Re: [PATCH] mm/sparsemem: pfn_to_page is not valid yet on SPARSEMEM

[Wei Yang]

On Thu, Feb 06, 2020 at 02:59:50PM +0100, David Hildenbrand wrote:
>On 06.02.20 14:57, Wei Yang wrote:
>> On Thu, Feb 06, 2020 at 02:28:53PM +0100, David Hildenbrand wrote:
>>> On 06.02.20 13:53, Wei Yang wrote:
>>>> When we use SPARSEMEM instead of SPARSEMEM_VMEMMAP, pfn_to_page()
>>>> doesn't work before sparse_init_one_section() is called. This leads to a
>>>> crash when hotplug memory.
>>>>
>>>> We should use memmap as it did.
>>>>
>>>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>>>> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>>>> CC: Dan Williams <dan.j.williams@xxxxxxxxx>
>>>> ---
>>>>  mm/sparse.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>>> index 5a8599041a2a..2efb24ff8f96 100644
>>>> --- a/mm/sparse.c
>>>> +++ b/mm/sparse.c
>>>> @@ -882,7 +882,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
>>>>  	 * Poison uninitialized struct pages in order to catch invalid flags
>>>>  	 * combinations.
>>>>  	 */
>>>> -	page_init_poison(pfn_to_page(start_pfn), sizeof(struct page) * nr_pages);
>>>> +	page_init_poison(memmap, sizeof(struct page) * nr_pages);
>>>
>>> If you add sub-sections that don't fall onto the start of the section,
>>>
>>> pfn_to_page(start_pfn) != memmap
>>>
>>> and your patch would break that under SPARSEMEM_VMEMMAP if I am not wrong.
>>>
>>> Instead of memmap, there would have to be something like
>>>
>>> memmap + (start_pfn - SECTION_ALIGN_DOWN(start_pfn))
>>>
>>> If I am not wrong :)
>> 
>> Hi, David, Thanks for your comment.
>> 
>> To be hones, I am not familiar with SPARSEMEM_VMEMMAP. Here is my
>> understanding about section_activate() when SPARSEMEM_VMEMMAP is set.
>> 
>>   section_activate(nid, start_pfn, nr_pages, altmap)
>>     populate_section_mmemap(start_pfn, nr_pages, nid, altmap)
>>       __populate_section_mmemap(start_pfn, nr_pages, nid, altmap)
>>         return pfn_to_page(start_pfn)
>> 
>> So the memmap is the page struct for start_pfn when SPARSEMEM_VMEMMAP is set.
>> 
>> Maybe I missed some critical part?
>
>I was assuming that memmap is the memmap of the section, not of the
>sub-section. (judging from the change in the original patch)
>
>If the right memmap pointer to the sub-section is returned, then we are
>fine. Will double check :)
>

Thanks, your comments are valuable :-)

>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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