Kyungsan Kim <ks0204.kim@xxxxxxxxxxx> writes: > 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. Sorry, I don't get your idea. You want the memory range 1. can be hot-removed 2. allow kernel context allocation This appears impossible for me. Why cannot you just use ZONE_MOVABLE? Best Regards, Huang, Ying > 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.
