Re: FW: [LSF/MM/BPF TOPIC] SMDK inspired MM changes for CXL

[David Hildenbrand <david@redhat.com>]

On 24.03.23 10:27, Kyungsan Kim wrote:
>> On 24.03.23 10:09, Kyungsan Kim wrote:
>>> 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.
>>
>> So how is it different that it would justify a different (more confusing
>> IMHO) name? :) Of course, names don't matter that much, but I'd be
>> interested in which other aspect that zone would be "special".
> 
> FYI for the first time I named it as ZONE_CXLMEM, but we thought it would be needed to cover other extended memory types as well.
> So I changed it as ZONE_EXMEM.
> We also would like to point out a "special" zone aspeact, which is different from ZONE_NORMAL for tranditional DDR DRAM.
> Of course, a symbol naming is important more or less to represent it very nicely, though.
> Do you prefer ZONE_SPECIAL? :)

I called it ZONE_PREFER_MOVABLE. If you studied that approach there must 
be a good reason to name it differently?

-- 
Thanks,

David / dhildenb