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

[Dan Williams]

On Wed, May 3, 2017 at 11:46 AM, Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote:
> 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.  It 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.  For
>> > > > checking badblocks, it needs to call dax_pmem to check with
>> > > > region-level badblocks.
>> > > >
>> > > > This patch attempts to keep device_dax self-contained.  It 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.  I 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?  Can 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?

No, I had started that way, but badblocks really needs write(2) or
fallocate(PUNCH_HOLE) support for clearing errors. Since we don't want
to support write(2) and were NAKd from supporting fallocate() the only
interface that was left was sending clear-error-DSM ioctls directly to
 the nvdimm bus. Since that is a very libnvdimm specific interface it
made sense to then add badblocks at the libnvdimm-region level. The
"ndctl clear-error" command is there to do the translation of error
offsets in user space and supersedes the need for the kernel to carry
a badblocks file for device-dax.