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); |
qtarget request queue
bioThe bio describing the memory mappings that will be submitted for IO. It may be a chained-bio properly constructed by block/bio layer.
gfp_maskgfp 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 bio's 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 hasn't 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.