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Subject[PATCH v4 1/2] bio: limit bio max size
Date
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 | 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

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