On 01/13/2020 02:44 PM, David Hildenbrand wrote: > > >> 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 :) They are so similarly named :) > > Not on my notebook right now, grep for MEM_GOING_OFFLINE, that should be it. > Got it, thanks ! But we will still need boot memory enumeration via MEMBLOCK_BOOT to reject affected offline requests in the callback.
