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

[Jens Axboe <axboe@kernel.dk>]

On 9/28/22 11:31 AM, Christoph Hellwig wrote:
> 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..

I don't think there are necessarily big problems with the slab side,
it's just that the per-cpu freeing there needs to be IRQ safe. And the
double cmpxchg() used for that isn't that fast compared to being able
to cache these locally with just preempt protection.

>> +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?

Good question - I think we'd need to ensure that once we get
past the initial alloc that we clear any gfp flags that'd make
the mempool_alloc() wait for completions.

-- 
Jens Axboe