On 01/13/2020 04:07 PM, David Hildenbrand wrote: > On 13.01.20 10:50, Anshuman Khandual wrote: >> >> >> 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. > > Do you really need that? > > We have SECTION_IS_EARLY. You could iterate all involved sections (for > which you are getting notified) and check if any one of these is marked > SECTION_IS_EARLY. then, it was added during boot and not via add_memory(). Seems to be a better approach than adding a new memblock flag. > >
