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    Subject[PATCH 14/14] fs: don't change the address limit for ->read_iter in __kernel_read
    Date
    If we read to a file that implements ->read_iter there is no need
    to change the address limit if we send a kvec down. Implement that
    case, and prefer it over using plain ->read with a changed address
    limit if available.

    Signed-off-by: Christoph Hellwig <hch@lst.de>
    ---
    fs/read_write.c | 24 +++++++++++++++++-------
    1 file changed, 17 insertions(+), 7 deletions(-)

    diff --git a/fs/read_write.c b/fs/read_write.c
    index 46ddfce17e839..c93acbd8bf5a3 100644
    --- a/fs/read_write.c
    +++ b/fs/read_write.c
    @@ -421,7 +421,6 @@ static ssize_t new_sync_read(struct file *filp, char __user *buf, size_t len, lo

    ssize_t __kernel_read(struct file *file, void *buf, size_t count, loff_t *pos)
    {
    - mm_segment_t old_fs = get_fs();
    ssize_t ret;

    if (!(file->f_mode & FMODE_CAN_READ))
    @@ -429,14 +428,25 @@ ssize_t __kernel_read(struct file *file, void *buf, size_t count, loff_t *pos)

    if (count > MAX_RW_COUNT)
    count = MAX_RW_COUNT;
    - set_fs(KERNEL_DS);
    - if (file->f_op->read)
    + if (file->f_op->read_iter) {
    + struct kvec iov = { .iov_base = buf, .iov_len = count };
    + struct kiocb kiocb;
    + struct iov_iter iter;
    +
    + init_sync_kiocb(&kiocb, file);
    + kiocb.ki_pos = *pos;
    + iov_iter_kvec(&iter, READ, &iov, 1, count);
    + ret = file->f_op->read_iter(&kiocb, &iter);
    + *pos = kiocb.ki_pos;
    + } else if (file->f_op->read) {
    + mm_segment_t old_fs = get_fs();
    +
    + set_fs(KERNEL_DS);
    ret = file->f_op->read(file, (void __user *)buf, count, pos);
    - else if (file->f_op->read_iter)
    - ret = new_sync_read(file, (void __user *)buf, count, pos);
    - else
    + set_fs(old_fs);
    + } else {
    ret = -EINVAL;
    - set_fs(old_fs);
    + }
    if (ret > 0) {
    fsnotify_access(file);
    add_rchar(current, ret);
    --
    2.26.2
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