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Message-ID: <c83b7e80-0a16-d83f-f3f4-ab0b32c7753b@quicinc.com>
Date:   Mon, 27 Sep 2021 23:00:25 +0300
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Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug
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To:     David Hildenbrand <david@redhat.com>,
        Chris Goldsworthy <quic_cgoldswo@quicinc.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        "Will Deacon" <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
CC:     <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
References: <cover.1632437225.git.quic_cgoldswo@quicinc.com>
 <595d09279824faf1f54961cef52b745609b05d97.1632437225.git.quic_cgoldswo@quicinc.com>
 <4f4f6e93-ddc2-c9bb-ef47-4a40f2c3d0d7@quicinc.com>
 <1b9c4377-0a6b-b7d4-7a6b-a69f9469bb70@redhat.com>
From:   Georgi Djakov <quic_c_gdjako@quicinc.com>
In-Reply-To: <1b9c4377-0a6b-b7d4-7a6b-a69f9469bb70@redhat.com>
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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.

Thanks,
Georgi

> wouldn't really do what you would expect either. But you could 
> force-read beyond max_pfn, yes, because the count computation was just 
> weird.
> 
> 
> I think the real issue is not properly adjusting max_pfn in the first 
> place when we introduced memoruy hotplug on arm64