Re: [PATCH] lightnvm: pblk: recover chunk state on 1.2 devices

[Matias BjÃrling]

On 08/03/2018 02:37 PM, Javier Gonzalez wrote:
> > On 3 Aug 2018, at 14.30, Matias BjÃrling <mb@xxxxxxxxxxx> wrote:
> >
> > On 08/03/2018 02:02 PM, Javier Gonzalez wrote:
> > > > On 3 Aug 2018, at 13.57, Matias BjÃrling <mb@xxxxxxxxxxx> wrote:
> > > >
> > > > On 07/24/2018 09:54 AM, Javier Gonzalez wrote:
> > > > > > On 29 Jun 2018, at 13.28, Matias BjÃrling <mb@xxxxxxxxxxx> wrote:
> > > > > >
> > > > > > On 06/29/2018 01:22 PM, Javier Gonzalez wrote:
> > > > > > > > On 29 Jun 2018, at 13.14, Matias BjÃrling <mb@xxxxxxxxxxx> wrote:
> > > > > > > >
> > > > > > > > On 06/28/2018 11:12 AM, Javier GonzÃlez wrote:
> > > > > > > > > The Open-Channel 1.2 spec does not define a mechanism for the host to
> > > > > > > > > recover the block (chunk) state. As a consequence, a newly format device
> > > > > > > > > will need to reconstruct the state. Currently, pblk assumes that blocks
> > > > > > > > > are not erased, which might cause double-erases in case that the device
> > > > > > > > > does not protect itself against them (which is not specified in the spec
> > > > > > > > > either).
> > > > > > > >
> > > > > > > > It should not be specified in the spec. It is up to the device to handle
> > > > > > > > double erases and not do it.
> > > > > > > >
> > > > > > > > > This patch, reconstructs the state based on read errors. If the first
> > > > > > > > > sector of a block returns and empty page (NVM_RSP_ERR_EMPTYPAGE), then
> > > > > > > > > the block s marked free, i.e., erased and ready to be used
> > > > > > > > > (NVM_CHK_ST_FREE). Otherwise, the block is marked as closed
> > > > > > > > > (NVM_CHK_ST_CLOSED). Note that even if a block is open and not fully
> > > > > > > > > written, it has to be erased in order to be used again.
> > > > > > > >
> > > > > > > > Should we extend it to do the scan, and update the write pointer as
> > > > > > > > well? I think this kind of feature already is baked into pblk?
> > > > > > > This is already in place: we scan until empty page and take it from
> > > > > > > there. This patch is only for the case in which we start a pblk instance
> > > > > > > form scratch. On a device already owned by pblk, we would not have the
> > > > > > > problem we are trying to solve here because we know the state.
> > > > > >
> > > > > > Agree. What I meant was that when we anyway are recovering the state,
> > > > > > we could just as well update ->wp and set to NVM_CHK_ST_OPEN and so
> > > > > > forth for the initialization phase.
> > > > > In 1.2 the use of chunk metadata is purely fictional. We respect the
> > > > > chunk state machine as we transition lines, but all the write pointers
> > > > > are ignored. Instead, we use the line bitmap to point to the next
> > > > > writable entry. This is BTW the same way we it in open lines on 2.0 too.
> > > >
> > > > Now I understand where you are coming from. I had the understanding
> > > > that we where using the write pointer now that we moved to 2.0,
> > > > looking through the code, that wasn't the case. :) Which means that
> > > > pblk doesn't work with a devices that implements 2.0. Yikes... I knew
> > > > I had forgot a detail when support was added into pblk.
> > > I think you misunderstood; pblk does support 2.0 devices. What happens
> > > is that we transform the per chunk WP in 2.0 into the line bitmap to
> > > simplify the lookup. The point being that we do not need to create a
> > > fictional chunk for 1.2 devices since we do the translation to the
> > > bitmap directly. Does this make sense?
> >
> > The chunk->wp isn't used anywhere. So it can't take wp into account.
> > It uses the EMPTYPAGE marker from 1.2 instead. See pblk-core and
> > pblk-recovery.
>
> I see that the patches for this are still internal. Will post for 4.20

Thanks. Please also put a Fixes: on, so it gets backported appropriately.