* Zhang Yanfei <zhangyanfei.yes@xxxxxxxxx> wrote: > Hello Andrew > > On 12/04/2013 07:48 AM, Andrew Morton wrote: > > On Tue, 03 Dec 2013 10:19:44 +0800 Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx> wrote: > > > >> The current Linux cannot migrate pages used by the kerenl because > >> of the kernel direct mapping. In Linux kernel space, va = pa + PAGE_OFFSET. > >> When the pa is changed, we cannot simply update the pagetable and > >> keep the va unmodified. So the kernel pages are not migratable. > >> > >> There are also some other issues will cause the kernel pages not migratable. > >> For example, the physical address may be cached somewhere and will be used. > >> It is not to update all the caches. > >> > >> When doing memory hotplug in Linux, we first migrate all the pages in one > >> memory device somewhere else, and then remove the device. But if pages are > >> used by the kernel, they are not migratable. As a result, memory used by > >> the kernel cannot be hot-removed. > >> > >> Modifying the kernel direct mapping mechanism is too difficult to do. And > >> it may cause the kernel performance down and unstable. So we use the following > >> way to do memory hotplug. > >> > >> > >> [What we are doing] > >> > >> In Linux, memory in one numa node is divided into several zones. One of the > >> zones is ZONE_MOVABLE, which the kernel won't use. > >> > >> In order to implement memory hotplug in Linux, we are going to arrange all > >> hotpluggable memory in ZONE_MOVABLE so that the kernel won't use these memory. > > > > How does the user enable this? I didn't spot a Kconfig variable which > > enables it. Is there a boot option? > > Yeah, there is a Kconfig variable "MOVABLE_NODE" and a boot option "movable_node" > > mm/Kconfig > > config MOVABLE_NODE Some bikeshedding: I suspect 'movable nodes' is the right idiom to use here, unless the feature is restricted to a single node only. So the option should be 'CONFIG_MOVABLE_NODES=y' and 'movable_nodes=...'. Thanks, Ingo -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>