Man page 9 : blk_make_request

blk_make_request - given a bio, allocate a corresponding struct request.

struct request * blk_make_request(struct request_queue *  q , struct bio *  bio , gfp_t  gfp_mask );

q
    target request queue

bio
    The bio describing the memory mappings that will be submitted for IO. It may be a chained-bio properly constructed by block/bio layer.

gfp_mask
    gfp flags to be used for memory allocation

blk_make_request is the parallel of generic_make_request for BLOCK_PC type commands. Where the struct request needs to be farther initialized by the caller. It is passed a struct bio, which describes the memory info of the I/O transfer.

The caller of blk_make_request must make sure that bi_io_vec are set to describe the memory buffers. That bio_data_dir will return the needed direction of the request. (And all bioAqs in the passed bio-chain are properly set accordingly)

If called under none-sleepable conditions, mapped bio buffers must not need bouncing, by calling the appropriate masked or flagged allocator, suitable for the target device. Otherwise the call to blk_queue_bounce will BUG.

When allocating/cloning a bio-chain, careful consideration should be given to how you allocate bios. In particular, you cannot use __GFP_WAIT for anything but the first bio in the chain. Otherwise you risk waiting for IO completion of a bio that hasnAqt been submitted yet, thus resulting in a deadlock. Alternatively bios should be allocated using bio_kmalloc instead of bio_alloc, as that avoids the mempool deadlock. If possible a big IO should be split into smaller parts when allocation fails. Partial allocation should not be an error, or you risk a live-lock.