Re: [PATCH 0/3] dax: clear poison on the fly along pwrite

[Dan Williams]

On Fri, Sep 17, 2021 at 5:57 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> On Thu, Sep 16, 2021 at 11:40:28AM -0700, Dan Williams wrote:
> > > That was my gut feeling.  If everyone feels 100% comfortable with
> > > zeroingas the mechanism to clear poisoning I'll cave in.  The most
> > > important bit is that we do that through a dedicated DAX path instead
> > > of abusing the block layer even more.
> >
> > ...or just rename dax_zero_page_range() to dax_reset_page_range()?
> > Where reset == "zero + clear-poison"?
>
> I'd say that naming is more confusing than overloading zero.

Ok, I see Darrick has a better suggestion for the shed color.

>
> > > I'm really worried about both patartitions on DAX and DM passing through
> > > DAX because they deeply bind DAX to the block layer, which is just a bad
> > > idea.  I think we also need to sort that whole story out before removing
> > > the EXPERIMENTAL tags.
> >
> > I do think it was a mistake to allow for DAX on partitions of a pmemX
> > block-device.
> >
> > DAX-reflink support may be the opportunity to start deprecating that
> > support. Only enable DAX-reflink for direct mounting on /dev/pmemX
> > without partitions (later add dax-device direct mounting),
>
> I think we need to fully or almost fully sort this out.
>
> Here is my bold suggestions:
>
>  1) drop no drop the EXPERMINTAL on the current block layer overload
>     at all

s/drop no drop/do not drop/?

>  2) add direct mounting of the nvdimm namespaces ASAP.  Because all
>     the filesystem currently also need the /dev/pmem0 device add a way
>     to open the block device by the dax_device instead of our current
>     way of doing the reverse

Oh, interesting. I can get on board with that. There's currently no
/dev entry for namespaces. It's either /dev/pmemX, or /dev/daxX.Y as a
child of /sys/bus/nd/devices/namespaceX.Y. However, I see nothing
glaringly wrong with having /dev/daxX.Y always published regardless of
whether /dev/pmemX is also present.

>  3) deprecate DAX support through block layer mounts with a say 2 year
>     deprecation period
>  4) add DAX remapping devices as needed
>
> I'll volunteer to write the initial code for 2).  And I think we should
> not allow DAX+reflink on the block device shim at all.

Yeah, I think this can fly.