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

[Hans Holmberg <hans@owltronix.com>]

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?

>
> >
> > 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.

over-and-out,
Hans
>
> Javier