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

[Dan Williams]

On Thu, Nov 4, 2021 at 1:27 PM Jane Chu <jane.chu@xxxxxxxxxx> wrote:
>
> On 11/4/2021 12:00 PM, Dan Williams wrote:
>
> >>
> >> If this understanding is in the right direction, then I'd like to
> >> propose below changes to
> >>     dax_direct_access(), dax_copy_to/from_iter(), pmem_copy_to/from_iter()
> >>     and the dm layer copy_to/from_iter, dax_iomap_iter().
> >>
> >> 1. dax_iomap_iter() rely on dax_direct_access() to decide whether there
> >>      is likely media error: if the API without DAX_F_RECOVERY returns
> >>      -EIO, then switch to recovery-read/write code.  In recovery code,
> >>      supply DAX_F_RECOVERY to dax_direct_access() in order to obtain
> >>      'kaddr', and then call dax_copy_to/from_iter() with DAX_F_RECOVERY.
> >
> > I like it. It allows for an atomic write+clear implementation on
> > capable platforms and coordinates with potentially unmapped pages. The
> > best of both worlds from the dax_clear_poison() proposal and my "take
> > a fault and do a slow-path copy".
> >
> >> 2. the _copy_to/from_iter implementation would be largely the same
> >>      as in my recent patch, but some changes in Christoph's
> >>      'dax-devirtualize' maybe kept, such as DAX_F_VIRTUAL, obviously
> >>      virtual devices don't have the ability to clear poison, so no need
> >>      to complicate them.  And this also means that not every endpoint
> >>      dax device has to provide dax_op.copy_to/from_iter, they may use the
> >>      default.
> >
> > Did I miss this series or are you talking about this one?
> > https://lore.kernel.org/all/20211018044054.1779424-1-hch@xxxxxx/
>
> I was referring to
>
> http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dax-devirtualize
> that has not come out yet, I said early on that I'll rebase on it,
> but looks like we still need pmem_copy_to/from_iter(), so.

Yeah, since the block-layer divorce gets rid of the old poison
clearing path, then we're back to pmem_copy_to_iter() (or something
like it) needing to pick up the slack for poison clearing. I do agree
it would be nice to clean up all the unnecessary boilerplate, but the
error-list coordination requires a driver specific callback. At least
the DAX_F_VIRTUAL flag can eliminate the virtiofs and fuse callbacks.