Changelog: v6 - A few minor changes and added reviewed-by's. - Spent time studying lock ordering issue that was reported by Vishal Verma, but that issue already exists in Linux, and can be reproduced with exactly the same steps with ACPI memory hotplugging. v5 - Addressed comments from Dan Williams: made remove_memory() to return an error code, and use this function from dax. v4 - Addressed comments from Dave Hansen v3 - Addressed comments from David Hildenbrand. Don't release lock_device_hotplug after checking memory status, and rename memblock_offlined_cb() to check_memblock_offlined_cb() v2 - Dan Williams mentioned that drv->remove() return is ignored by unbind. Unbind always succeeds. Because we cannot guarantee that memory can be offlined from the driver, don't even attempt to do so. Simply check that every section is offlined beforehand and only then proceed with removing dax memory. --- 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 an 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. Copy the state, and hotadd the persistent memory so machine still has all 8G available for runtime. Before reboot, offline and 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 ramdisk to be consumed by apps. and free ramdisk. 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 echo online_movable > /sys/devices/system/memoryXXX/state 4. Before reboot hotremove device-dax memory from System RAM echo offline > /sys/devices/system/memoryXXX/state 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 that was stored by apps to ramdisk to pmem device 7. Do kexec reboot or reboot through firmware if firmware does not zero memory in pmem0 region (These machines have only regular volatile memory). So to have pmem0 device either memmap kernel parameter is used, or devices nodes in dtb are specified. Pavel Tatashin (3): device-dax: fix memory and resource leak if hotplug fails mm/hotplug: make remove_memory() interface useable device-dax: "Hotremove" persistent memory that is used like normal RAM drivers/dax/dax-private.h | 2 ++ drivers/dax/kmem.c | 46 +++++++++++++++++++++--- include/linux/memory_hotplug.h | 8 +++-- mm/memory_hotplug.c | 64 +++++++++++++++++++++++----------- 4 files changed, 92 insertions(+), 28 deletions(-) -- 2.21.0
