Re: [PATCH V3 6/6] nvmet: use inline bio for passthru fast path

[Logan Gunthorpe <logang@deltatee.com>]

On 2020-10-21 7:02 p.m., Chaitanya Kulkarni wrote:
> In nvmet_passthru_execute_cmd() which is a high frequency function
> it uses bio_alloc() which leads to memory allocation from the fs pool
> for each I/O.
> 
> For NVMeoF nvmet_req we already have inline_bvec allocated as a part of
> request allocation that can be used with preallocated bio when we
> already know the size of request before bio allocation with bio_alloc(),
> which we already do.
> 
> Introduce a bio member for the nvmet_req passthru anon union. In the
> fast path, check if we can get away with inline bvec and bio from
> nvmet_req with bio_init() call before actually allocating from the
> bio_alloc().
> 
> This will be useful to avoid any new memory allocation under high
> memory pressure situation and get rid of any extra work of
> allocation (bio_alloc()) vs initialization (bio_init()) when
> transfer len is < NVMET_MAX_INLINE_DATA_LEN that user can configure at
> compile time.
> 
> Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
> ---
>  drivers/nvme/target/nvmet.h    |  1 +
>  drivers/nvme/target/passthru.c | 20 ++++++++++++++++++--
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/nvme/target/nvmet.h b/drivers/nvme/target/nvmet.h
> index 559a15ccc322..408a13084fb4 100644
> --- a/drivers/nvme/target/nvmet.h
> +++ b/drivers/nvme/target/nvmet.h
> @@ -330,6 +330,7 @@ struct nvmet_req {
>  			struct work_struct      work;
>  		} f;
>  		struct {
> +			struct bio		inline_bio;
>  			struct request		*rq;
>  			struct work_struct      work;
>  			bool			use_workqueue;
> diff --git a/drivers/nvme/target/passthru.c b/drivers/nvme/target/passthru.c
> index 496ffedb77dc..32498b4302cc 100644
> --- a/drivers/nvme/target/passthru.c
> +++ b/drivers/nvme/target/passthru.c
> @@ -178,6 +178,14 @@ static void nvmet_passthru_req_done(struct request *rq,
>  	blk_mq_free_request(rq);
>  }
>  
> +static void nvmet_passthru_bio_done(struct bio *bio)
> +{
> +	struct nvmet_req *req = bio->bi_private;
> +
> +	if (bio != &req->p.inline_bio)
> +		bio_put(bio);
> +}
> +
>  static int nvmet_passthru_map_sg(struct nvmet_req *req, struct request *rq)
>  {
>  	int sg_cnt = req->sg_cnt;
> @@ -186,13 +194,21 @@ static int nvmet_passthru_map_sg(struct nvmet_req *req, struct request *rq)
>  	int i;
>  
>  	bio = bio_alloc(GFP_KERNEL, min(sg_cnt, BIO_MAX_PAGES));
> -	bio->bi_end_io = bio_put;
> +	if (req->transfer_len <= NVMET_MAX_INLINE_DATA_LEN) {
> +		bio = &req->p.inline_bio;
> +		bio_init(bio, req->inline_bvec, ARRAY_SIZE(req->inline_bvec));
> +	} else {
> +		bio = bio_alloc(GFP_KERNEL, min(sg_cnt, BIO_MAX_PAGES));
> +	}
> +
> +	bio->bi_end_io = nvmet_passthru_bio_done;

I still think it's cleaner to change bi_endio for the inline/alloc'd
cases by simply setting bi_endi_io to bio_put() only in the bio_alloc
case. This should also be more efficient as it's one less indirect call
and condition for the inline case.

Besides that, the entire series looks good to me.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>

Logan

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