- struct bio * bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set * bs);
- the GFP_ mask given to the slab allocator
- number of iovecs to pre-allocate
- the bio_set to allocate from.
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.
Pointer to new bio on success, NULL on failure.