Re: [PATCHv2 2/5] nvme: simplify passthrough bio cleanup

[Kanchan Joshi <joshi.k@samsung.com>]

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On Fri, Apr 07, 2023 at 12:16:33PM -0700, Keith Busch wrote:
>From: Keith Busch <kbusch@kernel.org>
>
>Set the bio's bi_end_io to handle the cleanup so that uring_cmd doesn't
>need this complex pdu->{bio,req} switchero and restore.
>
>Signed-off-by: Keith Busch <kbusch@kernel.org>
>---
> drivers/nvme/host/ioctl.c | 26 +++++++++-----------------
> 1 file changed, 9 insertions(+), 17 deletions(-)
>
>diff --git a/drivers/nvme/host/ioctl.c b/drivers/nvme/host/ioctl.c
>index d24ea2e051564..278c57ee0db91 100644
>--- a/drivers/nvme/host/ioctl.c
>+++ b/drivers/nvme/host/ioctl.c
>@@ -159,6 +159,11 @@ static struct request *nvme_alloc_user_request(struct request_queue *q,
> 	return req;
> }
>
>+static void nvme_uring_bio_end_io(struct bio *bio)
>+{
>+	blk_rq_unmap_user(bio);
>+}
>+
> static int nvme_map_user_request(struct request *req, u64 ubuffer,
> 		unsigned bufflen, void __user *meta_buffer, unsigned meta_len,
> 		u32 meta_seed, void **metap, struct io_uring_cmd *ioucmd,
>@@ -204,6 +209,7 @@ static int nvme_map_user_request(struct request *req, u64 ubuffer,
> 		*metap = meta;
> 	}
>
>+	bio->bi_end_io = nvme_uring_bio_end_io;
> 	return ret;
>
> out_unmap:
>@@ -249,8 +255,6 @@ static int nvme_submit_user_cmd(struct request_queue *q,
> 	if (meta)
> 		ret = nvme_finish_user_metadata(req, meta_buffer, meta,
> 						meta_len, ret);
>-	if (bio)
>-		blk_rq_unmap_user(bio);

Is it safe to call blk_rq_unamp_user in irq context?
Agree that current code does some complex stuff, but that's to ensure
what the code-comment [1] says.

Also for polled-io, new-code will hit this warn-on [2] on calling
bio_put_percpu_cache.

[1] 
623  *    A matching blk_rq_unmap_user() must be issued at the end of I/O, while
624  *    still in process context.
625  */
626 int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
627                         struct rq_map_data *map_data,


[2]
773         if ((bio->bi_opf & REQ_POLLED) && !WARN_ON_ONCE(in_interrupt())) {
774                 bio->bi_next = cache->free_list;

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