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Subject: Re: [PATCH 2/6] arm64/mm: Enable memory hot remove
To:     Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mm@kvack.org, akpm@linux-foundation.org, will.deacon@arm.com,
        catalin.marinas@arm.com
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        arunks@codeaurora.org, dan.j.williams@intel.com, osalvador@suse.de,
        pasha.tatashin@oracle.com, david@redhat.com, cai@lca.pw,
        Stephen Bates <sbates@raithlin.com>
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From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <45afb99f-5785-4048-a748-4e0f06b06b31@arm.com>
Date:   Thu, 4 Apr 2019 12:37:58 +0530
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On 04/03/2019 11:02 PM, Logan Gunthorpe wrote:
> 
> 
> On 2019-04-02 10:30 p.m., Anshuman Khandual wrote:
>> Memory removal from an arch perspective involves tearing down two different
>> kernel based mappings i.e vmemmap and linear while releasing related page
>> table pages allocated for the physical memory range to be removed.
>>
>> Define a common kernel page table tear down helper remove_pagetable() which
>> can be used to unmap given kernel virtual address range. In effect it can
>> tear down both vmemap or kernel linear mappings. This new helper is called
>> from both vmemamp_free() and ___remove_pgd_mapping() during memory removal.
>> The argument 'direct' here identifies kernel linear mappings.
>>
>> Vmemmap mappings page table pages are allocated through sparse mem helper
>> functions like vmemmap_alloc_block() which does not cycle the pages through
>> pgtable_page_ctor() constructs. Hence while removing it skips corresponding
>> destructor construct pgtable_page_dtor().
>>
>> While here update arch_add_mempory() to handle __add_pages() failures by
>> just unmapping recently added kernel linear mapping. Now enable memory hot
>> remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE.
>>
>> This implementation is overall inspired from kernel page table tear down
>> procedure on X86 architecture.
> 
> I've been working on very similar things for RISC-V. In fact, I'm
> currently in progress on a very similar stripped down version of
> remove_pagetable(). (Though I'm fairly certain I've done a bunch of
> stuff wrong.)
> 
> Would it be possible to move this work into common code that can be used
> by all arches? Seems like, to start, we should be able to support both
> arm64 and RISC-V... and maybe even x86 too.
> 
> I'd be happy to help integrate and test such functions in RISC-V.

Sure that will be great. The only impediment is pgtable_page_ctor() for kernel
linear mapping. This series is based on current arm64 where linear mapping
pgtable pages go through pgtable_page_ctor() init sequence but that might be
changing soon. If RISC-V does not have pgtable_page_ctor() init for linear
mapping and no other arch specific stuff later on we can try to consolidate
remove_pagetable() atleast for both the architectures.

Then I wondering whether I can transition pud|pmd_large() to pud|pmd_sect().