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

Hannes Reinecke
2023-03-04T11:14:40Z

On 3/4/23 11:22, Ming Lei wrote:
> On Sat, Mar 04, 2023 at 09:00:28AM +0100, Hannes Reinecke wrote:
>> 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?
>
> No, it isn't the case.
>
>> 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?
>
> But it is related with discard merge.
>
> If queue_max_discard_segments() is 1, block layer merges discard
> request/bios just like normal RW IO.
>
> However, if queue_max_discard_segments() is > 1, block layer simply
> 'merges' all bios into one request, no matter if the LBA is adjacent
> or not, and treat each bio as one discard segment, that is called
> multi range discard too.
>
But wouldn't the number of bios be subject to
'queue_max_discard_segment', too?
What guarantees we're not overflowing that for multi-segment discard merge?

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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