Re: [PATCH 4/6] block: clear BIO_PERCPU_CACHE flag if polling isn't supported

[Jens Axboe <axboe@kernel.dk>]

On 8/13/21 2:31 PM, Keith Busch wrote:
> On Fri, Aug 13, 2021 at 02:19:11PM -0600, Jens Axboe wrote:
>> On 8/13/21 2:17 PM, Keith Busch wrote:
>>> On Thu, Aug 12, 2021 at 11:41:58AM -0600, Jens Axboe wrote:
>>>> Indeed. Wonder if we should make that a small helper, as any clear of
>>>> REQ_HIPRI should clear BIO_PERCPU_CACHE as well.
>>>>
>>>>
>>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>>> index 7e852242f4cc..d2722ecd4d9b 100644
>>>> --- a/block/blk-core.c
>>>> +++ b/block/blk-core.c
>>>> @@ -821,11 +821,8 @@ static noinline_for_stack bool submit_bio_checks(struct bio *bio)
>>>>  		}
>>>>  	}
>>>>  
>>>> -	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags)) {
>>>> -		/* can't support alloc cache if we turn off polling */
>>>> -		bio_clear_flag(bio, BIO_PERCPU_CACHE);
>>>> -		bio->bi_opf &= ~REQ_HIPRI;
>>>> -	}
>>>> +	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
>>>> +		bio_clear_hipri(bio);
>>>
>>> Since BIO_PERCPU_CACHE doesn't work without REQ_HIRPI, should this check
>>> look more like this?
>>>
>>> 	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
>>> 		bio->bi_opf &= ~REQ_HIPRI;
>>> 	if (!(bio->bi_opf & REQ_HIPRI))
>>> 		bio_clear_flag(bio, BIO_PERCPU_CACHE);
>>>
>>> I realise the only BIO_PERCPU_CACHE user in this series never sets it
>>> without REQ_HIPRI, but it looks like a problem waiting to happen if
>>> nothing enforces this pairing: someone could set the CACHE flag on a
>>> QUEUE_FLAG_POLL enabled queue without setting HIPRI and get the wrong
>>> bio_put() action.
>>
>> I'd rather turn that into a WARN_ON or similar. But probably better to
>> do that on the freeing side, honestly. That'll be the most reliable way,
>> but a shame to add cycles to the hot path...
> 
> Yeah, it is a coding error if that happened, so a WARN sounds okay. I
> also don't like adding these kinds of checks, so please feel free to not
> include it if you think the usage is clear enough.

Just have to watch for new additions of IOCB_ALLOC_CACHE, which
thankfully shouldn't be too bad.

-- 
Jens Axboe