Re: [PATCH 6/6] arm64/mm: Enable ZONE_DEVICE

[Jerome Glisse]

On Wed, Apr 03, 2019 at 02:58:28PM +0100, Robin Murphy wrote:
> [ +Dan, Jerome ]
> 
> On 03/04/2019 05:30, Anshuman Khandual wrote:
> > Arch implementation for functions which create or destroy vmemmap mapping
> > (vmemmap_populate, vmemmap_free) can comprehend and allocate from inside
> > device memory range through driver provided vmem_altmap structure which
> > fulfils all requirements to enable ZONE_DEVICE on the platform. Hence just
> 
> ZONE_DEVICE is about more than just altmap support, no?
> 
> > enable ZONE_DEVICE by subscribing to ARCH_HAS_ZONE_DEVICE. But this is only
> > applicable for ARM64_4K_PAGES (ARM64_SWAPPER_USES_SECTION_MAPS) only which
> > creates vmemmap section mappings and utilize vmem_altmap structure.
> 
> What prevents it from working with other page sizes? One of the foremost
> use-cases for our 52-bit VA/PA support is to enable mapping large quantities
> of persistent memory, so we really do need this for 64K pages too. FWIW, it
> appears not to be an issue for PowerPC.
> 
> > Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > ---
> >   arch/arm64/Kconfig | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index db3e625..b5d8cf5 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -31,6 +31,7 @@ config ARM64
> >   	select ARCH_HAS_SYSCALL_WRAPPER
> >   	select ARCH_HAS_TEARDOWN_DMA_OPS if IOMMU_SUPPORT
> >   	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
> > +	select ARCH_HAS_ZONE_DEVICE if ARM64_4K_PAGES
> 
> IIRC certain configurations (HMM?) don't even build if you just turn this on
> alone (although of course things may have changed elsewhere in the meantime)
> - crucially, though, from previous discussions[1] it seems fundamentally
> unsafe, since I don't think we can guarantee that nobody will touch the
> corners of ZONE_DEVICE that also require pte_devmap in order not to go
> subtly wrong. I did get as far as cooking up some patches to sort that out
> [2][3] which I never got round to posting for their own sake, so please
> consider picking those up as part of this series.

Correct _do not_ enable ZONE_DEVICE without support for pte_devmap detection.
If you want some feature of ZONE_DEVICE. Like HMM as while DAX does require
pte_devmap, HMM device private does not. So you would first have to split
ZONE_DEVICE into more sub-features kconfig option.

What is the end use case you are looking for ? Persistent memory ?

Cheers,
Jérôme