Re: [HMM-CDM 5/5] mm/hmm: simplify kconfig and enable HMM and DEVICE_PUBLIC for ppc64

[John Hubbard]

On 06/14/2017 07:09 PM, Jerome Glisse wrote:
> On Wed, Jun 14, 2017 at 04:10:32PM -0700, John Hubbard wrote:
> > On 06/14/2017 01:11 PM, JÃrÃme Glisse wrote:
[...]
> > Hi Jerome,
> >
> > There are still some problems with using this configuration. First and
> > foremost, it is still possible (and likely, given the complete dissimilarity
> > in naming, and difference in location on the screen) to choose HMM_MIRROR,
> > and *not* to choose either DEVICE_PRIVATE or DEVICE_PUBLIC. And then we end
> > up with a swath of important page fault handling code being ifdef'd out, and
> > one ends up having to investigate why.
> >
> > As for solutions, at least for the x86 (DEVICE_PRIVATE)case, we could do this:
> >
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 7de939a29466..f64182d7b956 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -279,6 +279,7 @@ config HMM_MIRROR
> >          bool "HMM mirror CPU page table into a device page table"
> >          depends on ARCH_HAS_HMM && X86_64
> >          select MMU_NOTIFIER
> > +       select DEVICE_PRIVATE
> >          select HMM
> >          help
> >            Select HMM_MIRROR if you want to mirror range of the CPU page table of a
> >
> > ...and that is better than the other direction (having HMM_MIRROR depend on
> > DEVICE_PRIVATE), because in the latter case, HMM_MIRROR will disappear (and
> > it's several lines above) until you select DEVICE_PRIVATE. That is hard to
> > work with for the user.
> >
> > The user will tend to select HMM_MIRROR, but it is *not* obvious that he/she
> > should also select DEVICE_PRIVATE. So Kconfig should do it for them.
> >
> > In fact, I'm not even sure if the DEVICE_PRIVATE and DEVICE_PUBLIC actually
> > need Kconfig protection, but if they don't, then life would be easier for
> > whoever is configuring their kernel.
>
> We do need Kconfig for DEVICE_PRIVATE and DEVICE_PUBLIC. I can remove HMM_MIRROR
> and have HMM mirror code ifdef on DEVICE_PRIVATE.
>
> Cheers,
> JÃrÃme

That's probably fine.

(I see that you may have missed the rest of my response, but looks like Balbir 
covered it.)

thanks,
john h