Re: [PATCH 13/18] io_uring: add file set registration

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On 07/02/2019 19:55, Jens Axboe wrote:
We normally have to fget/fput for each IO we do on a file. Even with
the batching we do, the cost of the atomic inc/dec of the file usage
count adds up.

This adds IORING_REGISTER_FILES, and IORING_UNREGISTER_FILES opcodes
for the io_uring_register(2) system call. The arguments passed in must
be an array of __s32 holding file descriptors, and nr_args should hold
the number of file descriptors the application wishes to pin for the
duration of the io_uring context (or until IORING_UNREGISTER_FILES is
called).

When used, the application must set IOSQE_FIXED_FILE in the sqe->flags
member. Then, instead of setting sqe->fd to the real fd, it sets sqe->fd
to the index in the array passed in to IORING_REGISTER_FILES.

Files are automatically unregistered when the io_uring context is
torn down. An application need only unregister if it wishes to
register a new set of fds.

Signed-off-by: Jens Axboe<axboe@xxxxxxxxx>
---
  fs/io_uring.c                 | 207 +++++++++++++++++++++++++++++-----
  include/net/af_unix.h         |   1 +
  include/uapi/linux/io_uring.h |   9 +-
  net/unix/af_unix.c            |   2 +-
  4 files changed, 188 insertions(+), 31 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 9d6233dc35ca..f2550efec60d 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -29,6 +29,7 @@
  #include <linux/net.h>
  #include <net/sock.h>
  #include <net/af_unix.h>
+#include <net/scm.h>
  #include <linux/anon_inodes.h>
  #include <linux/sched/mm.h>
  #include <linux/uaccess.h>
@@ -101,6 +102,13 @@ struct io_ring_ctx {
  		struct fasync_struct	*cq_fasync;
  	} ____cacheline_aligned_in_smp;
+ /*
+	 * If used, fixed file set. Writers must ensure that ->refs is dead,
+	 * readers must ensure that ->refs is alive as long as the file* is
+	 * used. Only updated through io_uring_register(2).
+	 */
+	struct scm_fp_list	*user_files;
+
  	/* if used, fixed mapped user buffers */
  	unsigned		nr_user_bufs;
  	struct io_mapped_ubuf	*user_bufs;
@@ -148,6 +156,7 @@ struct io_kiocb {
  	unsigned int		flags;
  #define REQ_F_FORCE_NONBLOCK	1	/* inline submission attempt */
  #define REQ_F_IOPOLL_COMPLETED	2	/* polled IO has completed */
+#define REQ_F_FIXED_FILE	4	/* ctx owns file */
  	u64			user_data;
  	u64			error;

+static void __io_sqe_files_unregister(struct io_ring_ctx *ctx)
+{
+#if defined(CONFIG_NET)
+	if (ctx->ring_sock) {
+		struct sock *sock = ctx->ring_sock->sk;
+		struct sk_buff *skb;
+
+		while ((skb = skb_dequeue(&sock->sk_receive_queue)) != NULL)
+			kfree_skb(skb);
+	}
+#else
+	int i;
+
+	for (i = 0; i < ctx->user_files->count; i++)
+		fput(ctx->user_files->fp[i]);
+
+	kfree(ctx->user_files);
+#endif
+}
+
+static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
+{
+	if (!ctx->user_files)
+		return -ENXIO;
+
+	__io_sqe_files_unregister(ctx);
+	ctx->user_files = NULL;
+	return 0;
+}
+
+static int io_sqe_files_scm(struct io_ring_ctx *ctx)
+{
+#if defined(CONFIG_NET)
+	struct scm_fp_list *fpl = ctx->user_files;
+	struct sk_buff *skb;
+	int i;
+
+	skb =  __alloc_skb(0, GFP_KERNEL, 0, NUMA_NO_NODE);
+	if (!skb)
+		return -ENOMEM;
+
+	skb->sk = ctx->ring_sock->sk;
+	skb->destructor = unix_destruct_scm;
+
+	fpl->user = get_uid(ctx->user);
+	for (i = 0; i < fpl->count; i++) {
+		get_file(fpl->fp[i]);
+		unix_inflight(fpl->user, fpl->fp[i]);
+		fput(fpl->fp[i]);
+	}
+
+	UNIXCB(skb).fp = fpl;
+	skb_queue_head(&ctx->ring_sock->sk->sk_receive_queue, skb);

This code sounds elegant if you know about the existence of unix_gc(), but quite mysterious if you don't.  (E.g. why "inflight"?)  Could we have a brief comment, to comfort mortal readers on their journey?

/* A message on a unix socket can hold a reference to a file. This can cause a reference cycle. So there is a garbage collector for unix sockets, which we hook into here. */

I think this is bypassing too_many_unix_fds() though?  I understood that was intended to bound kernel memory allocation, at least in principle.

+#endif

Also, this code relies on CONFIG_NET.  To handle the case where CONFIG_NET is not enabled, don't you still need to forbid registering an io_uring fd ?

+	return 0;
+}
+
+static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
+				 unsigned nr_args)
+{
+	__s32 __user *fds = (__s32 __user *) arg;
+	struct scm_fp_list *fpl;
+	int fd, ret = 0;
+	unsigned i;
+
+	if (ctx->user_files)
+		return -EBUSY;
+	if (!nr_args || nr_args > SCM_MAX_FD)
+		return -EINVAL;
+
+	fpl = kzalloc(sizeof(*ctx->user_files), GFP_KERNEL);
+	if (!fpl)
+		return -ENOMEM;
+	fpl->max = nr_args;
+
+	for (i = 0; i < nr_args; i++) {
+		ret = -EFAULT;
+		if (copy_from_user(&fd, &fds[i], sizeof(fd)))
+			break;
+
+		fpl->fp[i] = fget(fd);
+
+		ret = -EBADF;
+		if (!fpl->fp[i])
+			break;
+		fpl->count++;
+		ret = 0;
+	}
+
+	ctx->user_files = fpl;
+	if (!ret)
+		ret = io_sqe_files_scm(ctx);
+	if (ret)
+		io_sqe_files_unregister(ctx);
+
+	return ret;
+}
+
  static int io_sq_offload_start(struct io_ring_ctx *ctx)
  {
  	int ret;
@@ -1520,14 +1658,16 @@ static void io_ring_ctx_free(struct io_ring_ctx *ctx)
  		destroy_workqueue(ctx->sqo_wq);
  	if (ctx->sqo_mm)
  		mmdrop(ctx->sqo_mm);
+
+	io_iopoll_reap_events(ctx);
+	io_sqe_buffer_unregister(ctx);
+	io_sqe_files_unregister(ctx);
+
  #if defined(CONFIG_NET)
  	if (ctx->ring_sock)
  		sock_release(ctx->ring_sock);
  #endif
- io_iopoll_reap_events(ctx);
-	io_sqe_buffer_unregister(ctx);
-
  	io_mem_free(ctx->sq_ring);
  	io_mem_free(ctx->sq_sqes);
  	io_mem_free(ctx->cq_ring);
@@ -1885,6 +2025,15 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
  			break;
  		ret = io_sqe_buffer_unregister(ctx);
  		break;
+	case IORING_REGISTER_FILES:
+		ret = io_sqe_files_register(ctx, arg, nr_args);
+		break;
+	case IORING_UNREGISTER_FILES:
+		ret = -EINVAL;
+		if (arg || nr_args)
+			break;
+		ret = io_sqe_files_unregister(ctx);
+		break;
  	default:
  		ret = -EINVAL;
  		break;



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