__get_request(9) [centos man page]

__GET_REQUEST(9)						   Block Devices						  __GET_REQUEST(9)

NAME

       __get_request - get a free request

SYNOPSIS

       struct request * __get_request(struct request_list * rl, int rw_flags, struct bio * bio, gfp_t gfp_mask);

ARGUMENTS

       rl
	   request list to allocate from

       rw_flags
	   RW and SYNC flags

       bio
	   bio to allocate request for (can be NULL)

       gfp_mask
	   allocation mask

DESCRIPTION

       Get a free request from q. This function may fail under memory pressure or if q is dead.

       Must be callled with q->queue_lock held and, Returns NULL on failure, with q->queue_lock held. Returns !NULL on success, with q->queue_lock
       *not held*.

COPYRIGHT

Kernel Hackers Manual 3.10					     June 2014							  __GET_REQUEST(9)

BLK_MAKE_REQUEST(9)						   Block Devices					       BLK_MAKE_REQUEST(9)

NAME

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

SYNOPSIS

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

ARGUMENTS

       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

DESCRIPTION

       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.

WARNING

       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.

COPYRIGHT

Kernel Hackers Manual 2.6.					     July 2010						       BLK_MAKE_REQUEST(9)