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    Subject[PATCH 4.17 296/336] block: bio_iov_iter_get_pages: fix size of last iovec
    Date
    4.17-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Martin Wilck <mwilck@suse.com>

    commit b403ea2404889e1227812fa9657667a1deb9c694 upstream.

    If the last page of the bio is not "full", the length of the last
    vector slot needs to be corrected. This slot has the index
    (bio->bi_vcnt - 1), but only in bio->bi_io_vec. In the "bv" helper
    array, which is shifted by the value of bio->bi_vcnt at function
    invocation, the correct index is (nr_pages - 1).

    v2: improved readability following suggestions from Ming Lei.
    v3: followed a formatting suggestion from Christoph Hellwig.

    Fixes: 2cefe4dbaadf ("block: add bio_iov_iter_get_pages()")
    Reviewed-by: Hannes Reinecke <hare@suse.com>
    Reviewed-by: Ming Lei <ming.lei@redhat.com>
    Reviewed-by: Jan Kara <jack@suse.cz>
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Signed-off-by: Martin Wilck <mwilck@suse.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    block/bio.c | 18 ++++++++----------
    1 file changed, 8 insertions(+), 10 deletions(-)

    --- a/block/bio.c
    +++ b/block/bio.c
    @@ -881,16 +881,16 @@ EXPORT_SYMBOL(bio_add_page);
    */
    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 nr_pages = bio->bi_max_vecs - bio->bi_vcnt, idx;
    struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
    struct page **pages = (struct page **)bv;
    - size_t offset, diff;
    + size_t offset;
    ssize_t size;

    size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
    if (unlikely(size <= 0))
    return size ? size : -EFAULT;
    - nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;
    + idx = nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;

    /*
    * Deep magic below: We need to walk the pinned pages backwards
    @@ -903,17 +903,15 @@ int bio_iov_iter_get_pages(struct bio *b
    bio->bi_iter.bi_size += size;
    bio->bi_vcnt += nr_pages;

    - diff = (nr_pages * PAGE_SIZE - offset) - size;
    - while (nr_pages--) {
    - bv[nr_pages].bv_page = pages[nr_pages];
    - bv[nr_pages].bv_len = PAGE_SIZE;
    - bv[nr_pages].bv_offset = 0;
    + while (idx--) {
    + bv[idx].bv_page = pages[idx];
    + bv[idx].bv_len = PAGE_SIZE;
    + bv[idx].bv_offset = 0;
    }

    bv[0].bv_offset += offset;
    bv[0].bv_len -= offset;
    - if (diff)
    - bv[bio->bi_vcnt - 1].bv_len -= diff;
    + bv[nr_pages - 1].bv_len -= nr_pages * PAGE_SIZE - offset - size;

    iov_iter_advance(iter, size);
    return 0;

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