Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while computing virtual address

[Dan Williams]

On Mon, May 20, 2019 at 12:27 PM Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>
> On Mon, May 20, 2019 at 11:07:38AM +0530, Anshuman Khandual wrote:
> > On 05/18/2019 03:20 AM, Andrew Morton wrote:
> > > On Fri, 17 May 2019 16:08:34 +0530 Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> > >
> > >> The presence of struct page does not guarantee linear mapping for the pfn
> > >> physical range. Device private memory which is non-coherent is excluded
> > >> from linear mapping during devm_memremap_pages() though they will still
> > >> have struct page coverage. Just check for device private memory before
> > >> giving out virtual address for a given pfn.
> > >
> > > I was going to give my standard "what are the user-visible runtime
> > > effects of this change?", but...
> > >
> > >> All these helper functions are all pfn_t related but could not figure out
> > >> another way of determining a private pfn without looking into it's struct
> > >> page. pfn_t_to_virt() is not getting used any where in mainline kernel.Is
> > >> it used by out of tree drivers ? Should we then drop it completely ?
> > >
> > > Yeah, let's kill it.
> > >
> > > But first, let's fix it so that if someone brings it back, they bring
> > > back a non-buggy version.
> >
> > Makes sense.
> >
> > >
> > > So...  what (would be) the user-visible runtime effects of this change?
> >
> > I am not very well aware about the user interaction with the drivers which
> > hotplug and manage ZONE_DEVICE memory in general. Hence will not be able to
> > comment on it's user visible runtime impact. I just figured this out from
> > code audit while testing ZONE_DEVICE on arm64 platform. But the fix makes
> > the function bit more expensive as it now involve some additional memory
> > references.
>
> A device private pfn can never leak outside code that does not understand it
> So this change is useless for any existing users and i would like to keep the
> existing behavior ie never leak device private pfn.

The issue is that only an HMM expert might know that such a pfn can
never leak, in other words the pfn concept from a code perspective is
already leaked / widespread. Ideally any developer familiar with a pfn
and the core-mm pfn helpers need only worry about pfn semantics
without being required to go audit HMM users.