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? Or is it always enabled? If so, that seems incautious - if it breaks in horrid ways we want people to be able to go back to the usual behavior. -- 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>