Re: [PATCH] block: don't allow multiple bios for IOCB_NOWAIT issue

[Jens Axboe <axboe@kernel.dk>]

On 1/16/23 11:03?AM, Jens Axboe wrote:
>>> +	/*
>>> +	 * We're doing more than a bio worth of IO (> 256 pages), and we
>>> +	 * cannot guarantee that one of the sub bios will not fail getting
>>> +	 * issued FOR NOWAIT as error results are coalesced across all of
>>> +	 * them. Be safe and ask for a retry of this from blocking context.
>>> +	 */
>>> +	if (iocb->ki_flags & IOCB_NOWAIT)
>>> +		return -EAGAIN;
>>>  	return __blkdev_direct_IO(iocb, iter, bio_max_segs(nr_pages));
>>
>> If the I/O is too a huge page we could easily end up with a single
>> bio here.
> 
> True - we can push the decision making further down potentially, but
> honestly not sure it's worth the effort.

And even for page merges too, fwiw. We could probably do something like
the below (totally untested), downside there would be that we've already
mapped and allocated a bio at that point.


diff --git a/block/fops.c b/block/fops.c
index a03cb732c2a7..859361011e43 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -221,6 +221,14 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 			bio_endio(bio);
 			break;
 		}
+		if (iocb->ki_flags & IOCB_NOWAIT) {
+			if (iov_iter_count(iter)) {
+				bio_release_pages(bio, false);
+				bio_put(bio);
+				return -EAGAIN;
+			}
+			bio->bi_opf |= REQ_NOWAIT;
+		}
 
 		if (is_read) {
 			if (dio->flags & DIO_SHOULD_DIRTY)
@@ -228,9 +236,6 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 		} else {
 			task_io_account_write(bio->bi_iter.bi_size);
 		}
-		if (iocb->ki_flags & IOCB_NOWAIT)
-			bio->bi_opf |= REQ_NOWAIT;
-
 		dio->size += bio->bi_iter.bi_size;
 		pos += bio->bi_iter.bi_size;
 

-- 
Jens Axboe