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Subject: Re: [PATCH 15/15] io_uring: support true async buffered reads, if
 file provides it
To:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org
References: <20200618144355.17324-1-axboe@kernel.dk>
 <20200618144355.17324-16-axboe@kernel.dk>
 <029947e3-7615-e446-3194-d48827730e1d@gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <9c368ff8-b867-d40e-cd3b-6dacbecc0515@kernel.dk>
Date:   Tue, 23 Jun 2020 08:38:30 -0600
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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