[PATCH for-next v10 5/7] block: factor out bio_map_get helper

[Christoph Hellwig]

On Tue, Sep 27, 2022 at 11:06:08PM +0530, Kanchan Joshi wrote:
> Move bio allocation logic from bio_map_user_iov to a new helper
> bio_map_get. It is named so because functionality is opposite of what is
> done inside bio_map_put. This is a prep patch.

I'm still not a fan of using bio_sets for passthrough and would be
much happier if we could drill down what the problems with the
slab per-cpu allocator are, but it seems like I've lost that fight
against Jens..

> +static struct bio *bio_map_get(struct request *rq, unsigned int nr_vecs,
>  		gfp_t gfp_mask)

But these names just seems rather misleading.  Why not someting
like blk_rq_map_bio_alloc and blk_mq_map_bio_put?

Not really new in this code but a question to Jens:  The existing
bio_map_user_iov has no real upper bounds on the number of bios
allocated, how does that fit with the very limited pool size of
fs_bio_set?