linux-ext4 - Re: [PATCH 5/8] nowait aio: return on congested block device

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Date:   Fri, 24 Mar 2017 06:32:11 -0500
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
To:     Jens Axboe <axboe@...nel.dk>, linux-fsdevel@...r.kernel.org
Cc:     jack@...e.com, hch@...radead.org, linux-block@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, sagi@...mberg.me, avi@...lladb.com,
        linux-api@...r.kernel.org, willy@...radead.org,
        Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 5/8] nowait aio: return on congested block device



On 03/16/2017 09:33 AM, Jens Axboe wrote:
> On 03/15/2017 03:51 PM, Goldwyn Rodrigues wrote:
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 0eeb99e..2e5cba2 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -2014,7 +2019,7 @@ blk_qc_t generic_make_request(struct bio *bio)
>>  	do {
>>  		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
>>  
>> -		if (likely(blk_queue_enter(q, false) == 0)) {
>> +		if (likely(blk_queue_enter(q, bio_flagged(bio, BIO_NOWAIT)) == 0)) {
>>  			struct bio_list hold;
>>  			struct bio_list lower, same;
>>  
>> @@ -2040,7 +2045,10 @@ blk_qc_t generic_make_request(struct bio *bio)
>>  			bio_list_merge(&bio_list_on_stack, &same);
>>  			bio_list_merge(&bio_list_on_stack, &hold);
>>  		} else {
>> -			bio_io_error(bio);
>> +			if (unlikely(bio_flagged(bio, BIO_NOWAIT)))
>> +				bio_wouldblock_error(bio);
>> +			else
>> +				bio_io_error(bio);
> 
> This doesn't look right. What if the queue is dying, and BIO_NOWAIT just
> happened to be set?
> 

Yes, I need to add a condition here to check for blk_queue_dying(). Thanks.

> And you're missing wbt_wait() as well as a blocking point. Ditto in
> blk-mq.

wbt_wait() does not apply to WRITE_ODIRECT


> 
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 159187a..942ce8c 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -1518,6 +1518,8 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
>>  	rq = blk_mq_sched_get_request(q, bio, bio->bi_opf, &data);
>>  	if (unlikely(!rq)) {
>>  		__wbt_done(q->rq_wb, wb_acct);
>> +		if (bio && bio_flagged(bio, BIO_NOWAIT))
>> +			bio_wouldblock_error(bio);
>>  		return BLK_QC_T_NONE;
>>  	}
>>  
> 
> This seems a little fragile now, since not both paths free the bio.
> 

Direct I/O should free the bios in bio_dio_complete(). I am not sure why
it would not free bio here originally, but IIRC, this path is for
bio==NULL only. So, with this patch we would get a rq==NULL here and
hence the bio_wouldblock_error() call.

-- 
Goldwyn