Re: [PATCH] nvmet-passthru: Cleanup nvmet_passthru_map_sg()

[Douglas Gilbert <dgilbert@interlog.com>]

On 2020-10-09 7:18 p.m., Logan Gunthorpe wrote:
> Clean up some confusing elements of nvmet_passthru_map_sg() by returning
> early if the request is greater than the maximum bio size. This allows
> us to drop the sg_cnt variable.
> 
> This should not result in any functional change but makes the code
> clearer and more understandable. The original code allocated a truncated
> bio then would return EINVAL when bio_add_pc_page() filled that bio. The
> new code just returns EINVAL early if this would happen.
> 
> Fixes: c1fef73f793b ("nvmet: add passthru code to process commands")
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> Suggested-by: Douglas Gilbert <dgilbert@interlog.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Sagi Grimberg <sagi@grimberg.me>
> Cc: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>

Reviewed-by: Douglas Gilbert <dgilbert@interlog.com>


I have commented offline to Logan about the return from bio_alloc()
not being checked for NULL. He assures me that can't happen but I believe
that is true only for small scatter-gather lists. So I guess it depends
on the size of the data this is planned to be sent through this
pass-through.

> ---
>   drivers/nvme/target/passthru.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/nvme/target/passthru.c b/drivers/nvme/target/passthru.c
> index dacfa7435d0b..076e829490a1 100644
> --- a/drivers/nvme/target/passthru.c
> +++ b/drivers/nvme/target/passthru.c
> @@ -180,18 +180,20 @@ static void nvmet_passthru_req_done(struct request *rq,
>   
>   static int nvmet_passthru_map_sg(struct nvmet_req *req, struct request *rq)
>   {
> -	int sg_cnt = req->sg_cnt;
>   	struct scatterlist *sg;
>   	int op_flags = 0;
>   	struct bio *bio;
>   	int i, ret;
>   
> +	if (req->sg_cnt > BIO_MAX_PAGES)
> +		return -EINVAL;
> +
>   	if (req->cmd->common.opcode == nvme_cmd_flush)
>   		op_flags = REQ_FUA;
>   	else if (nvme_is_write(req->cmd))
>   		op_flags = REQ_SYNC | REQ_IDLE;
>   
> -	bio = bio_alloc(GFP_KERNEL, min(sg_cnt, BIO_MAX_PAGES));
> +	bio = bio_alloc(GFP_KERNEL, req->sg_cnt);
>   	bio->bi_end_io = bio_put;
>   	bio->bi_opf = req_op(rq) | op_flags;
>   
> @@ -201,7 +203,6 @@ static int nvmet_passthru_map_sg(struct nvmet_req *req, struct request *rq)
>   			bio_put(bio);
>   			return -EINVAL;
>   		}
> -		sg_cnt--;
>   	}
>   
>   	ret = blk_rq_append_bio(rq, &bio);
> 
> base-commit: 549738f15da0e5a00275977623be199fbbf7df50
> 


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