[PATCH 1/5] block: enable batched allocation for blk_mq_alloc_request()

[Pankaj Raghav]

On 2022-09-23 16:52, Pankaj Raghav wrote:
> On Thu, Sep 22, 2022 at 12:28:01PM -0600, Jens Axboe wrote:
>> The filesystem IO path can take advantage of allocating batches of
>> requests, if the underlying submitter tells the block layer about it
>> through the blk_plug. For passthrough IO, the exported API is the
>> blk_mq_alloc_request() helper, and that one does not allow for
>> request caching.
>>
>> Wire up request caching for blk_mq_alloc_request(), which is generally
>> done without having a bio available upfront.
>>
>> Signed-off-by: Jens Axboe <axboe at kernel.dk>
>> ---
>>  block/blk-mq.c | 80 ++++++++++++++++++++++++++++++++++++++++++++------
>>  1 file changed, 71 insertions(+), 9 deletions(-)
>>
> I think we need this patch to ensure correct behaviour for passthrough:
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c11949d66163..840541c1ab40 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1213,7 +1213,7 @@ void blk_execute_rq_nowait(struct request *rq, bool at_head)
>         WARN_ON(!blk_rq_is_passthrough(rq));
>  
>         blk_account_io_start(rq);
> -       if (current->plug)
> +       if (blk_mq_plug(rq->bio))
>                 blk_add_rq_to_plug(current->plug, rq);
>         else
>                 blk_mq_sched_insert_request(rq, at_head, true, false);
> 
> As the passthrough path can now support request caching via blk_mq_alloc_request(),
> and it uses blk_execute_rq_nowait(), bad things can happen at least for zoned
> devices:
> 
> static inline struct blk_plug *blk_mq_plug( struct bio *bio)
> {
> 	/* Zoned block device write operation case: do not plug the BIO */
> 	if (bdev_is_zoned(bio->bi_bdev) && op_is_write(bio_op(bio)))
> 		return NULL;
> ..

Thinking more about it, even this will not fix it because op is
REQ_OP_DRV_OUT if it is a NVMe write for passthrough requests.

@Damien Should the condition in blk_mq_plug() be changed to:

static inline struct blk_plug *blk_mq_plug( struct bio *bio)
{
	/* Zoned block device write operation case: do not plug the BIO */
	if (bdev_is_zoned(bio->bi_bdev) && !op_is_read(bio_op(bio)))
		return NULL;
> 
> Am I missing something?