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From:   Changheun Lee <nanich.lee@samsung.com>
To:     damien.lemoal@wdc.com
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Subject: Re: [PATCH v3 1/2] bio: limit bio max size
Date:   Wed, 27 Jan 2021 09:21:13 +0900
Message-Id: <20210127002113.8280-1-nanich.lee@samsung.com>
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> On 2021/01/26 18:37, Changheun Lee wrote:
> > 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. 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@samsung.com>
> > ---
> >  block/bio.c            | 17 ++++++++++++++++-
> >  include/linux/bio.h    |  4 +++-
> >  include/linux/blkdev.h |  3 +++
> >  3 files changed, 22 insertions(+), 2 deletions(-)
> > 
> > diff --git a/block/bio.c b/block/bio.c
> > index 1f2cc1fbe283..ec0281889045 100644
> > --- a/block/bio.c
> > +++ b/block/bio.c
> > @@ -287,6 +287,21 @@ 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;
> > +
> > +	if (!bio->bi_disk)
> > +		return UINT_MAX;
> > +
> > +	q = bio->bi_disk->queue;
> > +	if (!blk_queue_limit_bio_size(q))
> > +		return UINT_MAX;
> > +
> > +	return blk_queue_get_max_sectors(q, bio_op(bio)) << SECTOR_SHIFT;
> > +}
> > +EXPORT_SYMBOL(bio_max_size);
> 
> Why export this ? This is only used in this file and in bio.h in bio_full().

OK. I'll remove it.

> 
> > +
> >  /**
> >   * bio_reset - reinitialize a bio
> >   * @bio:	bio to reset
> > @@ -877,7 +892,7 @@ 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..cdb134ca7bf5 100644
> > --- a/include/linux/bio.h
> > +++ b/include/linux/bio.h
> > @@ -100,6 +100,8 @@ static inline void *bio_data(struct bio *bio)
> >  	return NULL;
> >  }
> >  
> > +extern unsigned int bio_max_size(struct bio *);
> 
> No need for extern.

It's just for compile warning in my test environment.
I'll remove it too. But I think compile warning could be in the other
.c file which includes bio.h. Is it OK?

> 
> > +
> >  /**
> >   * bio_full - check if the bio is full
> >   * @bio:	bio to check
> > @@ -113,7 +115,7 @@ 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);
> > 
> 
> 
> -- 
> Damien Le Moal
> Western Digital Research

-- 
Changheun Lee
Samsung Electronics