Re: RE: RE(2): FW: [LSF/MM/BPF TOPIC] SMDK inspired MM changes for CXL

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Thank you Mike Rapoport for participating discussion and adding your thought.

>Hi,
>
>On Thu, Mar 23, 2023 at 07:51:05PM +0900, Kyungsan Kim wrote:
>> I appreciate dan for the careful advice.
>> 
>> >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.
>> 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.
>> 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.
>
>This still does not describe what are the use cases that require having
>kernel allocations on CXL.mem. 
>
>I believe it's important to start with explanation *why* it is important to
>have kernel allocations on removable devices.
> 

In general, a memory system with DDR/CXL DRAM will have near/far memory.
And, we think kernel already includes memory tiering solutions - Meta TPP, zswap, and pagecache.
Some kernel contexts would prefer fast memory. For example, a hot data with time locality or a data for fast processing such as metadata or indexing.
Others would enough with slow memory. For example, a zswap page which is being used while swapping. 

>> Kindly let me know any advice or comment on our thoughts.
>> 
>> 
>
>-- 
>Sincerely yours,
>Mike.



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