Re: [PATCH v4 2/2] block,iomap: disable iopoll when split needed

[JeffleXu <jefflexu@linux.alibaba.com>]

Sorry for that, I will send a new version later.

On 11/24/20 7:25 PM, Christoph Hellwig wrote:
> On Fri, Nov 20, 2020 at 06:06:54PM +0800, JeffleXu wrote:
>>
>> On 11/20/20 1:55 AM, Christoph Hellwig wrote:
>>>> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
>>>> index 933f234d5bec..396ac0f91a43 100644
>>>> --- a/fs/iomap/direct-io.c
>>>> +++ b/fs/iomap/direct-io.c
>>>> @@ -309,6 +309,16 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>>>>   		copied += n;
>>>>   		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
>>>> +		/*
>>>> +		 * The current dio needs to be split into multiple bios here.
>>>> +		 * iopoll for split bio will cause subtle trouble such as
>>>> +		 * hang when doing sync polling, while iopoll is initially
>>>> +		 * for small size, latency sensitive IO. Thus disable iopoll
>>>> +		 * if split needed.
>>>> +		 */
>>>> +		if (nr_pages)
>>>> +			dio->iocb->ki_flags &= ~IOCB_HIPRI;
>>> I think this is confusing two things.
>>
>> Indeed there's two level of split concerning this issue when doing sync
>> iopoll.
>>
>>
>> The first is that one bio got split in block-core, and patch 1 of this patch
>> set just fixes this.
>>
>>
>> Second is that one dio got split into multiple bios in fs layer, and patch 2
>> fixes this.
>>
>>
>>>   One is that we don't handle
>>> polling well when there are multiple bios.  For this I think we should
>>> only call bio_set_polled when we know there is a single bio.
>>
>>
>> How about the following patch:
>>
>>
>> --- a/fs/iomap/direct-io.c
>> +++ b/fs/iomap/direct-io.c
>> @@ -60,12 +60,12 @@ int iomap_dio_iopoll(struct kiocb *kiocb, bool spin)
>> ??EXPORT_SYMBOL_GPL(iomap_dio_iopoll);
>>
>> ??static void iomap_dio_submit_bio(struct iomap_dio *dio, struct iomap
>> *iomap,
>> -???????????????????????????? struct bio *bio, loff_t pos)
>> +???????????????????????????? struct bio *bio, loff_t pos, bool split)
> 
> This seems pretty messed up by your mailer and I have a hard time
> reading it.  Can you resend it?
> 

-- 
Thanks,
Jeffle