Re: [PATCH 3/5] vfs: add a zero-initialization mode to fallocate

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Sep 22, 2021 at 7:43 PM Dan Williams <dan.j.williams@intel.com> wrote:
>
> On Wed, Sep 22, 2021 at 6:42 PM Dave Chinner <david@fromorbit.com> wrote:
> [..]
> > Hence this discussion leads me to conclude that fallocate() simply
> > isn't the right interface to clear storage hardware poison state and
> > it's much simpler for everyone - kernel and userspace - to provide a
> > pwritev2(RWF_CLEAR_HWERROR) flag to directly instruct the IO path to
> > clear hardware error state before issuing this user write to the
> > hardware.
>
> That flag would slot in nicely in dax_iomap_iter() as the gate for
> whether dax_direct_access() should allow mapping over error ranges,
> and then as a flag to dax_copy_from_iter() to indicate that it should
> compare the incoming write to known poison and clear it before
> proceeding.
>
> I like the distinction, because there's a chance the application did
> not know that the page had experienced data loss and might want the
> error behavior. The other service the driver could offer with this
> flag is to do a precise check of the incoming write to make sure it
> overlaps known poison and then repair the entire page. Repairing whole
> pages makes for a cleaner implementation of the code that tries to
> keep poison out of the CPU speculation path, {set,clear}_mce_nospec().

This flag could also be useful for preadv2() as there is currently no
way to read the good data in a PMEM page with poison via DAX. So the
flag would tell dax_direct_access() to again proceed in the face of
errors, but then the driver's dax_copy_to_iter() operation could
either read up to the precise byte offset of the error in the page, or
autoreplace error data with zero's to try to maximize data recovery.