Re: [PATCH v3 1/2] bio: limit bio max size

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jan 26, 2021 at 04:06:06AM +0000, Damien Le Moal wrote:
> On 2021/01/26 12:58, Ming Lei wrote:
> > On Tue, Jan 26, 2021 at 10:32:34AM +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. 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);
> >> +
> >>  /**
> >>   * 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;
> >>  			}
> > 
> > So far we don't need bio->bi_disk or bio->bi_bdev(will be changed in
> > Christoph's patch) during adding page to bio, so there is null ptr
> > refereance risk.
> > 
> >> 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 *);
> >> +
> >>  /**
> >>   * 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)
> > 
> > I don't think it is a good idea by adding queue flag for this purpose,
> > since this case just needs to limit bio size for not delay bio submission
> > too much, which is kind of logical thing, nothing to do with request queue.
> > 
> > Just wondering why you don't take the following way:
> > 
> > 
> > diff --git a/block/bio.c b/block/bio.c
> > index 99040a7e6656..35852f7f47d4 100644
> > --- a/block/bio.c
> > +++ b/block/bio.c
> > @@ -1081,7 +1081,7 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
> >   * It's intended for direct IO, so doesn't do PSI tracking, the caller is
> >   * responsible for setting BIO_WORKINGSET if necessary.
> >   */
> > -int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> > +int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter, bool sync)
> >  {
> >  	int ret = 0;
> >  
> > @@ -1092,12 +1092,20 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> >  		bio_set_flag(bio, BIO_NO_PAGE_REF);
> >  		return 0;
> >  	} else {
> > +		/*
> > +		 * Don't add too many pages in case of sync dio for
> > +		 * avoiding delay bio submission too much especially
> > +		 * pinning user pages in memory isn't cheap.
> > +		 */
> > +		const unsigned int max_size = sync ? (1U << 12) : UINT_MAX;
> 
> 4KB max bio size ? That is a little small :)

It should have been (1U << 20), :-(

> In any case, I am not a fan of using an arbitrary value not related to the
> actual device characteristics. Wouldn't it be better to us the device
> max_sectors limit ? And that limit would need to be zone_append_max_sectors for
> zone append writes. So some helper like Changheun bio_max_size() may be useful.

Firstly, bio->bi_disk may not be initialized when adding page to bio; secondly this
limit isn't really related with device, is it? Also if it is one queue limit, it has
to be stacked.


Thanks,
Ming