Re: [dm-devel] [PATCH 0/6] dax poison recovery with RWF_RECOVERY_DATA flag

Christoph Hellwig
2021-11-04T17:46:52Z

On Thu, Nov 04, 2021 at 09:08:41AM -0700, Dan Williams wrote:
> Yes, atomic clear+write new data. The ability to atomic clear requires
> either a CPU with the ability to overwrite cachelines without doing a
> RMW cycle (MOVDIR64B), or it requires a device with a suitable
> slow-path mailbox command like the one defined for CXL devices (see
> section 8.2.9.5.4.3 Clear Poison in CXL 2.0).
>
> I don't know why you think these devices don't perform wear-leveling
> with spare blocks?

Because the interface looks so broken. But yes, apparently it's not
the media management that is broken but just the inteface that fakes
up byte level access.

> All kernel accesses do use it. They either route to
> pmem_copy_to_iter(), or like dm-writecache, call it directly. Do you
> see a kernel path that does not use that helper?

No, sorry. My knowledge is out of date.
(nova does, but it is out of tree, and the lack of using
copy_mc_to_kernel is the least of its problems)