Re: [PATCH 18/22] mm: mark DEVICE_PUBLIC as broken

[Ira Weiny]

On Thu, Jun 13, 2019 at 07:58:29PM +0000, Jason Gunthorpe wrote:
> On Thu, Jun 13, 2019 at 12:53:02PM -0700, Ralph Campbell wrote:
> > 
> > On 6/13/19 12:44 PM, Jason Gunthorpe wrote:
> > > On Thu, Jun 13, 2019 at 11:43:21AM +0200, Christoph Hellwig wrote:
> > > > The code hasn't been used since it was added to the tree, and doesn't
> > > > appear to actually be usable.  Mark it as BROKEN until either a user
> > > > comes along or we finally give up on it.
> > > > 
> > > > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > > >   mm/Kconfig | 1 +
> > > >   1 file changed, 1 insertion(+)
> > > > 
> > > > diff --git a/mm/Kconfig b/mm/Kconfig
> > > > index 0d2ba7e1f43e..406fa45e9ecc 100644
> > > > +++ b/mm/Kconfig
> > > > @@ -721,6 +721,7 @@ config DEVICE_PRIVATE
> > > >   config DEVICE_PUBLIC
> > > >   	bool "Addressable device memory (like GPU memory)"
> > > >   	depends on ARCH_HAS_HMM
> > > > +	depends on BROKEN
> > > >   	select HMM
> > > >   	select DEV_PAGEMAP_OPS
> > > 
> > > This seems a bit harsh, we do have another kconfig that selects this
> > > one today:
> > > 
> > > config DRM_NOUVEAU_SVM
> > >          bool "(EXPERIMENTAL) Enable SVM (Shared Virtual Memory) support"
> > >          depends on ARCH_HAS_HMM
> > >          depends on DRM_NOUVEAU
> > >          depends on STAGING
> > >          select HMM_MIRROR
> > >          select DEVICE_PRIVATE
> > >          default n
> > >          help
> > >            Say Y here if you want to enable experimental support for
> > >            Shared Virtual Memory (SVM).
> > > 
> > > Maybe it should be depends on STAGING not broken?
> > > 
> > > or maybe nouveau_svm doesn't actually need DEVICE_PRIVATE?
> > > 
> > > Jason
> > 
> > I think you are confusing DEVICE_PRIVATE for DEVICE_PUBLIC.
> > DRM_NOUVEAU_SVM does use DEVICE_PRIVATE but not DEVICE_PUBLIC.
> 
> Indeed you are correct, never mind
> 
> Hum, so the only thing this config does is short circuit here:
> 
> static inline bool is_device_public_page(const struct page *page)
> {
>         return IS_ENABLED(CONFIG_DEV_PAGEMAP_OPS) &&
>                 IS_ENABLED(CONFIG_DEVICE_PUBLIC) &&
>                 is_zone_device_page(page) &&
>                 page->pgmap->type == MEMORY_DEVICE_PUBLIC;
> }
> 
> Which is called all over the place.. 

<sigh>  yes but the earlier patch:

[PATCH 03/22] mm: remove hmm_devmem_add_resource

Removes the only place type is set to MEMORY_DEVICE_PUBLIC.

So I think it is ok.  Frankly I was wondering if we should remove the public
type altogether but conceptually it seems ok.  But I don't see any users of it
so...  should we get rid of it in the code rather than turning the config off?

Ira

> 
> So, yes, we really don't want any distro or something to turn this on
> until it has a use.
> 
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> 
> Jason
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