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

[Wei Yang]

On Thu, Feb 06, 2020 at 03:37:40PM +0100, David Hildenbrand wrote:
>On 06.02.20 15:07, Baoquan He wrote:
>> On 02/06/20 at 02:55pm, David Hildenbrand wrote:
>>> On 06.02.20 14:50, Baoquan He wrote:
>>>> On 02/06/20 at 02:28pm, 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.
>>>>
>>>> It returns the pfn_to_page(pfn) from __populate_section_memmap() and
>>>> assign to memmap in vmemmap case, how come it breaks anything. Correct
>>>> me if I was wrong.
>>>
>>> I'm sorry, I can't follow :) Can you elaborate?
>>>
>>> Was your comment targeted at why the old code cannot be broken or why
>>> this patch cannot be broken?
>> 
>> Sorry for the confusion :-) the latter. I mean the returned memmap has been
>> at the pfn_to_page(start_pfn) in SPARSEMEM_VMEMMAP case.
>
>Yeah, at least for SPARSEMEM_VMEMMAP it is indeed right. Thanks :)
>
>
>Now, about SPARSEMEM:
>
>populate_section_memmap() does not care about nr_pages and will allocate
>a memmap for the whole section. So, whenever we add sub-sections to a
>section, we allocate a new memmap for the whole section. And we do
>overwrite the memmap pointer in our section. ( sparse_add_section() )
>
>That makes me assume that sub-section hot-add under SPARSEMEM is either
>
>a) never enabled and only works with SPARSEMEM_VMEMMAP
>b) horribly broken
>
>And I think a) applies (looking at pfn_section_valid()). Therefore, we
>don't have to care about sub-section hot-add specifics (and I would be
>broken already)

Yes, I am looking into this problem. Actually, there maybe another problem.

Just get my brain refreshed, need some time to dig into.

>
>Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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