Re: [RFC PATCH] dax: add badblocks check to Device DAX

[Kani, Toshimitsu]

On Wed, 2017-05-03 at 09:30 -0700, Dan Williams wrote:
> On Wed, May 3, 2017 at 9:09 AM, Kani, Toshimitsu <toshi.kani@xxxxxxx>
> wrote:
> > On Wed, 2017-05-03 at 08:52 -0700, Dan Williams wrote:
> > > On Wed, May 3, 2017 at 8:31 AM, Toshi Kani <toshi.kani@xxxxxxx>
> > > wrote:
> > > > This is a RFC patch for seeking suggestions.ÂÂIt adds support
> > > > of badblocks check in Device DAX by using region-level
> > > > badblocks list.  This patch is only briefly tested.
> > > > 
> > > > device_dax is a well-isolated self-contained module as it calls
> > > > alloc_dax() with dev_dax, which is private to device_dax.ÂÂFor
> > > > checking badblocks, it needs to call dax_pmem to check with
> > > > region-level badblocks.
> > > > 
> > > > This patch attempts to keep device_dax self-contained.ÂÂIt adds
> > > > check_error() to dax_operations, and dax_check_error() as a
> > > > stub with *dev_dax and *dev pointers to convey it to
> > > > dax_pmem.ÂÂI am wondering if this is the right direction, or we
> > > > should change the modularity to let dax_pmem call alloc_dax()
> > > > with its dax_pmem (or I completely missed something).
> > > 
> > > The problem is that device-dax guarantees a given fault
> > > granularity. To make that guarantee we can't fallback from 1G or
> > > 2M mappings due to an error. We also can't reasonably go the
> > > other way and fail mappings that contain a badblock because that
> > > would change the blast radius of a media error to the fault size.
> > 
> > Does it mean we expect users to have CPUs with MCE recovery for
> > Device DAX?ÂÂCan we add an attributes like allow error-check &
> > fall-back?
> 
> Yes, without MCE recovery device-dax mappings that consume errors
> will reboot. If an application needs the kernel protection it should
> be using filesystem-dax.

Understood.  Are we going to provide sysfs "badblocks" for Device DAX
as it is also needed for ndctl clear-error?

Thanks,
-Toshi