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

[Jens Axboe <axboe@kernel.dk>]

On 1/16/23 10:51 AM, Christoph Hellwig wrote:
> On Mon, Jan 16, 2023 at 09:01:37AM -0700, Jens Axboe wrote:
>> This depends on: 613b14884b85 ("block: handle bio_split_to_limits() NULL return")
> 
> Can we sort the NUL vs ERR_PTR thing there first?

Which thing?

>> +	/*
>> +	 * 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.

-- 
Jens Axboe