RE(4): FW: [LSF/MM/BPF TOPIC] SMDK inspired MM changes for CXL

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Thank you David Hinderbrand for your interest on this topic.

>>
>>> Kyungsan Kim wrote:
>>> [..]
>>>>> In addition to CXL memory, we may have other kind of memory in the
>>>>> system, for example, HBM (High Bandwidth Memory), memory in FPGA card,
>>>>> memory in GPU card, etc.  I guess that we need to consider them
>>>>> together.  Do we need to add one zone type for each kind of memory?
>>>>
>>>> We also don't think a new zone is needed for every single memory
>>>> device.  Our viewpoint is the sole ZONE_NORMAL becomes not enough to
>>>> manage multiple volatile memory devices due to the increased device
>>>> types.  Including CXL DRAM, we think the ZONE_EXMEM can be used to
>>>> represent extended volatile memories that have different HW
>>>> characteristics.
>>>
>>> Some advice for the LSF/MM discussion, the rationale will need to be
>>> more than "we think the ZONE_EXMEM can be used to represent extended
>>> volatile memories that have different HW characteristics". It needs to
>>> be along the lines of "yes, to date Linux has been able to describe DDR
>>> with NUMA effects, PMEM with high write overhead, and HBM with improved
>>> bandwidth not necessarily latency, all without adding a new ZONE, but a
>>> new ZONE is absolutely required now to enable use case FOO, or address
>>> unfixable NUMA problem BAR." Without FOO and BAR to discuss the code
>>> maintainability concern of "fewer degress of freedom in the ZONE
>>> dimension" starts to dominate.
>>
>> One problem we experienced was occured in the combination of hot-remove and kerelspace allocation usecases.
>> ZONE_NORMAL allows kernel context allocation, but it does not allow hot-remove because kernel resides all the time.
>> ZONE_MOVABLE allows hot-remove due to the page migration, but it only allows userspace allocation.
>> Alternatively, we allocated a kernel context out of ZONE_MOVABLE by adding GFP_MOVABLE flag.

>That sounds like a bad hack :) .
I consent you.

>> In case, oops and system hang has occasionally occured because ZONE_MOVABLE can be swapped.
>> We resolved the issue using ZONE_EXMEM by allowing seletively choice of the two usecases.

>I once raised the idea of a ZONE_PREFER_MOVABLE [1], maybe that's
>similar to what you have in mind here. In general, adding new zones is
>frowned upon.

Actually, we have already studied your idea and thought it is similar with us in 2 aspects.
1. ZONE_PREFER_MOVABLE allows a kernelspace allocation using a new zone
2. ZONE_PREFER_MOVABLE helps less fragmentation by splitting zones, and ordering allocation requests from the zones.

We think ZONE_EXMEM also helps less fragmentation.
Because it is a separated zone and handles a page allocation as movable by default.

>> As you well know, among heterogeneous DRAM devices, CXL DRAM is the first PCIe basis device, which allows hot-pluggability, different RAS, and extended connectivity.
>> So, we thought it could be a graceful approach adding a new zone and separately manage the new features.
>>
>> Kindly let me know any advice or comment on our thoughts.
>
>[1] https://www.lkml.org/lkml/2020/9/9/667
>
>--
>Thanks,
>
>David / dhildenb



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