Re: [PATCH 05/18] Add io_uring IO interface

[Jann Horn <jannh@google.com>]

On Mon, Jan 28, 2019 at 10:35 PM Jens Axboe <axboe@kernel.dk> wrote:
> The submission queue (SQ) and completion queue (CQ) rings are shared
> between the application and the kernel. This eliminates the need to
> copy data back and forth to submit and complete IO.
>
> IO submissions use the io_uring_sqe data structure, and completions
> are generated in the form of io_uring_sqe data structures. The SQ
> ring is an index into the io_uring_sqe array, which makes it possible
> to submit a batch of IOs without them being contiguous in the ring.
> The CQ ring is always contiguous, as completion events are inherently
> unordered, and hence any io_uring_cqe entry can point back to an
> arbitrary submission.
>
> Two new system calls are added for this:
>
> io_uring_setup(entries, params)
>         Sets up a context for doing async IO. On success, returns a file
>         descriptor that the application can mmap to gain access to the
>         SQ ring, CQ ring, and io_uring_sqes.
>
> io_uring_enter(fd, to_submit, min_complete, flags, sigset, sigsetsize)
>         Initiates IO against the rings mapped to this fd, or waits for
>         them to complete, or both. The behavior is controlled by the
>         parameters passed in. If 'to_submit' is non-zero, then we'll
>         try and submit new IO. If IORING_ENTER_GETEVENTS is set, the
>         kernel will wait for 'min_complete' events, if they aren't
>         already available. It's valid to set IORING_ENTER_GETEVENTS
>         and 'min_complete' == 0 at the same time, this allows the
>         kernel to return already completed events without waiting
>         for them. This is useful only for polling, as for IRQ
>         driven IO, the application can just check the CQ ring
>         without entering the kernel.
>
> With this setup, it's possible to do async IO with a single system
> call. Future developments will enable polled IO with this interface,
> and polled submission as well. The latter will enable an application
> to do IO without doing ANY system calls at all.
>
> For IRQ driven IO, an application only needs to enter the kernel for
> completions if it wants to wait for them to occur.
>
> Each io_uring is backed by a workqueue, to support buffered async IO
> as well. We will only punt to an async context if the command would
> need to wait for IO on the device side. Any data that can be accessed
> directly in the page cache is done inline. This avoids the slowness
> issue of usual threadpools, since cached data is accessed as quickly
> as a sync interface.
>
> Sample application: http://git.kernel.dk/cgit/fio/plain/t/io_uring.c
[...]
> +static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
> +                     bool force_nonblock)
> +{
> +       struct kiocb *kiocb = &req->rw;
> +       int ret;
> +
> +       kiocb->ki_filp = fget(sqe->fd);
> +       if (unlikely(!kiocb->ki_filp))
> +               return -EBADF;
> +       kiocb->ki_pos = sqe->off;
> +       kiocb->ki_flags = iocb_flags(kiocb->ki_filp);
> +       kiocb->ki_hint = ki_hint_validate(file_write_hint(kiocb->ki_filp));
> +       if (sqe->ioprio) {
> +               ret = ioprio_check_cap(sqe->ioprio);
> +               if (ret)
> +                       goto out_fput;
> +
> +               kiocb->ki_ioprio = sqe->ioprio;
> +       } else
> +               kiocb->ki_ioprio = get_current_ioprio();
> +
> +       ret = kiocb_set_rw_flags(kiocb, sqe->rw_flags);
> +       if (unlikely(ret))
> +               goto out_fput;
> +       if (force_nonblock) {
> +               kiocb->ki_flags |= IOCB_NOWAIT;
> +               req->flags |= REQ_F_FORCE_NONBLOCK;
> +       }
> +       if (kiocb->ki_flags & IOCB_HIPRI) {
> +               ret = -EINVAL;
> +               goto out_fput;
> +       }
> +
> +       kiocb->ki_complete = io_complete_rw;
> +       return 0;
> +out_fput:
> +       fput(kiocb->ki_filp);
> +       return ret;
> +}
[...]
> +static ssize_t io_read(struct io_kiocb *req, const struct io_uring_sqe *sqe,
> +                      bool force_nonblock)
> +{
> +       struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
> +       struct kiocb *kiocb = &req->rw;
> +       struct iov_iter iter;
> +       struct file *file;
> +       ssize_t ret;
> +
> +       ret = io_prep_rw(req, sqe, force_nonblock);
> +       if (ret)
> +               return ret;
> +       file = kiocb->ki_filp;
> +
> +       ret = -EBADF;
> +       if (unlikely(!(file->f_mode & FMODE_READ)))
> +               goto out_fput;
> +       ret = -EINVAL;
> +       if (unlikely(!file->f_op->read_iter))
> +               goto out_fput;
> +
> +       ret = io_import_iovec(req->ctx, READ, sqe, &iovec, &iter);
> +       if (ret)
> +               goto out_fput;
> +
> +       ret = rw_verify_area(READ, file, &kiocb->ki_pos, iov_iter_count(&iter));
> +       if (!ret) {
> +               ssize_t ret2;
> +
> +               /* Catch -EAGAIN return for forced non-blocking submission */
> +               ret2 = call_read_iter(file, kiocb, &iter);
> +               if (!force_nonblock || ret2 != -EAGAIN)
> +                       io_rw_done(kiocb, ret2);
> +               else
> +                       ret = -EAGAIN;
> +       }
> +       kfree(iovec);
> +out_fput:
> +       if (unlikely(ret))
> +               fput(file);
> +       return ret;
> +}
[...]
> +static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
> +                          struct sqe_submit *s, bool force_nonblock)
> +{
> +       const struct io_uring_sqe *sqe = s->sqe;
> +       ssize_t ret;
> +
> +       if (unlikely(s->index >= ctx->sq_entries))
> +               return -EINVAL;
> +       req->user_data = sqe->user_data;
> +
> +       ret = -EINVAL;
> +       switch (sqe->opcode) {
> +       case IORING_OP_NOP:
> +               ret = io_nop(req, sqe);
> +               break;
> +       case IORING_OP_READV:
> +               ret = io_read(req, sqe, force_nonblock);
> +               break;
> +       case IORING_OP_WRITEV:
> +               ret = io_write(req, sqe, force_nonblock);
> +               break;
> +       default:
> +               ret = -EINVAL;
> +               break;
> +       }
> +
> +       return ret;
> +}
> +
> +static void io_sq_wq_submit_work(struct work_struct *work)
> +{
> +       struct io_kiocb *req = container_of(work, struct io_kiocb, work);
> +       struct sqe_submit *s = &req->submit;
> +       u64 user_data = s->sqe->user_data;
> +       struct io_ring_ctx *ctx = req->ctx;
> +       mm_segment_t old_fs = get_fs();
> +       struct files_struct *old_files;
> +       int ret;
> +
> +        /* Ensure we clear previously set forced non-block flag */
> +       req->flags &= ~REQ_F_FORCE_NONBLOCK;
> +
> +       old_files = current->files;
> +       current->files = ctx->sqo_files;

