Re: [PATCH v2] block: don't allow multiple bios for IOCB_NOWAIT issue

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 1/17/23 08:29, Jens Axboe wrote:
> On 1/16/23 4:28?PM, Jens Axboe wrote:
>> On 1/16/23 4:20?PM, Damien Le Moal wrote:
>>> On 1/17/23 06:06, Jens Axboe wrote:
>>>> If we're doing a large IO request which needs to be split into multiple
>>>> bios for issue, then we can run into the same situation as the below
>>>> marked commit fixes - parts will complete just fine, one or more parts
>>>> will fail to allocate a request. This will result in a partially
>>>> completed read or write request, where the caller gets EAGAIN even though
>>>> parts of the IO completed just fine.
>>>>
>>>> Do the same for large bios as we do for splits - fail a NOWAIT request
>>>> with EAGAIN. This isn't technically fixing an issue in the below marked
>>>> patch, but for stable purposes, we should have either none of them or
>>>> both.
>>>>
>>>> This depends on: 613b14884b85 ("block: handle bio_split_to_limits() NULL return")
>>>>
>>>> Cc: stable@vger.kernel.org # 5.15+
>>>> Fixes: 9cea62b2cbab ("block: don't allow splitting of a REQ_NOWAIT bio")
>>>> Link: https://github.com/axboe/liburing/issues/766
>>>> Reported-and-tested-by: Michael Kelley <mikelley@microsoft.com>
>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>
>>>> ---
>>>>
>>>> Since v1: catch this at submit time instead, since we can have various
>>>> valid cases where the number of single page segments will not take a
>>>> bio segment (page merging, huge pages).
>>>>
>>>> diff --git a/block/fops.c b/block/fops.c
>>>> index 50d245e8c913..d2e6be4e3d1c 100644
>>>> --- a/block/fops.c
>>>> +++ b/block/fops.c
>>>> @@ -221,6 +221,24 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
>>>>  			bio_endio(bio);
>>>>  			break;
>>>>  		}
>>>> +		if (iocb->ki_flags & IOCB_NOWAIT) {
>>>> +			/*
>>>> +			 * This is nonblocking IO, and we need to allocate
>>>> +			 * another bio if we have data left to map. As we
>>>> +			 * cannot guarantee that one of the sub bios will not
>>>> +			 * fail getting issued FOR NOWAIT and as error results
>>>> +			 * are coalesced across all of them, be safe and ask for
>>>> +			 * a retry of this from blocking context.
>>>> +			 */
>>>> +			if (unlikely(iov_iter_count(iter))) {
>>>> +				bio_release_pages(bio, false);
>>>> +				bio_clear_flag(bio, BIO_REFFED);
>>>> +				bio_put(bio);
>>>> +				blk_finish_plug(&plug);
>>>> +				return -EAGAIN;
>>>
>>> Doesn't this mean that for a really very large IO request that has 100%
>>> chance of being split, the user will always get -EAGAIN ? Not that I mind,
>>> doing super large IOs with NOWAIT is not a smart thing to do in the first
>>> place... But as a user interface, it seems that this will prevent any
>>> forward progress for such really large NOWAIT IOs. Is that OK ?
>>
>> Right, if you asked for NOWAIT, then it would not necessarily succeed if
>> it:
>>
>> 1) Needs multiple bios
>> 2) Needs splitting
>>
>> You're expected to attempt blocking issue at that point. Reasoning is
>> explained in this (and the previous commit related to the issue),
>> otherwise you end up with potentially various amounts of the request
>> being written to disk or read from disk, but EAGAIN being returned for
>> the request as a whole.
> 
> BTW, this is no different than eg doing a buffered read and needing to
> read in the data. You'd get EAGAIN, and no amount of repeated retries
> would change that. You need to either block for the IO at that point, or
> otherwise start it so it will become available directly at some later
> point (eg readahead).

Indeed.

-- 
Damien Le Moal
Western Digital Research