Re: [PATCH v5 1/2] bio: limit bio max size

[Changheun Lee]

> 
> On Tue, Feb 16, 2021 at 11:00:32AM +0900, Changheun Lee wrote:
> > Please feedback to me if more modification is needed to apply. :)
> 
> No context here :(
> 

I'm so sorry. I missed it.

> bio size can grow up to 4GB when muli-page bvec is enabled.
> but sometimes it would lead to inefficient behaviors.
> in case of large chunk direct I/O, - 32MB chunk read in user space -
> all pages for 32MB would be merged to a bio structure if the pages
> physical addresses are contiguous. it makes some delay to submit
> until merge complete. bio max size should be limited to a proper size.
> 
> When 32MB chunk read with direct I/O option is coming from userspace,
> kernel behavior is below now in do_direct_IO() loop. it's timeline.
> 
>  | bio merge for 32MB. total 8,192 pages are merged.
>  | total elapsed time is over 2ms.
>  |------------------ ... ----------------------->|
>                                                  | 8,192 pages merged a bio.
>                                                  | at this time, first bio submit is done.
>                                                  | 1 bio is split to 32 read request and issue.
>                                                  |--------------->
>                                                   |--------------->
>                                                    |--------------->
>                                                               ......
>                                                                    |--------------->
>                                                                     |--------------->|
>                           total 19ms elapsed to complete 32MB read done from device. |
> 
> If bio max size is limited with 1MB, behavior is changed below.
> 
>  | bio merge for 1MB. 256 pages are merged for each bio.
>  | total 32 bio will be made.
>  | total elapsed time is over 2ms. it's same.
>  | but, first bio submit timing is fast. about 100us.
>  |--->|--->|--->|---> ... -->|--->|--->|--->|--->|
>       | 256 pages merged a bio.
>       | at this time, first bio submit is done.
>       | and 1 read request is issued for 1 bio.
>       |--------------->
>            |--------------->
>                 |--------------->
>                                       ......
>                                                  |--------------->
>                                                   |--------------->|
>         total 17ms elapsed to complete 32MB read done from device. |
> 
> As a result, read request issue timing is faster if bio max size is limited.
> Current kernel behavior with multipage bvec, super large bio can be created.
> And it lead to delay first I/O request issue.
> 
> Signed-off-by: Changheun Lee <nanich.lee@xxxxxxxxxxx>
> ---
>  block/bio.c            | 13 ++++++++++++-
>  include/linux/bio.h    |  2 +-
>  include/linux/blkdev.h |  3 +++
>  3 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 1f2cc1fbe283..c528e1f944c7 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -287,6 +287,17 @@ void bio_init(struct bio *bio, struct bio_vec *table,
>  }
>  EXPORT_SYMBOL(bio_init);
>  
> +unsigned int bio_max_size(struct bio *bio)
> +{
> +	struct request_queue *q = bio->bi_disk->queue;
> +
> +	if (blk_queue_limit_bio_size(q))
> +		return blk_queue_get_max_sectors(q, bio_op(bio))
> +			<< SECTOR_SHIFT;
> +
> +	return UINT_MAX;
> +}
> +
>  /**
>   * bio_reset - reinitialize a bio
>   * @bio:	bio to reset
> @@ -877,7 +888,7 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
>  		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
>  
>  		if (page_is_mergeable(bv, page, len, off, same_page)) {
> -			if (bio->bi_iter.bi_size > UINT_MAX - len) {
> +			if (bio->bi_iter.bi_size > bio_max_size(bio) - len) {
>  				*same_page = false;
>  				return false;
>  			}
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 1edda614f7ce..13b6f6562a5b 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -113,7 +113,7 @@ static inline bool bio_full(struct bio *bio, unsigned len)
>  	if (bio->bi_vcnt >= bio->bi_max_vecs)
>  		return true;
>  
> -	if (bio->bi_iter.bi_size > UINT_MAX - len)
> +	if (bio->bi_iter.bi_size > bio_max_size(bio) - len)
>  		return true;
>  
>  	return false;
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index f94ee3089e01..3aeab9e7e97b 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -621,6 +621,7 @@ struct request_queue {
>  #define QUEUE_FLAG_RQ_ALLOC_TIME 27	/* record rq->alloc_time_ns */
>  #define QUEUE_FLAG_HCTX_ACTIVE	28	/* at least one blk-mq hctx is active */
>  #define QUEUE_FLAG_NOWAIT       29	/* device supports NOWAIT */
> +#define QUEUE_FLAG_LIMIT_BIO_SIZE 30	/* limit bio size */
>  
>  #define QUEUE_FLAG_MQ_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\
>  				 (1 << QUEUE_FLAG_SAME_COMP) |		\
> @@ -667,6 +668,8 @@ bool blk_queue_flag_test_and_set(unsigned int flag, struct request_queue *q);
>  #define blk_queue_fua(q)	test_bit(QUEUE_FLAG_FUA, &(q)->queue_flags)
>  #define blk_queue_registered(q)	test_bit(QUEUE_FLAG_REGISTERED, &(q)->queue_flags)
>  #define blk_queue_nowait(q)	test_bit(QUEUE_FLAG_NOWAIT, &(q)->queue_flags)
> +#define blk_queue_limit_bio_size(q)	\
> +	test_bit(QUEUE_FLAG_LIMIT_BIO_SIZE, &(q)->queue_flags)
>  
>  extern void blk_set_pm_only(struct request_queue *q);
>  extern void blk_clear_pm_only(struct request_queue *q);
> -- 
> 2.28.0
> 

Please feedback to me if more modification is needed to apply. :)

---
Changheun Lee