> Am 13.01.2020 um 10:10 schrieb Anshuman Khandual <anshuman.khandual@xxxxxxx>: > > > >> On 01/10/2020 02:12 PM, David Hildenbrand wrote: >>> On 10.01.20 04:09, Anshuman Khandual wrote: >>> Currently there are two interfaces to initiate memory range hot removal i.e >>> remove_memory() and __remove_memory() which then calls try_remove_memory(). >>> Platform gets called with arch_remove_memory() to tear down required kernel >>> page tables and other arch specific procedures. But there are platforms >>> like arm64 which might want to prevent removal of certain specific memory >>> ranges irrespective of their present usage or movability properties. >> >> Why? Is this only relevant for boot memory? I hope so, otherwise the >> arch code needs fixing IMHO. > > Right, it is relevant only for the boot memory on arm64 platform. But this > new arch callback makes it flexible to reject any given memory range. > >> >> If it's only boot memory, we should disallow offlining instead via a >> memory notifier - much cleaner. > > Dont have much detail understanding of MMU notifier mechanism but from some > initial reading, it seems like we need to have a mm_struct for a notifier > to monitor various events on the page table. Just wondering how a physical > memory range like boot memory can be monitored because it can be used both > for for kernel (init_mm) or user space process at same time. Is there some > mechanism we could do this ? > >> >>> >>> Current arch call back arch_remove_memory() is too late in the process to >>> abort memory hot removal as memory block devices and firmware memory map >>> entries would have already been removed. Platforms should be able to abort >>> the process before taking the mem_hotplug_lock with mem_hotplug_begin(). >>> This essentially requires a new arch callback for memory range validation. >> >> I somewhat dislike this very much. Memory removal should never fail if >> used sanely. See e.g., __remove_memory(), it will BUG() whenever >> something like that would strike. >> >>> >>> This differentiates memory range validation between memory hot add and hot >>> remove paths before carving out a new helper check_hotremove_memory_range() >>> which incorporates a new arch callback. This call back provides platforms >>> an opportunity to refuse memory removal at the very onset. In future the >>> same principle can be extended for memory hot add path if required. >>> >>> Platforms can choose to override this callback in order to reject specific >>> memory ranges from removal or can just fallback to a default implementation >>> which allows removal of all memory ranges. >> >> I suspect we want really want to disallow offlining instead. E.g., I > > If boot memory pages can be prevented from being offlined for sure, then it > would indirectly definitely prevent hot remove process as well. > >> remember s390x does that with certain areas needed for dumping/kexec. > > Could not find any references to mmu_notifier in arch/s390 or any other arch > for that matter apart from KVM (which has an user space component), could you > please give some pointers ? Memory (hotplug) notifier, not MMU notifier :) Not on my notebook right now, grep for MEM_GOING_OFFLINE, that should be it.
