linux-kernel - Re: [PATCH v2 2/5] mm,memory_hotplug: Introduce MHP_VMEMMAP

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Date:   Thu, 25 Jul 2019 11:30:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/5] mm,memory_hotplug: Introduce MHP_VMEMMAP_FLAGS

On 25.07.19 11:27, Oscar Salvador wrote:
> On Wed, Jul 24, 2019 at 01:11:52PM -0700, Dan Williams wrote:
>> On Tue, Jun 25, 2019 at 12:53 AM Oscar Salvador <osalvador@...e.de> wrote:
>>>
>>> This patch introduces MHP_MEMMAP_DEVICE and MHP_MEMMAP_MEMBLOCK flags,
>>> and prepares the callers that add memory to take a "flags" parameter.
>>> This "flags" parameter will be evaluated later on in Patch#3
>>> to init mhp_restrictions struct.
>>>
>>> The callers are:
>>>
>>> add_memory
>>> __add_memory
>>> add_memory_resource
>>>
>>> Unfortunately, we do not have a single entry point to add memory, as depending
>>> on the requisites of the caller, they want to hook up in different places,
>>> (e.g: Xen reserve_additional_memory()), so we have to spread the parameter
>>> in the three callers.
>>>
>>> The flags are either MHP_MEMMAP_DEVICE or MHP_MEMMAP_MEMBLOCK, and only differ
>>> in the way they allocate vmemmap pages within the memory blocks.
>>>
>>> MHP_MEMMAP_MEMBLOCK:
>>>         - With this flag, we will allocate vmemmap pages in each memory block.
>>>           This means that if we hot-add a range that spans multiple memory blocks,
>>>           we will use the beginning of each memory block for the vmemmap pages.
>>>           This strategy is good for cases where the caller wants the flexiblity
>>>           to hot-remove memory in a different granularity than when it was added.
>>>
>>>           E.g:
>>>                 We allocate a range (x,y], that spans 3 memory blocks, and given
>>>                 memory block size = 128MB.
>>>                 [memblock#0  ]
>>>                 [0 - 511 pfns      ] - vmemmaps for section#0
>>>                 [512 - 32767 pfns  ] - normal memory
>>>
>>>                 [memblock#1 ]
>>>                 [32768 - 33279 pfns] - vmemmaps for section#1
>>>                 [33280 - 65535 pfns] - normal memory
>>>
>>>                 [memblock#2 ]
>>>                 [65536 - 66047 pfns] - vmemmap for section#2
>>>                 [66048 - 98304 pfns] - normal memory
>>>
>>> MHP_MEMMAP_DEVICE:
>>>         - With this flag, we will store all vmemmap pages at the beginning of
>>>           hot-added memory.
>>>
>>>           E.g:
>>>                 We allocate a range (x,y], that spans 3 memory blocks, and given
>>>                 memory block size = 128MB.
>>>                 [memblock #0 ]
>>>                 [0 - 1533 pfns    ] - vmemmap for section#{0-2}
>>>                 [1534 - 98304 pfns] - normal memory
>>>
>>> When using larger memory blocks (1GB or 2GB), the principle is the same.
>>>
>>> Of course, MHP_MEMMAP_DEVICE is nicer when it comes to have a large contigous
>>> area, while MHP_MEMMAP_MEMBLOCK allows us to have flexibility when removing the
>>> memory.
>>
>> Concept and patch looks good to me, but I don't quite like the
>> proliferation of the _DEVICE naming, in theory it need not necessarily
>> be ZONE_DEVICE that is the only user of that flag. I also think it
>> might be useful to assign a flag for the default 'allocate from RAM'
>> case, just so the code is explicit. So, how about:
> 
> Well, MHP_MEMMAP_DEVICE is not tied to ZONE_DEVICE.
> MHP_MEMMAP_DEVICE was chosen to make a difference between:
> 
>  * allocate memmap pages for the whole memory-device
>  * allocate memmap pages on each memoryblock that this memory-device spans

I agree that DEVICE is misleading here, you are assuming a one-to-one
mapping between a device and add_memory(). You are actually taliing
about "allocate a single chunk of mmap pages for the whole memory range
that is added - which could consist of multiple memory blocks".

-- 

Thanks,

David / dhildenb