linux-kernel - Re: [PATCH] ARC: Use max_high

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Date:   Mon, 26 Apr 2021 16:31:28 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>,
        Vladimir Isaev <Vladimir.Isaev@...opsys.com>
CC:     "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARC: Use max_high_pfn as a HIGHMEM zone border

On 4/26/21 9:15 AM, Mike Rapoport wrote:
> On Mon, Apr 26, 2021 at 11:55:00AM +0000, Vladimir Isaev wrote:
>> Hi Mike,
>>
>> On Mon, April 26, 2021 2:29 PM, Mike Rapoport wrote:
>>> On Mon, Apr 26, 2021 at 01:10:04PM +0300, Vladimir Isaev wrote:
>>>> -	max_zone_pfn[ZONE_HIGHMEM] = min_low_pfn;
>>>> +	max_zone_pfn[ZONE_HIGHMEM] = max_high_pfn;
>>> This is correct with PAE40, but it will break !PAE40 when "highmem" has lower
>>> addresses than lowmem.
>>>
>>> It rather should be something like:
>>>
>>>          if (IS_ENABLED(CONFIG_ARC_HAS_PAE40))
>>>                  max_zone_pfn[ZONE_HIGHMEM] = max_high_pfn;
>>>          else
>>>              	max_zone_pfn[ZONE_HIGHMEM] = min_low_pfn;
>>>
>> Not sure if I understand why we should have min_low_pfn here. In !PAE40
>> case max_high_pfn just will be smaller than min_low_pfn.
> Hmm, actually, you are right. This should be fine.

But still worth adding a comment. If this could trip the person who did 
the massive cross-arch rework then I don't know what mere mortals would 
run into ;-)

-Vineet