为什么 bi_end_io 用带有/不带 biovecs 的 bio 调用？

我正在制作一个块设备驱动程序，在处理 READ 请求时，我在 bio.bi_end_io 字段中设置了我的处理程序（I/O 完成例程）。正在调用我的处理程序，但 bio 中没有 biovec，我需要在块 I/O 子系统将数据返回给应用程序之前处理数据。这里有一些代码片段来说明。*

static  void    __iob_enc_dec   (
        struct bio *    iob
            )
{
struct bio_vec  bvec = {0};
struct  bvec_iter iter = {0};
sector_t    lbn, nlbn;

    $TRACE("Start %scrypting ...", bio_data_dir(iob) == WRITE ? "En" : "De");

    $SHOW_PTR(bio_data(iob));

    /* Do each segment independently. */
    **bio_for_each_segment**(bvec, iob, iter)
        {
        char *  iobuf;

        iobuf = __bio_kmap_atomic(iob, iter);

        $TRACE("#%02d: page=%p, off=%u, len=%u, iobuf=%p, lbn=%lu",
            iter.bi_idx, bvec.bv_page, bvec.bv_offset, bvec.bv_len, iobuf, iter.bi_sector);

        iobuf += bvec.bv_offset;

        lbn = iter.bi_sector;
        nlbn    = iter.bi_size/DUDRV$K_BLKSZ;

        for ( ;nlbn; nlbn--, lbn++ , iobuf += DUDRV$K_BLKSZ)
            {
            **...**
            }

        __bio_kunmap_atomic(iob);
        }
}


static  void    dua_bio_end_io  (
            struct bio *    iob
                )
{
IOB_ARGS * iob_args;

    $TRACE("Entering to %s completion I/O, bio=%p ...", bio_data_dir(iob) == WRITE ? "WRITE" : "READ", iob);

    $SHOW_UNSIGNED(iob->bi_flags);
    $SHOW_INT(iob->bi_vcnt);

    $SHOW_BOOL(bio_has_data(iob));

    iob_args = iob->bi_private;

    iob->bi_end_io = iob_args->bi_end_io;
    iob->bi_private = iob_args->bi_private;

    bio_put(iob);

    __ret_iob_args (iob_args);

    /* In case of READ request - we should  decrypt data buffer right now */
    if ( bio_data_dir(iob) == READ )
        **__iob_enc_dec(iob)**;

    bio_endio (iob);

}

static blk_qc_t dua_make_request_fn (
        struct request_queue *  ioq,
            struct bio *    iob
                )
{
int status = 0;

    $TRACE("Starting (%s), bio=%p, op=%d ...", bio_data_dir(iob) == WRITE ? "WRITE" : "READ", iob, bio_op(iob));


    $SHOW_UNSIGNED(iob->bi_flags);
    $SHOW_INT(iob->bi_vcnt);

    $SHOW_BOOL(bio_has_data(iob));

    /* In case of WRITE request - we can encrypt data buffer right now */
    if ( bio_data_dir(iob) == WRITE )
        __iob_enc_dec(iob);

    /*
     * A handling of the READ request is require 'enqueue read request' & 'wait for read completion' paradigm,
     * so we need to allocate IOB_ARGS block to carry data to the "Read Completion I/O" routine.
     */
    else if ( bio_data_dir(iob) == READ )
        {
        IOB_ARGS *iob_args = NULL;

        if ( __get_iob_args (&iob_args) )
            {
            printk(KERN_ERR  __MODULE__ ": Buffered I/O quota limit has been exhausted\n");

            iob->bi_error = -EBUSY;
            bio_endio(iob);
            }

        iob_args->bi_end_io = iob->bi_end_io;
        iob_args->bi_private = iob->bi_private;

        /*
         * Replace an address of the Completion I/O routine for 'read' operation,
         * save original address.
         */
        iob->bi_private = iob_args;
        iob->bi_end_io = dua_bio_end_io;

        bio_get(iob);
        }

    /* Just for sanity check ... */
    else    {
        printk(KERN_WARNING  __MODULE__ ": Unhandled I/O request %d\n", bio_data_dir(iob) );
        }


    /* Call original make_reques_fn() to performs a main work ... */
    status = backend_make_request_fn(ioq, iob);

    return  status;
}




[6463.418222] [DUDRIVER\dua_make_request_fn:479] Starting (READ), bio=ffff95c5f9552100, op=0 ...
[ 6463.418222] [DUDRIVER\dua_make_request_fn:482] : iob->bi_flags = 0x00000000
[ 6463.418223] [DUDRIVER\dua_make_request_fn:483] : iob->bi_vcnt = 1
[ 6463.418223] [DUDRIVER\dua_make_request_fn:485] : bio_has_data(iob) = ENABLED(TRUE)

[ 6463.418266] [DUDRIVER\dua_bio_end_io:445] Entering to READ completion I/O, bio=ffff95c5f9552100 ...
[ 6463.418266] [DUDRIVER\dua_bio_end_io:447] : iob->bi_flags = 0x00000102
[ 6463.418267] [DUDRIVER\dua_bio_end_io:448] : iob->bi_vcnt = 1
[ 6463.418267] [DUDRIVER\dua_bio_end_io:450] : bio_has_data(iob) = DISABLED(FALSE)

我需要做什么才能访问 bio_end_io() 中的数据缓冲区？这发生在 Ubuntu 16.x、Linux 内核 4.10.y 中。

更新1：设置了 BIO_CLONED 标志的 bio_for_each_segment 宏可能不允许您运行缓冲区段，但那该怎么办？