		return 0;
+	/* properties that are always inherited */
+	p.flags |= (ctx->flags & COPY_FLAGS);
+
+	ret = io_uring_fill_params(p.sq_entries, &p);
+	if (unlikely(ret))
+		return ret;
+
+	size = rings_size(p.flags, p.sq_entries, p.cq_entries,
+				&sq_array_offset);
+	if (size == SIZE_MAX)
+		return -EOVERFLOW;
+
+	if (!(p.flags & IORING_SETUP_NO_MMAP))
+		n.rings = io_pages_map(&n.ring_pages, &n.n_ring_pages, size);
+	else
+		n.rings = __io_uaddr_map(&n.ring_pages, &n.n_ring_pages,
+						p.cq_off.user_addr, size);
+	if (IS_ERR(n.rings))
+		return PTR_ERR(n.rings);
+
+	n.rings->sq_ring_mask = p.sq_entries - 1;
+	n.rings->cq_ring_mask = p.cq_entries - 1;
+	n.rings->sq_ring_entries = p.sq_entries;
+	n.rings->cq_ring_entries = p.cq_entries;
+
+	if (copy_to_user(arg, &p, sizeof(p))) {
+		io_register_free_rings(&p, &n);
+		return -EFAULT;
+	}
+
+	if (p.flags & IORING_SETUP_SQE128)
+		size = array_size(2 * sizeof(struct io_uring_sqe), p.sq_entries);
+	else
+		size = array_size(sizeof(struct io_uring_sqe), p.sq_entries);
+	if (size == SIZE_MAX) {
+		io_register_free_rings(&p, &n);
+		return -EOVERFLOW;
+	}
+
+	if (!(p.flags & IORING_SETUP_NO_MMAP))
+		ptr = io_pages_map(&n.sqe_pages, &n.n_sqe_pages, size);
+	else
+		ptr = __io_uaddr_map(&n.sqe_pages, &n.n_sqe_pages,
+					p.sq_off.user_addr,
+					size);
+	if (IS_ERR(ptr)) {
+		io_register_free_rings(&p, &n);
+		return PTR_ERR(ptr);
+	}
+
+	/* now copy entries, if any */
+	n.sq_sqes = ptr;
+	tail = ctx->rings->sq.tail;
+	for (i = ctx->rings->sq.head; i < tail; i++) {
+		unsigned src_head = i & (ctx->sq_entries - 1);
+		unsigned dst_head = i & n.rings->sq_ring_mask;
+
+		n.sq_sqes[dst_head] = ctx->sq_sqes[src_head];
+	}
+	n.rings->sq.head = ctx->rings->sq.head;
+	n.rings->sq.tail = ctx->rings->sq.tail;
+
+	spin_lock(&ctx->completion_lock);
+	tail = ctx->rings->cq.tail;
+	for (i = ctx->rings->cq.head; i < tail; i++) {
+		unsigned src_head = i & (ctx->cq_entries - 1);
+		unsigned dst_head = i & n.rings->cq_ring_mask;
+
+		n.rings->cqes[dst_head] = ctx->rings->cqes[src_head];
+	}
+	n.rings->cq.head = ctx->rings->cq.head;
+	n.rings->cq.tail = ctx->rings->cq.tail;
+	/* invalidate cached cqe refill */
+	ctx->cqe_cached = ctx->cqe_sentinel = NULL;
+
+	n.rings->sq_dropped = ctx->rings->sq_dropped;
+	n.rings->sq_flags = ctx->rings->sq_flags;
+	n.rings->cq_flags = ctx->rings->cq_flags;
+	n.rings->cq_overflow = ctx->rings->cq_overflow;
+
+	/* all done, store old pointers and assign new ones */
+	if (!(ctx->flags & IORING_SETUP_NO_SQARRAY))
+		ctx->sq_array = (u32 *)((char *)n.rings + sq_array_offset);
+
+	ctx->sq_entries = p.sq_entries;
+	ctx->cq_entries = p.cq_entries;
+
+	swap_old(ctx, o, n, rings);
+	swap_old(ctx, o, n, n_ring_pages);
+	swap_old(ctx, o, n, n_sqe_pages);
+	swap_old(ctx, o, n, ring_pages);
+	swap_old(ctx, o, n, sqe_pages);
+	swap_old(ctx, o, n, sq_sqes);
+	spin_unlock(&ctx->completion_lock);
+
+	io_register_free_rings(&p, &o);
+	return 0;
+}
+
 static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 			       void __user *arg, unsigned nr_args)
 	__releases(ctx->uring_lock)
@@ -549,6 +704,12 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 			break;
 		ret = io_register_clone_buffers(ctx, arg);
 		break;
+	case IORING_REGISTER_RESIZE_RINGS:
+		ret = -EINVAL;
+		if (!arg || nr_args != 1)
+			break;
+		ret = io_register_resize_rings(ctx, arg);
+		break;
 	default:
 		ret = -EINVAL;
 		break;
-- 
2.45.2