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

Jens Axboe
2020-06-23T14:38:34Z

On 6/23/20 6:39 AM, Pavel Begunkov wrote:
> 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?

This was tweaked and rebased on top of the REQ_F_NEED_CLEANUP change,
it should be correct in the tree:

https://git.kernel.dk/cgit/linux-block/tree/fs/io_uring.c?h=for-5.9/io_uring#n2908

--
Jens Axboe