[PATCH] nvme: fix handling single range discard request

[Sagi Grimberg]

On 3/7/23 14:14, Ming Lei wrote:
> On Tue, Mar 07, 2023 at 01:39:27PM +0200, Sagi Grimberg wrote:
>>
>>
>> On 3/6/23 23:49, Ming Lei wrote:
>>> On Mon, Mar 06, 2023 at 04:21:08PM +0200, Sagi Grimberg wrote:
>>>>
>>>>
>>>> On 3/4/23 01:13, 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 at 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)) {
>>>>
>>>>
>>>> Maybe just set segments to min(blk_rq_nr_discard_segments(req),
>>>> queue_max_discard_segments(req->q)) and let the existing code do
>>>> its thing?
>>>
>>> What is the existing code for applying min()?
>>
>> Was referring to this:
>> --
>> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
>> index 3345f866178e..dbc402587431 100644
>> --- a/drivers/nvme/host/core.c
>> +++ b/drivers/nvme/host/core.c
>> @@ -781,6 +781,7 @@ static blk_status_t nvme_setup_discard(struct nvme_ns
>> *ns, struct request *req,
>>                  range = page_address(ns->ctrl->discard_page);
>>          }
>>
>> +       segments = min(segments, queue_max_discard_segments(req->q));
> 
> That can't work.
> 
> In case of queue_max_discard_segments(req->q) == 1, the request still
> can have more than one bios since the normal merge is taken for discard
> IOs.

Ah, I see, the bios are contiguous though right?
We could add a contiguity check in the loop and conditionally
increment n, but maybe that would probably be more complicated...