Re: [PATCH v4 3/6] mm: migrate: split folio_migrate_mapping()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/6/6 8:54, Jane Chu wrote:
> [..]
>> -int folio_migrate_mapping(struct address_space *mapping,
>> -		struct folio *newfolio, struct folio *folio, int extra_count)
>> +static void __folio_migrate_mapping(struct address_space *mapping,
>> +		struct folio *newfolio, struct folio *folio, int expected_cnt)
>>   {
>>   	XA_STATE(xas, &mapping->i_pages, folio_index(folio));
>>   	struct zone *oldzone, *newzone;
>> -	int dirty;
>> -	int expected_count = folio_expected_refs(mapping, folio) + extra_count;
>>   	long nr = folio_nr_pages(folio);
>>   	long entries, i;
>> +	int dirty;
>>   
>>   	if (!mapping) {
>> -		/* Anonymous page without mapping */
> 
> If 'mapping' was NULL, the first line would blow up while dereferencing 
> 'mapping->i_pages'.

0->i_pages is wrong, but &(0->i_pages) is legal, and then xas->xa = NULL.
> 
> The ordering change was introduced by commit
> 
> 89eb946a7432b "(mm: Convert page migration to XArray)"
> 
> Matthew, please correct me if I'm missing something.
> 
> thanks,
> 
> -jane
>