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    SubjectRe: [PATCH] bio: limit bio max size.
    Date
    On 2021/01/12 17:52, Changheun Lee wrote:
    > From: "Changheun Lee" <nanich.lee@samsung.com>
    >
    > 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, - 64MB chunk read in user space -
    > all pages for 64MB would be merged to a bio structure if memory address is
    > continued phsycally. it makes some delay to submit until merge complete.
    > bio max size should be limited as a proper size.

    But merging physically contiguous pages into the same bvec + later automatic bio
    split on submit should give you better throughput for large IOs compared to
    having to issue a bio chain of smaller BIOs that are arbitrarily sized and will
    likely need splitting anyway (because of DMA boundaries etc).

    Do you have a specific case where you see higher performance with this patch
    applied ? On Intel, BIO_MAX_SIZE would be 1MB... That is arbitrary and too small
    considering that many hardware can execute larger IOs than that.


    >
    > Signed-off-by: Changheun Lee <nanich.lee@samsung.com>
    > ---
    > block/bio.c | 2 +-
    > include/linux/bio.h | 3 ++-
    > 2 files changed, 3 insertions(+), 2 deletions(-)
    >
    > diff --git a/block/bio.c b/block/bio.c
    > index 1f2cc1fbe283..dbe14d675f28 100644
    > --- a/block/bio.c
    > +++ b/block/bio.c
    > @@ -877,7 +877,7 @@ 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 - len) {
    > *same_page = false;
    > return false;
    > }
    > diff --git a/include/linux/bio.h b/include/linux/bio.h
    > index 1edda614f7ce..0f49b354b1f6 100644
    > --- a/include/linux/bio.h
    > +++ b/include/linux/bio.h
    > @@ -20,6 +20,7 @@
    > #endif
    >
    > #define BIO_MAX_PAGES 256
    > +#define BIO_MAX_SIZE (BIO_MAX_PAGES * PAGE_SIZE)
    >
    > #define bio_prio(bio) (bio)->bi_ioprio
    > #define bio_set_prio(bio, prio) ((bio)->bi_ioprio = prio)
    > @@ -113,7 +114,7 @@ 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 - len)
    > return true;
    >
    > return false;
    >


    --
    Damien Le Moal
    Western Digital Research

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