[PATCH v9] bio: limit bio max size

From: Changheun Lee
Date: Mon May 03 2021 - 06:10:15 EST

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 +++++++++++--
block/blk-settings.c | 5 +++++
include/linux/bio.h | 4 +++-
include/linux/blkdev.h | 2 ++
4 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 44205dfb6b60..221dc56ba22f 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -255,6 +255,13 @@ void bio_init(struct bio *bio, struct bio_vec *table,
}
EXPORT_SYMBOL(bio_init);

+unsigned int bio_max_size(struct bio *bio)
+{
+ struct block_device *bdev = bio->bi_bdev;
+
+ return bdev ? bdev->bd_disk->queue->limits.bio_max_bytes : UINT_MAX;
+}
+
/**
* bio_reset - reinitialize a bio
* @bio: bio to reset
@@ -866,7 +873,7 @@ 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;
}
@@ -995,6 +1002,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
{
unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
unsigned short entries_left = bio->bi_max_vecs - bio->bi_vcnt;
+ unsigned int bytes_left = bio_max_size(bio) - bio->bi_iter.bi_size;
struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
struct page **pages = (struct page **)bv;
bool same_page = false;
@@ -1010,7 +1018,8 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);

- size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
+ size = iov_iter_get_pages(iter, pages, bytes_left, nr_pages,
+ &offset);
if (unlikely(size <= 0))
return size ? size : -EFAULT;

diff --git a/block/blk-settings.c b/block/blk-settings.c
index 9c009090c4b5..c646503e55d2 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -31,6 +31,7 @@ EXPORT_SYMBOL_GPL(blk_queue_rq_timeout);
*/
void blk_set_default_limits(struct queue_limits *lim)
{
+ lim->bio_max_bytes = UINT_MAX;
lim->max_segments = BLK_MAX_SEGMENTS;
lim->max_discard_segments = 1;
lim->max_integrity_segments = 0;
@@ -139,6 +140,10 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
limits->logical_block_size >> SECTOR_SHIFT);
limits->max_sectors = max_sectors;

+ if (check_shl_overflow(max_sectors, SECTOR_SHIFT,
+ &limits->bio_max_bytes))
+ limits->bio_max_bytes = UINT_MAX;
+
q->backing_dev_info->io_pages = max_sectors >> (PAGE_SHIFT - 9);
}
EXPORT_SYMBOL(blk_queue_max_hw_sectors);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index a0b4cfdf62a4..f1a99f0a240c 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -106,6 +106,8 @@ static inline void *bio_data(struct bio *bio)
return NULL;
}

+extern unsigned int bio_max_size(struct bio *bio);
+
/**
* bio_full - check if the bio is full
* @bio: bio to check
@@ -119,7 +121,7 @@ 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 b91ba6207365..40c7c4d87aa1 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -327,6 +327,8 @@ enum blk_bounce {
};

struct queue_limits {
+ unsigned int bio_max_bytes;
+
enum blk_bounce bounce;
unsigned long seg_boundary_mask;
unsigned long virt_boundary_mask;
--
2.29.0