Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating the hot remove range

[David Hildenbrand]

> Am 13.01.2020 um 10:10 schrieb Anshuman Khandual <anshuman.khandual@xxxxxxx>:
> 
> ï
> 
>> 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.