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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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