linux-kernel - Re: [PATCH v5] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

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Message-ID: <dc69cc0e-c3ca-4e2a-8d0e-998643f31ccf@quicinc.com>
Date: Thu, 13 Feb 2025 15:45:47 +0800
From: Zhenhua Huang <quic_zhenhuah@...cinc.com>
To: Catalin Marinas <catalin.marinas@....com>
CC: <anshuman.khandual@....com>, <will@...nel.org>, <ardb@...nel.org>,
        <ryan.roberts@....com>, <mark.rutland@....com>, <joey.gouly@....com>,
        <dave.hansen@...ux.intel.com>, <akpm@...ux-foundation.org>,
        <chenfeiyang@...ngson.cn>, <chenhuacai@...nel.org>,
        <linux-mm@...ck.org>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <quic_tingweiz@...cinc.com>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH v5] arm64: mm: Populate vmemmap/linear at the page level
 for hotplugged sections



On 2025/2/13 2:28, Catalin Marinas wrote:
>> @@ -1339,9 +1349,27 @@ int arch_add_memory(int nid, u64 start, u64 size,
>>   		    struct mhp_params *params)
>>   {
>>   	int ret, flags = NO_EXEC_MAPPINGS;
>> +	unsigned long start_pfn = PFN_DOWN(start);
>> +	struct mem_section *ms = __pfn_to_section(start_pfn);
>>   
>>   	VM_BUG_ON(!mhp_range_allowed(start, size, true));
>>   
>> +	/* should not be invoked by early section */
>> +	WARN_ON(early_section(ms));
> I don't remember the discussion, do we still need this warning here if
> the sections are not marked as early? I guess we can keep it if one does
> an arch_add_memory() on an early section.
> 
> I think I suggested to use a WARN_ON_ONCE(!present_section()) but I
> completely forgot the memory hotplug code paths.

Dear Catalin,

The previous discussion can be found at 
https://lore.kernel.org/lkml/aedbbc4f-8f6c-46d8-a8d7-53103675a816@quicinc.com/, 
I highlighted the key points from conversation between me and Anshuman 
for your reference:
"
 >>
 >> BTW, shall we remove the check for !early_section since 
arch_add_memory is only called during hotplugging case? Correct me 
please if I'm mistaken :)
 >
 > While this is true, still might be a good idea to keep the 
early_section()
 > check in place just to be extra careful here. Otherwise an WARN_ON() 
might
 > be needed.

Make sense. I would like to add some comments and WARN_ON() if
early_section().
"
Regarding your suggestion, I believed it was intended for the 
vmemmap_populate() function ?(Discussion: 
https://lore.kernel.org/linux-mm/Z3_d59kp4CuHQp97@arm.com/), but as 
workflow below indicates:
Hot plug:
1. section_activate -> vmemmap_populate
2. mark PRESENT

In contrast, the early flow:
1. memblocks_present -> mark PRESENT
2. __populate_section_memmap -> vmemmap_populate

Could this result in a false warning during hotplugging? I replied with 
the doubt in above link before but seems you missed :) Could you please 
share your thoughts if you have a different idea ?

I will include your tags, correct capitalization nit and post one new 
version.