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Subject: Re: [PATCH 2/6] arm64/mm: Enable memory hot remove
To:     Anshuman Khandual <anshuman.khandual@arm.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
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        catalin.marinas@arm.com
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        robin.murphy@arm.com, cai@lca.pw, pasha.tatashin@oracle.com,
        Stephen Bates <sbates@raithlin.com>, james.morse@arm.com,
        cpandya@codeaurora.org, arunks@codeaurora.org,
        dan.j.williams@intel.com, mgorman@techsingularity.net,
        osalvador@suse.de
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From:   Steven Price <steven.price@arm.com>
Message-ID: <1d1d69d1-06e6-f429-f22b-00ca922a314d@arm.com>
Date:   Thu, 4 Apr 2019 10:16:48 +0100
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On 04/04/2019 08:07, Anshuman Khandual wrote:
> 
> 
> 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().

The first 10 patches of my generic page walk series[1] adds p?d_large()
as a common feature, so probably best sticking with p?d_large() if this
is going to be common and basing on top of those patches.

[1]
https://lore.kernel.org/lkml/20190403141627.11664-1-steven.price@arm.com/T/

Steve