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    Subject[PATCH v3 6/7] bio: add a helper calculating nr segments to alloc
    Date
    Add a helper function calculating the number of bvec segments we need to
    allocate to construct a bio. It doesn't change anything functionally,
    but will be used to not duplicate special cases in the future.

    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
    ---
    fs/block_dev.c | 7 ++++---
    fs/iomap/direct-io.c | 9 ++++-----
    include/linux/bio.h | 10 ++++++++++
    3 files changed, 18 insertions(+), 8 deletions(-)

    diff --git a/fs/block_dev.c b/fs/block_dev.c
    index 3b8963e228a1..6f5bd9950baf 100644
    --- a/fs/block_dev.c
    +++ b/fs/block_dev.c
    @@ -416,7 +416,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
    dio->size += bio->bi_iter.bi_size;
    pos += bio->bi_iter.bi_size;

    - nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
    + nr_pages = bio_iov_vecs_to_alloc(iter, BIO_MAX_PAGES);
    if (!nr_pages) {
    bool polled = false;

    @@ -481,9 +481,10 @@ blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
    {
    int nr_pages;

    - nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES + 1);
    - if (!nr_pages)
    + if (!iov_iter_count(iter))
    return 0;
    +
    + nr_pages = bio_iov_vecs_to_alloc(iter, BIO_MAX_PAGES + 1);
    if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
    return __blkdev_direct_IO_simple(iocb, iter, nr_pages);

    diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
    index 933f234d5bec..ea1e8f696076 100644
    --- a/fs/iomap/direct-io.c
    +++ b/fs/iomap/direct-io.c
    @@ -250,11 +250,8 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
    orig_count = iov_iter_count(dio->submit.iter);
    iov_iter_truncate(dio->submit.iter, length);

    - nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
    - if (nr_pages <= 0) {
    - ret = nr_pages;
    + if (!iov_iter_count(dio->submit.iter))
    goto out;
    - }

    if (need_zeroout) {
    /* zero out from the start of the block to the write offset */
    @@ -263,6 +260,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
    iomap_dio_zero(dio, iomap, pos - pad, pad);
    }

    + nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter, BIO_MAX_PAGES);
    do {
    size_t n;
    if (dio->error) {
    @@ -308,7 +306,8 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
    dio->size += n;
    copied += n;

    - nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
    + nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter,
    + BIO_MAX_PAGES);
    iomap_dio_submit_bio(dio, iomap, bio, pos);
    pos += n;
    } while (nr_pages);
    diff --git a/include/linux/bio.h b/include/linux/bio.h
    index 1edda614f7ce..d8f9077c43ef 100644
    --- a/include/linux/bio.h
    +++ b/include/linux/bio.h
    @@ -10,6 +10,7 @@
    #include <linux/ioprio.h>
    /* struct bio, bio_vec and BIO_* flags are defined in blk_types.h */
    #include <linux/blk_types.h>
    +#include <linux/uio.h>

    #define BIO_DEBUG

    @@ -441,6 +442,15 @@ static inline void bio_wouldblock_error(struct bio *bio)
    bio_endio(bio);
    }

    +/*
    + * Calculate number of bvec segments that should be allocated to fit data
    + * pointed by @iter.
    + */
    +static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
    +{
    + return iov_iter_npages(iter, max_segs);
    +}
    +
    struct request_queue;

    extern int submit_bio_wait(struct bio *bio);
    --
    2.24.0
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