Re: [PATCH V3 2/3] arm64/mm/hotplug: Enable MEM_OFFLINE event handling

[Anshuman Khandual]

On 09/23/2020 12:01 PM, Gavin Shan wrote:
> Hi Anshuman,
> 
> On 9/21/20 10:05 PM, Anshuman Khandual wrote:
>> This enables MEM_OFFLINE memory event handling. It will help intercept any
>> possible error condition such as if boot memory some how still got offlined
>> even after an explicit notifier failure, potentially by a future change in
>> generic hot plug framework. This would help detect such scenarios and help
>> debug further.
>>
>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Cc: Will Deacon <will@xxxxxxxxxx>
>> Cc: Mark Rutland <mark.rutland@xxxxxxx>
>> Cc: Marc Zyngier <maz@xxxxxxxxxx>
>> Cc: Steve Capper <steve.capper@xxxxxxx>
>> Cc: Mark Brown <broonie@xxxxxxxxxx>
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> ---
> 
> I'm not sure if it makes sense since MEM_OFFLINE won't be triggered
> after NOTIFY_BAD is returned from MEM_GOING_OFFLINE. NOTIFY_BAD means
> the whole offline process is stopped. It would be guranteed by generic
> framework from syntax standpoint.

Right but the intent here is to catch any deviation in generic hotplug
semantics going forward.
 > 
> However, this looks good if MEM_OFFLINE is triggered without calling
> into MEM_GOING_OFFLINE previously, but it would be a bug from generic
> framework.

Exactly, this will just ensure that we know about any change or a bug
in the generic framework. But if required, this additional check can
be enabled only with DEBUG_VM.

> 
>>   arch/arm64/mm/mmu.c | 37 ++++++++++++++++++++++++++++++++-----
>>   1 file changed, 32 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index df3b7415b128..6b171bd88bcf 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -1482,13 +1482,40 @@ static int prevent_bootmem_remove_notifier(struct notifier_block *nb,
>>       unsigned long end_pfn = arg->start_pfn + arg->nr_pages;
>>       unsigned long pfn = arg->start_pfn;
>>   -    if (action != MEM_GOING_OFFLINE)
>> +    if ((action != MEM_GOING_OFFLINE) && (action != MEM_OFFLINE))
>>           return NOTIFY_OK;
>>   -    for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
>> -        ms = __pfn_to_section(pfn);
>> -        if (early_section(ms))
>> -            return NOTIFY_BAD;
>> +    if (action == MEM_GOING_OFFLINE) {
>> +        for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
>> +            ms = __pfn_to_section(pfn);
>> +            if (early_section(ms)) {
>> +                pr_warn("Boot memory offlining attempted\n");
>> +                return NOTIFY_BAD;
>> +            }
>> +        }
>> +    } else if (action == MEM_OFFLINE) {
>> +        for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
>> +            ms = __pfn_to_section(pfn);
>> +            if (early_section(ms)) {
>> +
>> +                /*
>> +                 * This should have never happened. Boot memory
>> +                 * offlining should have been prevented by this
>> +                 * very notifier. Probably some memory removal
>> +                 * procedure might have changed which would then
>> +                 * require further debug.
>> +                 */
>> +                pr_err("Boot memory offlined\n");
>> +
>> +                /*
>> +                 * Core memory hotplug does not process a return
>> +                 * code from the notifier for MEM_OFFLINE event.
>> +                 * Error condition has been reported. Report as
>> +                 * ignored.
>> +                 */
>> +                return NOTIFY_DONE;
>> +            }
>> +        }
>>       }
>>       return NOTIFY_OK;
>>   }
>>
> 
> It's pretty much irrelevant comment if the patch doesn't make sense:
> the logical block for MEM_GOING_OFFLINE would be reused by MEM_OFFLINE
> as they looks similar except the return value and error message :)

This can be reorganized in the above mentioned format as well. Without
much additional code or iteration, it might not need DEBUG_VM as well.

for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
	ms = __pfn_to_section(pfn);
	if (!early_section(ms))
		continue;

	if (action == MEM_GOING_OFFLINE) {
		pr_warn("Boot memory offlining attempted\n");
		return NOTIFY_BAD;
	}
	else if (action == MEM_OFFLINE) {
		pr_err("Boot memory offlined\n");
		return NOTIFY_DONE;
	}
}
return NOTIFY_OK;