BLK_QUEUE_MERGE_BVEC(9) Block Devices BLK_QUEUE_MERGE_BVEC(9)NAME
blk_queue_merge_bvec - set a merge_bvec function for queue
SYNOPSIS
void blk_queue_merge_bvec(struct request_queue * q, merge_bvec_fn * mbfn);
ARGUMENTS
q
queue
mbfn
merge_bvec_fn
DESCRIPTION
Usually queues have static limitations on the max sectors or segments that we can put in a request. Stacking drivers may have some settings
that are dynamic, and thus we have to query the queue whether it is ok to add a new bio_vec to a bio at a given offset or not. If the block
device has such limitations, it needs to register a merge_bvec_fn to control the size of bio's sent to it. Note that a block device *must*
allow a single page to be added to an empty bio. The block device driver may want to use the bio_split function to deal with these bio's.
By default no merge_bvec_fn is defined for a queue, and only the fixed limits are honored.
COPYRIGHT Kernel Hackers Manual 2.6. July 2010 BLK_QUEUE_MERGE_BVEC(9)
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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)
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