BLK_MAKE_REQUEST - Online Linux Manual Page

Section : 9
Updated : June 2018
Source : Kernel Hackers Manual 2​.6​.
Note : Block Devices
NAMEblk_make_request − given a bio, allocate a corresponding struct request​.
SYNOPSISstruct request * blk_make_request(struct request_queue * q, struct bio * bio, gfp_t gfp_mask);
ARGUMENTSq 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
DESCRIPTIONblk_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​.
WARNINGWhen 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​.
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