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(). -- Thanks, David / dhildenb
