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Subject: Re: [PATCH 0/6] arm64/mm: Enable memory hot remove and ZONE_DEVICE
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Michal Hocko <mhocko@suse.com>,
        Mel Gorman <mgorman@techsingularity.net>, james.morse@arm.com,
        Mark Rutland <mark.rutland@arm.com>,
        Robin Murphy <robin.murphy@arm.com>, cpandya@codeaurora.org,
        arunks@codeaurora.org, osalvador@suse.de,
        Logan Gunthorpe <logang@deltatee.com>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        David Hildenbrand <david@redhat.com>, cai@lca.pw
References: <1554265806-11501-1-git-send-email-anshuman.khandual@arm.com>
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From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <6d46fc37-91e2-c44c-6e01-bfbd5c022f39@arm.com>
Date:   Thu, 4 Apr 2019 18:41:25 +0530
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On 04/03/2019 11:38 PM, Dan Williams wrote:
> On Tue, Apr 2, 2019 at 9:30 PM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>>
>> This series enables memory hot remove on arm64, fixes a memblock removal
>> ordering problem in generic __remove_memory(), enables sysfs memory probe
>> interface on arm64. It also enables ZONE_DEVICE with struct vmem_altmap
>> support.
>>
>> Testing:
>>
>> Tested hot remove on arm64 for all 4K, 16K, 64K page config options with
>> all possible VA_BITS and PGTABLE_LEVELS combinations. Tested ZONE_DEVICE
>> with ARM64_4K_PAGES through a dummy driver.
>>
>> Build tested on non arm64 platforms. I will appreciate if folks can test
>> arch_remove_memory() re-ordering in __remove_memory() on other platforms.
>>
>> Dependency:
>>
>> V5 series in the thread (https://lkml.org/lkml/2019/2/14/1096) will make
>> kernel linear mapping loose pgtable_page_ctor() init. When this happens
>> the proposed functions free_pte|pmd|pud_table() in [PATCH 2/6] will have
>> to stop calling pgtable_page_dtor().
> 
> Hi Anshuman,

Hello Dan,

> 
> I'd be interested to integrate this with the sub-section hotplug
> support [1]. Otherwise the padding implementation in libnvdimm can't
> be removed unless all ZONE_DEVICE capable archs also agree on the
> minimum arch_add_memory() granularity. I'd prefer not to special case
> which archs support which granularity, but it unfortunately
> complicates what you're trying to achieve.

Sorry I have not been following your series on sub-section hotplug support.
Hence might not have the full context here. Could you please give some more
details on what exactly might be a problem.

> 
> I think at a minimum we, mm hotplug co-travellers, need to come to a
> consensus on whether sub-section support is viable for v5.2 and / or a
> pre-requisite for new arch-ZONE_DEVICE implementations

I would need to go through sub-section hotplug series first to understand
the pre-requisite. Do we need to support sub-section hotplug first before
being able to enable ZONE_DEVICE ?

- Anshuman