Re: [PATCH 2/2] zloop: clear REQ_NOWAIT before workqueue submission

[Damien Le Moal <dlemoal@kernel.org>]

On 11/19/25 09:36, Chaitanya Kulkarni wrote:
> zloop advertises REQ_NOWAIT support via BLK_FEAT_NOWAIT (set by default
> for all blk-mq devices), but delegates I/O processing to workqueues
> where blocking operations are allowed.
> 
> Since REQ_NOWAIT is not valid in the workqueue context, clear the
> REQ_NOWAIT flag before handing the request over to the workqueue. This
> avoids unnecessary non-blocking constraints in a context where blocking
> is acceptable.
> 
> Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
> ---
>  drivers/block/zloop.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/block/zloop.c b/drivers/block/zloop.c
> index 92be9f0af00a..22a245259622 100644
> --- a/drivers/block/zloop.c
> +++ b/drivers/block/zloop.c
> @@ -620,6 +620,8 @@ static blk_status_t zloop_queue_rq(struct blk_mq_hw_ctx *hctx,
>  
>  	blk_mq_start_request(rq);
>  
> +	rq->cmd_flags &= ~REQ_NOWAIT;
> +

To be able to catch any potential issue with this function blocking, I think it
is preferable to unset REQ_NOWAIT only once we are in the context of the worker
thread.

So something like this:

diff --git a/drivers/block/zloop.c b/drivers/block/zloop.c
index d6dc672bda5f..9e4256824336 100644
--- a/drivers/block/zloop.c
+++ b/drivers/block/zloop.c
@@ -547,6 +547,10 @@ static void zloop_handle_cmd(struct zloop_cmd *cmd)
        struct request *rq = blk_mq_rq_from_pdu(cmd);
        struct zloop_device *zlo = rq->q->queuedata;

+       /* We can block in this context, so ignore REQ_NOWAIT. */
+        if (rq->cmd_flags & REQ_NOWAIT)
+               rq->cmd_flags &= ~REQ_NOWAIT
+
        switch (req_op(rq)) {
        case REQ_OP_READ:
        case REQ_OP_WRITE:

The same comment applies to loop as well. I think it is better to clear
REQ_NOWAIT only once the request is owned by the worker thread context.

-- 
Damien Le Moal
Western Digital Research