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Subject: Re: [PATCH -next] mm/hotplug: skip bad PFNs from pfn_to_online_page()
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Oscar Salvador <osalvador@suse.de>, Qian Cai <cai@lca.pw>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        jmoyer <jmoyer@redhat.com>
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From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Date:   Fri, 14 Jun 2019 21:56:00 +0530
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On 6/14/19 9:52 PM, Dan Williams wrote:
> On Fri, Jun 14, 2019 at 9:18 AM Aneesh Kumar K.V
> <aneesh.kumar@linux.ibm.com> wrote:
>>
>> 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

That should be SECTION_SIZE_SHIFT

>> #endif
>>
>> ie, if SUBSECTION is not supported by arch we have one sub-section per
>> section?
> 
> A couple comments:
> 
> The only reason ARCH_SUBSECTION_SHIFT exists is because PMD_SHIFT on
> PowerPC was a non-constant value. However, I'm planning to remove the
> distinction in the next rev of the patches. Jeff rightly points out
> that having a variable subsection size per arch will lead to
> situations where persistent memory namespaces are not portable across
> archs. So I plan to just make SUBSECTION_SHIFT 21 everywhere.
> 


persistent memory namespaces are not portable across archs because they 
have PAGE_SIZE dependency. Then we have dependencies like the page size 
with which we map the vmemmap area. Why not let the arch
arch decide the SUBSECTION_SHIFT and default to one subsection per 
section if arch is not enabled to work with subsection.

-aneesh