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SubjectRe: [PATCH v3 3/7] block: add write-hint to stream-id conversion
On Fri, Mar 29, 2019 at 01:23:48PM +0530, Kanchan Joshi wrote:
> Earlier this conversion was done by driver (nvme). Current conversion
> is of the form "streamid = write-hint - 1", for both user and kernel
> streams (note that existing infra takes care that user-streams do not
> bump into kernel ones).

Unless we add new user streams, then all the kernel streams change
ID. I'll deal with this in more detail in a later patch.

> Conversion takes stream limit (maintained in
> request queue) into account. Write-hints beyond the queue-limit turn
> to 0.
> New field 'streamid' has been added in request. While 'write-hint' field
> continues to exist. It keeps original value passed from upper layer, and
> used during merging checks.
>
> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
> ---
> block/blk-core.c | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 3c5f61c..c86daed 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -727,6 +727,25 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
> return false;
> }
>
> +enum rw_hint blk_write_hint_to_streamid(struct request *req,
> + struct bio *bio)
> +{
> + enum rw_hint streamid, nr_streams;
> + struct request_queue *q = req->q;
> + nr_streams = q->limits.nr_streams;
> +
> + streamid = bio->bi_write_hint;
> + if (!nr_streams || streamid == WRITE_LIFE_NOT_SET ||
> + streamid == WRITE_LIFE_NONE)
> + streamid = 0;
> + else {
> + --streamid;

What's this magic thing do?

> + if(streamid > nr_streams)
> + streamid = 0;

So, basically, we'll compress all the kernel hints down to "no hint"
if there are more user streams than the device supports?

Surely we should be reserving a stream for the kernel hints separate
from the user and "none" streams when we have limited device streams
available...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com

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