linux-kernel - Re: [RFC] arm64: mm: update max

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Date:   Mon, 27 Sep 2021 22:14:24 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Georgi Djakov <quic_c_gdjako@...cinc.com>,
        Chris Goldsworthy <quic_cgoldswo@...cinc.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org,
        Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug

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@...cinc.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@...cinc.com>
>>>> Signed-off-by: Chris Goldsworthy <quic_cgoldswo@...cinc.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@...cinc.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.

-- 
Thanks,

David / dhildenb