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SubjectRe: [PATCH 6/6] arm64/mm: Enable ZONE_DEVICE
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[ +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@arm.com>
> ---
> 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.

Robin.

> select ARCH_HAVE_NMI_SAFE_CMPXCHG
> select ARCH_INLINE_READ_LOCK if !PREEMPT
> select ARCH_INLINE_READ_LOCK_BH if !PREEMPT
>


[1]
https://lore.kernel.org/linux-mm/CAA9_cmfA9GS+1M1aSyv1ty5jKY3iho3CERhnRAruWJW3PfmpgA@mail.gmail.com/#t
[2]
http://linux-arm.org/git?p=linux-rm.git;a=commitdiff;h=61816b833afdb56b49c2e58f5289ae18809e5d67
[3]
http://linux-arm.org/git?p=linux-rm.git;a=commitdiff;h=a5a16560eb1becf9a1d4cc0d03d6b5e76da4f4e1
(apologies to anyone if the linux-arm.org server is being flaky as usual
and requires a few tries to respond properly)

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