Re: [RFC] arm64: mm: update max_pfn after memory hotplug

[Georgi Djakov <quic_c_gdjako@quicinc.com>]

On 9/27/2021 11:14 PM, David Hildenbrand wrote:
> On 27.09.21 22:00, Georgi Djakov wrote:
>> On 9/27/2021 8:34 PM, David Hildenbrand wrote:
>>> On 27.09.21 19:22, Georgi Djakov wrote:
>>>> On 9/24/2021 1:54 AM, Chris Goldsworthy wrote:
>>>>> From: Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
>>>>>
>>>>> After new memory blocks have been hotplugged, max_pfn and max_low_pfn
>>>>> needs updating to reflect on new PFNs being hot added to system.
>>>>>
>>>>> Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
>>>>> Signed-off-by: Chris Goldsworthy <quic_cgoldswo@quicinc.com>
>>>>
>>>> Thanks for the patch, Chris!
>>>>
>>>> With this patch, the data in /proc/kpageflags appears to be correct and
>>>> memory tools like procrank work again on arm64 platforms.
>>>>
>>>> Tested-by: Georgi Djakov <quic_c_gdjako@quicinc.com>
>>>>
>>>> Maybe we should add fixes tag, as it has been broken since the 
>>>> following
>>>> commit:
>>>> Fixes: abec749facff ("fs/proc/page.c: allow inspection of last section
>>>> and fix end detection")
>>>
>>> Are you sure that that commit broke it?
>>
>> Reverting the above commit also "fixes" kpageflags, otherwise
>> kpageflags_read() returns 0 in the following check:
>>     if (src >= max_dump_pfn * KPMSIZE)
>>         return 0;
>>
>>> I recall that we would naturally run into the limit, because
>>>
>>> count = min_t(size_t, count, (max_pfn * KPMSIZE) - src);
>>
>> The function returns before we reach this line.
> 
> That is the old code. I don't see how the behavior of the old code with 
> wrong max_pfn was doing what it's supposed to do.
> 
> page_idle and page_owner also rely on max_pfn. The root issue is that 
> max_pfn wasn't updated properly.

Ok, i agree that this is not really fixing abec749facff, as it only 
reveals the real issue, so then the fixes tag should be even further
back to:
Fixes: 4ab215061554 ("arm64: Add memory hotplug support")

Thanks,
Georgi