I think you're not supposed to twiddle with current->files without
holding task_lock(current).

> +       if (!mmget_not_zero(ctx->sqo_mm)) {
> +               ret = -EFAULT;
> +               goto err;
> +       }
> +
> +       use_mm(ctx->sqo_mm);
> +       set_fs(USER_DS);
> +
> +       ret = __io_submit_sqe(ctx, req, s, false);
> +
> +       set_fs(old_fs);
> +       unuse_mm(ctx->sqo_mm);
> +       mmput(ctx->sqo_mm);
> +err:
> +       if (ret) {
> +               io_cqring_add_event(ctx, user_data, ret, 0);
> +               io_free_req(req);
> +       }
> +       current->files = old_files;
> +}
[...]
> +static int io_sq_offload_start(struct io_ring_ctx *ctx)
> +{
> +       int ret;
> +
> +       ctx->sqo_mm = current->mm;

What keeps this thing alive?

> +       /*
> +        * This is safe since 'current' has the fd installed, and if that gets
> +        * closed on exit, then fops->release() is invoked which waits for the
> +        * async contexts to flush and exit before exiting.
> +        */
> +       ret = -EBADF;
> +       ctx->sqo_files = current->files;
> +       if (!ctx->sqo_files)
> +               goto err;

That's gnarly. Adding Al Viro to the thread.

I think you misunderstand the semantics of f_op->release. The ->flush
handler is invoked whenever a file descriptor is closed through
filp_close() (via deletion of the files_struct, sys_close(),
sys_dup2(), ...), so if you had used that one, _maybe_ this would
work. But the ->release handler only runs when the _last_ reference to
a struct file has been dropped - so you can, for example, fork() a
child, then exit() in the parent, and the ->release handler isn't
invoked. So I don't see how this can work.

But even if you had abused ->flush for this instead: close_files()
currently has a comment in it that claims that "this is the last
reference to the files structure"; this change would make that claim
untrue.

> +       /* Do QD, or 2 * CPUS, whatever is smallest */
> +       ctx->sqo_wq = alloc_workqueue("io_ring-wq", WQ_UNBOUND | WQ_FREEZABLE,
> +                       min(ctx->sq_entries - 1, 2 * num_online_cpus()));
> +       if (!ctx->sqo_wq) {
> +               ret = -ENOMEM;
> +               goto err;
> +       }
> +
> +       return 0;
> +err:
> +       if (ctx->sqo_files)
> +               ctx->sqo_files = NULL;
> +       ctx->sqo_mm = NULL;
> +       return ret;
> +}
[...]
> +static const struct file_operations io_uring_fops = {
> +       .release        = io_uring_release,
> +       .mmap           = io_uring_mmap,
> +       .poll           = io_uring_poll,
> +       .fasync         = io_uring_fasync,
> +};
[...]

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