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Subject: Re: [PATCH 6/6] arm64/mm: Enable ZONE_DEVICE
To:     Anshuman Khandual <anshuman.khandual@arm.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
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 <1554265806-11501-7-git-send-email-anshuman.khandual@arm.com>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <ea5567c7-caad-8a4e-7c6f-cec4b772a526@arm.com>
Date:   Wed, 3 Apr 2019 14:58:28 +0100
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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)