Re: [PATCH 13/13] lightnvm: Inherit mdts from the parent nvme device

Javier González
2019-03-04T11:44:10Z

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> On 4 Mar 2019, at 12.30, Hans Holmberg <hans@owltronix.com> wrote:
>
> On Mon, Mar 4, 2019 at 10:05 AM Javier González <javier@javigon.com> wrote:
>>> On 27 Feb 2019, at 18.14, Igor Konopko <igor.j.konopko@intel.com> wrote:
>>>
>>> Current lightnvm and pblk implementation does not care
>>> about NVMe max data transfer size, which can be smaller
>>> than 64*K=256K. This patch fixes issues related to that.
>
> Could you describe *what* issues you are fixing?
>
>>> Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
>>> ---
>>> drivers/lightnvm/core.c | 9 +++++++--
>>> drivers/nvme/host/lightnvm.c | 1 +
>>> include/linux/lightnvm.h | 1 +
>>> 3 files changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/lightnvm/core.c b/drivers/lightnvm/core.c
>>> index 5f82036fe322..c01f83b8fbaf 100644
>>> --- a/drivers/lightnvm/core.c
>>> +++ b/drivers/lightnvm/core.c
>>> @@ -325,6 +325,7 @@ static int nvm_create_tgt(struct nvm_dev *dev, struct nvm_ioctl_create *create)
>>> struct nvm_target *t;
>>> struct nvm_tgt_dev *tgt_dev;
>>> void *targetdata;
>>> + unsigned int mdts;
>>> int ret;
>>>
>>> switch (create->conf.type) {
>>> @@ -412,8 +413,12 @@ static int nvm_create_tgt(struct nvm_dev *dev, struct nvm_ioctl_create *create)
>>> tdisk->private_data = targetdata;
>>> tqueue->queuedata = targetdata;
>>>
>>> - blk_queue_max_hw_sectors(tqueue,
>>> - (dev->geo.csecs >> 9) * NVM_MAX_VLBA);
>>> + mdts = (dev->geo.csecs >> 9) * NVM_MAX_VLBA;
>>> + if (dev->geo.mdts) {
>>> + mdts = min_t(u32, dev->geo.mdts,
>>> + (dev->geo.csecs >> 9) * NVM_MAX_VLBA);
>>> + }
>>> + blk_queue_max_hw_sectors(tqueue, mdts);
>>>
>>> set_capacity(tdisk, tt->capacity(targetdata));
>>> add_disk(tdisk);
>>> diff --git a/drivers/nvme/host/lightnvm.c b/drivers/nvme/host/lightnvm.c
>>> index b759c25c89c8..b88a39a3cbd1 100644
>>> --- a/drivers/nvme/host/lightnvm.c
>>> +++ b/drivers/nvme/host/lightnvm.c
>>> @@ -991,6 +991,7 @@ int nvme_nvm_register(struct nvme_ns *ns, char *disk_name, int node)
>>> geo->csecs = 1 << ns->lba_shift;
>>> geo->sos = ns->ms;
>>> geo->ext = ns->ext;
>>> + geo->mdts = ns->ctrl->max_hw_sectors;
>>>
>>> dev->q = q;
>>> memcpy(dev->name, disk_name, DISK_NAME_LEN);
>>> diff --git a/include/linux/lightnvm.h b/include/linux/lightnvm.h
>>> index 5d865a5d5cdc..d3b02708e5f0 100644
>>> --- a/include/linux/lightnvm.h
>>> +++ b/include/linux/lightnvm.h
>>> @@ -358,6 +358,7 @@ struct nvm_geo {
>>> u16 csecs; /* sector size */
>>> u16 sos; /* out-of-band area size */
>>> bool ext; /* metadata in extended data buffer */
>>> + u32 mdts; /* Max data transfer size*/
>>>
>>> /* device write constrains */
>>> u32 ws_min; /* minimum write size */
>>> --
>>> 2.17.1
>>
>> I see where you are going with this and I partially agree, but none of
>> the OCSSD specs define a way to define this parameter. Thus, adding this
>> behavior taken from NVMe in Linux can break current implementations. Is
>> this a real life problem for you? Or this is just for NVMe “correctness”?
>>
>> Javier
>
> Hmm.Looking into the 2.0 spec what it says about vector reads:
>
> (figure 28):"The number of Logical Blocks (NLB): This field indicates
> the number of logical blocks to be read. This is a 0’s based value.
> Maximum of 64 LBAs is supported."
>
> You got the max limit covered, and the spec does not say anything
> about the minimum number of LBAs to support.
>
> Matias: any thoughts on this?
>
> Javier: How would this patch break current implementations?

Say an OCSSD controller that sets mdts to a value under 64 or does not
set it at all (maybe garbage). Think you can get to one pretty quickly...

>
> Igor: how does this patch fix the mdts restriction? There are no
> checks on i.e. the gc read path that ensures that a lower limit than
> NVM_MAX_VLBA is enforced.

This is the other part where the implementation breaks.

Javier

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