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blk_make_request(9) [suse man page]

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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BIO_ALLOC_BIOSET(9)						   The Linux VFS					       BIO_ALLOC_BIOSET(9)

NAME
bio_alloc_bioset - allocate a bio for I/O SYNOPSIS
struct bio * bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set * bs); ARGUMENTS
gfp_mask the GFP_ mask given to the slab allocator nr_iovecs number of iovecs to pre-allocate bs the bio_set to allocate from. DESCRIPTION
If bs is NULL, uses kmalloc to allocate the bio; else the allocation is backed by the bs's mempool. When bs is not NULL, if __GFP_WAIT is set then bio_alloc will always be able to allocate a bio. This is due to the mempool guarantees. To make this work, callers must never allocate more than 1 bio at a time from this pool. Callers that need to allocate more than 1 bio must always submit the previously allocated bio for IO before attempting to allocate a new one. Failure to do so can cause deadlocks under memory pressure. Note that when running under generic_make_request (i.e. any block driver), bios are not submitted until after you return - see the code in generic_make_request that converts recursion into iteration, to prevent stack overflows. This would normally mean allocating multiple bios under generic_make_request would be susceptible to deadlocks, but we have deadlock avoidance code that resubmits any blocked bios from a rescuer thread. However, we do not guarantee forward progress for allocations from other mempools. Doing multiple allocations from the same mempool under generic_make_request should be avoided - instead, use bio_set's front_pad for per bio allocations. RETURNS
Pointer to new bio on success, NULL on failure. COPYRIGHT
Kernel Hackers Manual 3.10 June 2014 BIO_ALLOC_BIOSET(9)

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