Re: [PATCH v7 4/6] dax: add DAX_RECOVERY flag and .recovery_write dev_pgmap_ops

[Dan Williams]

On Wed, Apr 6, 2022 at 10:31 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> On Wed, Apr 06, 2022 at 05:32:31PM +0000, Jane Chu wrote:
> > Yes, I believe Dan was motivated by avoiding the dm dance as a result of
> > adding .recovery_write to dax_operations.
> >
> > I understand your point about .recovery_write is device specific and
> > thus not something appropriate for device agnostic ops.
> >
> > I can see 2 options so far -
> >
> > 1)  add .recovery_write to dax_operations and do the dm dance to hunt
> > down to the base device that actually provides the recovery action
>
> That would be my preference.  But I'll wait for Dan to chime in.

Yeah, so the motivation was avoiding plumbing recovery through stacked
lookups when the recovery is specific to a pfn and the provider of
that pfn, but I also see it from Christoph's perspective that the only
agent that cares about recovery is the fsdax I/O path. Certainly
having ->dax_direct_access() take a DAX_RECOVERY flag and the op
itself go through the pgmap is a confusing split that I did not
anticipate when I made the suggestion. Since that flag must be there,
then the ->recovery_write() should also stay relative to a dax device.

Apologies for the thrash Jane.

One ask though, please separate plumbing the new flag argument to
->dax_direct_access() and plumbing the new operation into preparation
patches before filling them in with the new goodness.