Re: [PATCH v2 3/5] libnvdimm/nvdimm/flush: Allow architecture to override the flush barrier

[Mikulas Patocka <mpatocka@redhat.com>]

On Thu, 21 May 2020, Dan Williams wrote:

> On Thu, May 21, 2020 at 10:03 AM Aneesh Kumar K.V
> <aneesh.kumar@linux.ibm.com> wrote:
> >
> > > Moving on to the patch itself--Aneesh, have you audited other persistent
> > > memory users in the kernel?  For example, drivers/md/dm-writecache.c does
> > > this:
> > >
> > > static void writecache_commit_flushed(struct dm_writecache *wc, bool wait_for_ios)
> > > {
> > >       if (WC_MODE_PMEM(wc))
> > >               wmb(); <==========
> > >          else
> > >                  ssd_commit_flushed(wc, wait_for_ios);
> > > }
> > >
> > > I believe you'll need to make modifications there.
> > >
> >
> > Correct. Thanks for catching that.
> >
> >
> > I don't understand dm much, wondering how this will work with
> > non-synchronous DAX device?
> 
> That's a good point. DM-writecache needs to be cognizant of things
> like virtio-pmem that violate the rule that persisent memory writes
> can be flushed by CPU functions rather than calling back into the
> driver. It seems we need to always make the flush case a dax_operation
> callback to account for this.

dm-writecache is normally sitting on the top of dm-linear, so it would 
need to pass the wmb() call through the dm core and dm-linear target ... 
that would slow it down ... I remember that you already did it this way 
some times ago and then removed it.

What's the exact problem with POWER? Could the POWER system have two types 
of persistent memory that need two different ways of flushing?

Mikulas
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