Re: [PATCH 15/15] io_uring: support true async buffered reads, if file provides it

Pavel Begunkov
2020-06-23T12:41:33Z

On 18/06/2020 17:43, Jens Axboe wrote:
> If the file is flagged with FMODE_BUF_RASYNC, then we don't have to punt
> the buffered read to an io-wq worker. Instead we can rely on page
> unlocking callbacks to support retry based async IO. This is a lot more
> efficient than doing async thread offload.
>
> The retry is done similarly to how we handle poll based retry. From
> the unlock callback, we simply queue the retry to a task_work based
> handler.
>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
> fs/io_uring.c | 145 +++++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 137 insertions(+), 8 deletions(-)
>
...
> static int io_read(struct io_kiocb *req, bool force_nonblock)
> {
> struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
> @@ -2784,10 +2907,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
> unsigned long nr_segs = iter.nr_segs;
> ssize_t ret2 = 0;
>
> - if (req->file->f_op->read_iter)
> - ret2 = call_read_iter(req->file, kiocb, &iter);
> - else
> - ret2 = loop_rw_iter(READ, req->file, kiocb, &iter);
> + ret2 = io_iter_do_read(req, &iter);
>
> /* Catch -EAGAIN return for forced non-blocking submission */
> if (!force_nonblock || (ret2 != -EAGAIN && ret2 != -EIO)) {
> @@ -2799,17 +2919,26 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
> ret = io_setup_async_rw(req, io_size, iovec,
> inline_vecs, &iter);
> if (ret)
> - goto out_free;
> + goto out;
> /* any defer here is final, must blocking retry */
> if (!(req->flags & REQ_F_NOWAIT) &&
> !file_can_poll(req->file))
> req->flags |= REQ_F_MUST_PUNT;
> + /* if we can retry, do so with the callbacks armed */
> + if (io_rw_should_retry(req)) {
> + ret2 = io_iter_do_read(req, &iter);
> + if (ret2 == -EIOCBQUEUED) {
> + goto out;
> + } else if (ret2 != -EAGAIN) {
> + kiocb_done(kiocb, ret2);
> + goto out;
> + }
> + }
> + kiocb->ki_flags &= ~IOCB_WAITQ;
> return -EAGAIN;
> }
> }
> -out_free:
> - kfree(iovec);
> - req->flags &= ~REQ_F_NEED_CLEANUP;

This looks fishy. For instance, if it fails early on rw_verify_area(), how would
it free yet on-stack iovec? Is it handled somehow?

> +out:
> return ret;
> }
>
>

--
Pavel Begunkov