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

[Wei Yang]

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?

>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me