On Sat, Apr 20, 2019 at 8:32 AM Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> wrote: > > Recently, adding a persistent memory to be used like a regular RAM was > added to Linux. This work extends this functionality to also allow hot > removing persistent memory. > > We (Microsoft) have a very important use case for this functionality. > > The requirement is for physical machines with small amount of RAM (~8G) > to be able to reboot in a very short period of time (<1s). Yet, there is > a userland state that is expensive to recreate (~2G). > > The solution is to boot machines with 2G preserved for persistent > memory. Makes sense, but I have some questions about the details. > > Copy the state, and hotadd the persistent memory so machine still has all > 8G for runtime. Before reboot, hotremove device-dax 2G, copy the memory > that is needed to be preserved to pmem0 device, and reboot. > > The series of operations look like this: > > 1. After boot restore /dev/pmem0 to boot > 2. Convert raw pmem0 to devdax > ndctl create-namespace --mode devdax --map mem -e namespace0.0 -f > 3. Hotadd to System RAM > echo dax0.0 > /sys/bus/dax/drivers/device_dax/unbind > echo dax0.0 > /sys/bus/dax/drivers/kmem/new_id > 4. Before reboot hotremove device-dax memory from System RAM > echo dax0.0 > /sys/bus/dax/drivers/kmem/unbind > 5. Create raw pmem0 device > ndctl create-namespace --mode raw -e namespace0.0 -f > 6. Copy the state to this device What is the source of this copy? The state that was in the hot-added memory? Isn't it "already there" since you effectively renamed dax0.0 to pmem0? > 7. Do kexec reboot, or reboot through firmware, is firmware does not > zero memory in pmem region. Wouldn't the dax0.0 contents be preserved regardless? How does the guest recover the pre-initialized state / how does the kernel know to give out the same pages to the application as the previous boot?
