linux-kernel - Re: [PATCH 18/22] mm: mark DEVICE

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Message-ID: <20190613195819.GA22062@mellanox.com>
Date:   Thu, 13 Jun 2019 19:58:29 +0000
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Ralph Campbell <rcampbell@...dia.com>
CC:     Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 18/22] mm: mark DEVICE_PUBLIC as broken

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@....de>
> > >   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.. 

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@...lanox.com>

Jason