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

[Dan Williams <dan.j.williams@intel.com>]

On Sun, May 19, 2019 at 10:37 PM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
> On 05/18/2019 03:20 AM, Andrew Morton wrote:
> > On Fri, 17 May 2019 16:08:34 +0530 Anshuman Khandual <anshuman.khandual@arm.com> 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.

+1 to killing it, since there has been a paucity of 'unsigned long
pfn' code path conversions to 'pfn_t', and it continues to go unused.

> > But first, let's fix it so that if someone brings it back, they bring
> > back a non-buggy version.

Not sure this can be solved without a rethink of who owns the virtual
address space corresponding to MEMORY_DEVICE_PRIVATE, and clawing back
some of the special-ness of HMM.

>
> 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.

MEMORY_DEVICE_PRIVATE semantics were part of the package of the
initial HMM submission that landed in the kernel without an upstream
user. While pfn_t_to_virt() also does not have an upstream user it was
at least modeled after the existing pfn_to_virt() api to allow for
future 'unsigned long pfn' to 'pfn_t' conversions. As for what a fix
might look like, it seems to me that we should try to unify 'pfn_t'
and 'hmm_pfn's. I don't see why 'hmm_pfn's need to exist as their own
concept vs trying consume more flag space out of pfn_t. That would at
least allow the pfn_t_has_page() helper to detect the HMM case.