Re: [v2 PATCH] mm: migrate: handle freed page at the first place

[Yang Shi <yang.shi@linux.alibaba.com>]

On 11/14/19 10:56 AM, Michal Hocko wrote:
> On Fri 15-11-19 02:24:29, Yang Shi wrote:
>> When doing migration if the freed page is met, we just return without
>> migrating it since it is pointless to migrate a freed page.  But, the
>> current code allocates target page unconditionally before handling freed
>> page, if the page is freed, the newly allocated will be just freed.  It
>> doesn't make too much sense and is just a waste of time although
>> migrating freed page is rare.
>>
>> So, handle freed page at the before that to avoid unnecessary page
>> allocation and free.
>>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> I would be really surprised if this led to any runtime visible effect
> but I do agree that one less put_page path looks slightly better. For
> that reason
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

>
>> ---
>> v2: * Keep thp migration support check before handling freed page per Michal Hocko
>>      * Fixed the build warning reported by 0-day
>>
>>   mm/migrate.c | 14 +++++---------
>>   1 file changed, 5 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 4fe45d1..a8f87cb 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1168,15 +1168,11 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>>   				   enum migrate_reason reason)
>>   {
>>   	int rc = MIGRATEPAGE_SUCCESS;
>> -	struct page *newpage;
>> +	struct page *newpage = NULL;
>>   
>>   	if (!thp_migration_supported() && PageTransHuge(page))
>>   		return -ENOMEM;
>>   
>> -	newpage = get_new_page(page, private);
>> -	if (!newpage)
>> -		return -ENOMEM;
>> -
>>   	if (page_count(page) == 1) {
>>   		/* page was freed from under us. So we are done. */
>>   		ClearPageActive(page);
>> @@ -1187,13 +1183,13 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>>   				__ClearPageIsolated(page);
>>   			unlock_page(page);
>>   		}
>> -		if (put_new_page)
>> -			put_new_page(newpage, private);
>> -		else
>> -			put_page(newpage);
>>   		goto out;
>>   	}
>>   
>> +	newpage = get_new_page(page, private);
>> +	if (!newpage)
>> +		return -ENOMEM;
>> +
>>   	rc = __unmap_and_move(page, newpage, force, mode);
>>   	if (rc == MIGRATEPAGE_SUCCESS)
>>   		set_page_owner_migrate_reason(newpage, reason);
>> -- 
>> 1.8.3.1
>>