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