Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug
To:     Georgi Djakov <quic_c_gdjako@quicinc.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>
 <c83b7e80-0a16-d83f-f3f4-ab0b32c7753b@quicinc.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <a2712ae7-ecbb-ef74-4b73-d31b152cbab1@redhat.com>
Date:   Mon, 27 Sep 2021 22:14:24 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.11.0
MIME-Version: 1.0
In-Reply-To: <c83b7e80-0a16-d83f-f3f4-ab0b32c7753b@quicinc.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.

-- 
Thanks,

David / dhildenb