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Subject: Re: [PATCH -next] mm/hotplug: skip bad PFNs from pfn_to_online_page()
To:     Oscar Salvador <osalvador@suse.de>
Cc:     Qian Cai <cai@lca.pw>, Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
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 <20190614153535.GA9900@linux>
From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Date:   Fri, 14 Jun 2019 21:48:13 +0530
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On 6/14/19 9:05 PM, Oscar Salvador wrote:
> On Fri, Jun 14, 2019 at 02:28:40PM +0530, Aneesh Kumar K.V wrote:
>> Can you check with this change on ppc64.  I haven't reviewed this series yet.
>> I did limited testing with change . Before merging this I need to go
>> through the full series again. The vmemmap poplulate on ppc64 needs to
>> handle two translation mode (hash and radix). With respect to vmemap
>> hash doesn't setup a translation in the linux page table. Hence we need
>> to make sure we don't try to setup a mapping for a range which is
>> arleady convered by an existing mapping.
>>
>> diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
>> index a4e17a979e45..15c342f0a543 100644
>> --- a/arch/powerpc/mm/init_64.c
>> +++ b/arch/powerpc/mm/init_64.c
>> @@ -88,16 +88,23 @@ static unsigned long __meminit vmemmap_section_start(unsigned long page)
>>    * which overlaps this vmemmap page is initialised then this page is
>>    * initialised already.
>>    */
>> -static int __meminit vmemmap_populated(unsigned long start, int page_size)
>> +static bool __meminit vmemmap_populated(unsigned long start, int page_size)
>>   {
>>   	unsigned long end = start + page_size;
>>   	start = (unsigned long)(pfn_to_page(vmemmap_section_start(start)));
>>   
>> -	for (; start < end; start += (PAGES_PER_SECTION * sizeof(struct page)))
>> -		if (pfn_valid(page_to_pfn((struct page *)start)))
>> -			return 1;
>> +	for (; start < end; start += (PAGES_PER_SECTION * sizeof(struct page))) {
>>   
>> -	return 0;
>> +		struct mem_section *ms;
>> +		unsigned long pfn = page_to_pfn((struct page *)start);
>> +
>> +		if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
>> +			return 0;
> 
> I might be missing something, but is this right?
> Having a section_nr above NR_MEM_SECTIONS is invalid, but if we return 0 here,
> vmemmap_populate will go on and populate it.

I should drop that completely. We should not hit that condition at all. 
I will send a final patch once I go through the full patch series making 
sure we are not breaking any ppc64 details.

Wondering why we did the below

#if defined(ARCH_SUBSECTION_SHIFT)
#define SUBSECTION_SHIFT (ARCH_SUBSECTION_SHIFT)
#elif defined(PMD_SHIFT)
#define SUBSECTION_SHIFT (PMD_SHIFT)
#else
/*
  * Memory hotplug enabled platforms avoid this default because they
  * either define ARCH_SUBSECTION_SHIFT, or PMD_SHIFT is a constant, but
  * this is kept as a backstop to allow compilation on
  * !ARCH_ENABLE_MEMORY_HOTPLUG archs.
  */
#define SUBSECTION_SHIFT 21
#endif

why not

#if defined(ARCH_SUBSECTION_SHIFT)
#define SUBSECTION_SHIFT (ARCH_SUBSECTION_SHIFT)
#else
#define SUBSECTION_SHIFT  SECTION_SHIFT
#endif

ie, if SUBSECTION is not supported by arch we have one sub-section per 
section?


-aneesh