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. Kindly let me know any advice or comment on our thoughts.
