Re: [PATCH v2 08/11] blk-mq: Add blk_mq_ops.init_request_no_hctx()

From: John Garry
Date: Wed Aug 18 2021 - 04:46:37 EST

On 18/08/2021 08:38, Ming Lei wrote:
On Mon, Aug 09, 2021 at 10:29:35PM +0800, John Garry wrote:
Add a variant of the init_request function which does not pass a hctx_idx

This is important for shared sbitmap support, as it needs to be ensured for
introducing shared static rqs that the LLDD cannot think that requests
are associated with a specific HW queue.

Signed-off-by: John Garry<john.garry@xxxxxxxxxx>
block/blk-mq.c | 15 ++++++++++-----
include/linux/blk-mq.h | 7 +++++++
2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f14cc2705f9b..4d6723cfa582 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2427,13 +2427,15 @@ struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
static int blk_mq_init_request(struct blk_mq_tag_set *set, struct request *rq,
unsigned int hctx_idx, int node)
- int ret;
+ int ret = 0;
- if (set->ops->init_request) {
+ if (set->ops->init_request)
ret = set->ops->init_request(set, rq, hctx_idx, node);
- if (ret)
- return ret;
- }
+ else if (set->ops->init_request_no_hctx)
+ ret = set->ops->init_request_no_hctx(set, rq, node);

Hi Ming,

The only shared sbitmap user of SCSI does not use passed hctx_idx, not
sure we need such new callback.

Sure, actually most versions of init_request callback don't use hctx_idx. Or numa_node arg.
If you really want to do this, just wondering why not pass '-1' as
hctx_idx in case of shared sbitmap?

Yeah, I did consider that. hctx_idx is an unsigned, and I generally don't like -1U - but that's no big deal. But I also didn't like how it relies on the driver init_request callback to check the value, which changes the semantics.

Obviously we don't add new versions of init_request for new block drivers which use shared sbitmap everyday, and any new ones would get it right.

I suppose I can go the way you suggest - I just thought that this method was neat as well.