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From:   Changheun Lee <nanich.lee@samsung.com>
To:     gregkh@linuxfoundation.org
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Subject: Re: [RESEND PATCH v5 1/2] bio: limit bio max size
Date:   Wed,  7 Apr 2021 09:16:12 +0900
Message-Id: <20210407001612.17631-1-nanich.lee@samsung.com>
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        <CGME20210407003345epcas1p21376df37d3dfe933684139d3beaf9c51@epcas1p2.samsung.com>
Precedence: bulk

> On Tue, Apr 06, 2021 at 10:31:28AM +0900, 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 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@samsung.com>
> > > ---
> > >  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. :)
> 
> You are adding code that tests for a value to be set, yet you never set
> it in this code so why is it needed at all?

This patch is a solution for some inefficient case of multipage bvec like
as current DIO scenario. So it's not set as a default.
It will be set when bio size limitation is needed in runtime.

> 
> thanks,
> 
> greg k-h

---
Changheun Lee