Re: [PATCH] nvme: fix handling single range discard request

Hannes Reinecke
2023-03-04T08:00:40Z

On 3/4/23 00:13, Ming Lei wrote:
> When investigating one customer report on warning in nvme_setup_discard,
> we observed the controller(nvme/tcp) actually exposes
> queue_max_discard_segments(req->q) == 1.
>
> Obviously the current code can't handle this situation, since contiguity
> merge like normal RW request is taken.
>
> Fix the issue by building range from request sector/nr_sectors directly.
>
> Fixes: b35ba01ea697 ("nvme: support ranged discard requests")
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> drivers/nvme/host/core.c | 28 +++++++++++++++++++---------
> 1 file changed, 19 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index c2730b116dc6..d4be525f8100 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -781,16 +781,26 @@ static blk_status_t nvme_setup_discard(struct nvme_ns *ns, struct request *req,
> range = page_address(ns->ctrl->discard_page);
> }
>
> - __rq_for_each_bio(bio, req) {
> - u64 slba = nvme_sect_to_lba(ns, bio->bi_iter.bi_sector);
> - u32 nlb = bio->bi_iter.bi_size >> ns->lba_shift;
> -
> - if (n < segments) {
> - range[n].cattr = cpu_to_le32(0);
> - range[n].nlb = cpu_to_le32(nlb);
> - range[n].slba = cpu_to_le64(slba);
> + if (queue_max_discard_segments(req->q) == 1) {
> + u64 slba = nvme_sect_to_lba(ns, blk_rq_pos(req));
> + u32 nlb = blk_rq_sectors(req) >> (ns->lba_shift - 9);
> +
> + range[0].cattr = cpu_to_le32(0);
> + range[0].nlb = cpu_to_le32(nlb);
> + range[0].slba = cpu_to_le64(slba);
> + n = 1;
> + } else { > + __rq_for_each_bio(bio, req) {
> + u64 slba = nvme_sect_to_lba(ns, bio->bi_iter.bi_sector);
> + u32 nlb = bio->bi_iter.bi_size >> ns->lba_shift;
> +
> + if (n < segments) {
> + range[n].cattr = cpu_to_le32(0);
> + range[n].nlb = cpu_to_le32(nlb);
> + range[n].slba = cpu_to_le64(slba);
> + }
> + n++;
> }
> - n++;
> }
>
> if (WARN_ON_ONCE(n != segments)) {
Now _that_ is odd.
Looks like 'req' is not formatted according to the 'max_discard_sectors'
setting.
But if that's the case, then this 'fix' would fail whenever
'max_discard_sectors' < 'max_hw_sectors', right?
Shouldn't we rather modify the merge algorithm to check for
max_discard_sectors for DISCARD requests, such that we never _have_
mis-matched requests and this patch would be pointless?

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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