On Mon 31-07-17 19:58:30, Gerald Schaefer wrote: > On Mon, 31 Jul 2017 17:53:50 +0200 > Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > On Mon 31-07-17 17:04:59, Gerald Schaefer wrote: [...] > > > Well, it still looks like we cannot do w/o splitting up add_memory(): > > > 1) (only) set up section map during our initial memory detection, w/o > > > allocating struct pages, so that the sysfs entries get created also for > > > our offline memory (or else we have no way to online it later) > > > 2) set up vmemmap and allocate struct pages with your new altmap approach > > > during our MEM_GOING_ONLINE callback, because only now the memory is really > > > accessible > > > > As I've tried to mentioned in my other response. This is not possible > > because there are memory hotplug usecases which never do an explicit > > online. > > Of course the default behaviour should not change, we only need an option > to do the "2-stage-approach". E.g. we would call __add_pages() from our > MEM_GOING_ONLINE handler, and not from arch_add_memory() as before, but > then we would need some way to add memory sections (for generating sysfs > memory blocks) only, without allocating struct pages. See also below. I would have to check that more deeply. I am not sure some parts of memblock infrastructure depends on struct page existence. > > I am sorry to ask again. But why exactly cannot we make the range > > accessible from arch_add_memory on s390? > > We have no acpi or other events to indicate new memory, both online and > offline memory needs to be (hypervisor) defined upfront, and then we want > to be able to use memory hotplug for ballooning during runtime. > > Making the range accessible is equivalent to a hypervisor call that assigns > the memory to the guest. The problem with arch_add_memory() is now that > this gets called from add_memory(), which we call during initial memory > detection for the offline memory ranges. At that time, assigning all > offline memory to the guest, and thus making it accessible, would break > the ballooning usecase (even if it is still offline in Linux, the > hypervisor could not use it for other guests any more). OK, I guess I see your point. Thanks for the clarification. I will try to think about this limitation but I will rule simply disable the feature for the initial inclusion. s390 can be done later. > The main difference to other architectures is that we can not simply > call add_memory() (and thus arch_add_memory()) at the time when the > offline memory is actually supposed to get online (e.g. triggered by acpi). > We rather need to somehow make sure that the offline memory is detected > early, and sysfs entries are created for it, so that it can be set online > later on demand. > > Maybe our design to use add_memory() for offline ranges during memory > detection was wrong, or overkill, since we actually only need to establish > a memory section, if I understood the sysfs code right. But I currently > see no other way to make sure that we get the sysfs attributes. And of > course the presence of users that work on offline struct pages, like > valid_zones, is also not helpful. Yeah, but I suspect that we can make the whole memory hotplug sysfs API independant on memory sections and struct page states. I am adding this to my todo list. -- Michal Hocko SUSE Labs -- 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>
