Re: [PATCH v2 1/5] mm: memory_hotplug: remove head variable in do_migrate_range()

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

On 2024/8/19 17:28, Jonathan Cameron wrote:
> On Sat, 17 Aug 2024 16:49:37 +0800
> Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
> 
>> Directly use a folio for HugeTLB and THP when calculate the next pfn,
>> then remove unused head variable.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> 
> Hi,
> 
> I was curious so had a read and have a quick comment
> on the comment.
> 
> 
>> ---
>>   mm/memory_hotplug.c | 22 ++++++++++++++--------
>>   1 file changed, 14 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index df291f2e509d..dc19b0e28fbc 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1773,7 +1773,7 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>>   static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>>   {
>>   	unsigned long pfn;
>> -	struct page *page, *head;
>> +	struct page *page;
>>   	LIST_HEAD(source);
>>   	static DEFINE_RATELIMIT_STATE(migrate_rs, DEFAULT_RATELIMIT_INTERVAL,
>>   				      DEFAULT_RATELIMIT_BURST);
>> @@ -1786,14 +1786,20 @@ static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>>   			continue;
>>   		page = pfn_to_page(pfn);
>>   		folio = page_folio(page);
>> -		head = &folio->page;
>>   
>> -		if (PageHuge(page)) {
>> -			pfn = page_to_pfn(head) + compound_nr(head) - 1;
>> -			isolate_hugetlb(folio, &source);
>> -			continue;
>> -		} else if (PageTransHuge(page))
>> -			pfn = page_to_pfn(head) + thp_nr_pages(page) - 1;
>> +		/*
>> +		 * The folio hold no reference or lock, it might be changed
>> +		 * concurrently(eg, split), folio_nr_pages() may read garbage,
>> +		 * but out loop could handle that as it revisits the split
>> +		 * folio later.
>> +		 */
> Perhaps reword this (it confused me a bit) I'm not sure I've retained
> the intended meaning!
> 
> 		* No reference or lock is held on the folio, so it might
> 		* be modified concurrently (e.g. split).  As such,
> 		* folio_nr_pages() may read garbage.  This is fine as the
> 		* outer loop will revisit the split folio later.

Thanks. hope Andrew could squash it if no more comments about this
patchset, or I will resend a new version with comment changes.


> 
> 
>> +		if (folio_test_large(folio)) {
>> +			pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1;
>> +			if (folio_test_hugetlb(folio)) {
>> +				isolate_hugetlb(folio, &source);
>> +				continue;
>> +			}
>> +		}
>>   
>>   		/*
>>   		 * HWPoison pages have elevated reference counts so the migration would
>