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

["Javier González" <javier@javigon.com>]

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> On 4 Mar 2019, at 13.22, Hans Holmberg <hans@owltronix.com> wrote:
> 
> On Mon, Mar 4, 2019 at 12:44 PM Javier González <javier@javigon.com> wrote:
>>> 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...
> 
> So we cant make use of a perfectly good, standardized, parameter
> because some hypothetical non-compliant device out there might not
> provide a sane value?

The OCSSD standard has never used NVMe parameters, so there is no
compliant / non-compliant. In fact, until we changed OCSSD 2.0 to
get the sector and OOB sizes from the standard identify
command, we used to have them in the geometry.

If you did not catch it in the first reference, this concern is explicitly
related to OCSSD controllers already out there - some of which you
should be caring about.

If we are to use this information in the future, I would advocate to
first make changes in the spec and then in the code, not the other way
around.

> 
>>> 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.
> 
> No, it just does not fix it.

It is broken in _this_ implementation.

> 
> over-and-out,
> Hans
>> Javier

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