linux-kernel - Re: [PATCH v2 1/7] mm,memory_hotplug: Allocate memmap from the added memory range

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Date:   Mon, 1 Mar 2021 09:32:19 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, VlastimilBabkavbabka@...e.cz,
        pasha.tatashin@...een.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2 1/7] mm,memory_hotplug: Allocate memmap from the added
 memory range


>>> +		(*online_page_callback)(pfn_to_page(pfn), pageblock_order);
>>> +		pfn += 1 << pageblock_order;
>>
>> pfn += pageblock_nr_pages;
>>
>> Can you add a comment why we can be sure that we are off by  a single pageblock? What about s390x where a MAX_ORDER_NR_PAGES == 4 * pageblock_nr_pages?
>>
>> Would it make thing simpler to just do a
>>
>> while (!IS_ALIGNED(pfn, MAX_ORDER_NR_PAGES)) {
>> 	(*online_page_callback)(pfn_to_page(pfn), 0);
>> 	pfn++;
>> }
> 
> Honestly, I did not spend much time thinking on other platforms other
> than arm64/x86_64.
> But I think that that would be the universal solution as we do not make
> any assumptions.
> 
> I will replace it.


I think you can safely go with

while (!IS_ALIGNED(pfn, MAX_ORDER_NR_PAGES)) {
	(*online_page_callback)(pfn_to_page(pfn), pageblock_order);
	pfn += pageblock_nr_pages;
}

and maybe add before the loop

VM_BUG_ON(!IS_ALIGNED(pfn, pageblock_nr_pages));

as help for the reader that this always holds.

-- 
Thanks,

David / dhildenb